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Purpose: We evaluated the hypothesis that repetitive gravitoinertial stress would augment the arterial-pressure response to peripheral sympathetic stimulation. Methods: Before and after a 5-weeks G-training regimen conducted in a human-use centrifuge, twenty healthy men performed a hand cold-pressor test, and nine of them also a foot cold-pressor test (4 min; 4 °C water). Arterial pressures and total peripheral resistance were monitored. Results: The cold-induced elevation (P ≤ 0.002) in arterial pressures and total peripheral resistance did not vary between testing periods, either in the hand [mean arterial pressure: Before = + 16% vs. After = + 17% and total peripheral resistance: Before = + 13% vs. After = + 15%], or in the foot [mean arterial pressure: Before = + 19% vs. After = + 21% and total peripheral resistance: Before = + 16% vs. After = + 16%] cold-pressor tests (P > 0.05). Conclusion: Present results demonstrate that 5 weeks of prolonged iterative exposure to hypergravity does not alter the responsiveness of sympathetically mediated circulatory reflexes. [ABSTRACT FROM AUTHOR]

We sought to investigate the effect of the α1-adrenergic receptor blockade during handgrip exercise (Grip), isolated metaboreflex activation (Metabo), and cold pressor test (CPT) on coronary circulation in young (YW) and postmenopausal women (PMW). Ten YW and 9 PMW underwent two protocols: (1) 3 min of baseline followed by 3 min of CPT and (2) 3 min of rest, 3 min of Grip followed by 3 min of Metabo. Protocols were carried out under control conditions and α1-adrenergic receptor blockade (oral prazosin 0.03 mg·kg−1). Coronary blood velocity (CBV) and vascular conductance (CCI) were lower in PMW. Grip increased CBV only in YW (YW: Δ18.0 ± 21.1% vs. PMW: Δ4.2 ± 10.1%; p < 0.05), and the blockade did not change the CBV response to Grip in YW and PMW. During the Metabo, CBV returned to resting levels in YW and was unchanged from rest in PMW, before (YW:Δ1.7 ± 8.7% vs. PMW: Δ− 1.5 ± 8.6) and under the blockade (YW: Δ4.5 ± 14.8% vs. PMW: Δ9.1 ± 29.5%). CPT did not change CBV in both groups (YW: Δ3.9 ± 8.0 vs. PMW: Δ− 4.1 ± 6.2%), following the α1-blockade, CPT increased CBV only in YW (YW: Δ11.2 ± 12.8% vs. PMW: Δ2.2 ± 7.1%; p < 0.05 for group and condition). CCI decreased during Grip, Metabo, and CPT in YW and PMW, while the blockade prevented that decrease only in YW. The α1-adrenergic receptor plays a role in the control of coronary circulation in young women, evoking stronger vasoconstriction during CPT than Grip and Metabo in YW. PMW have impaired vasomotor control in the coronary circulation, which seems not to be caused by the α1-adrenergic receptor. [ABSTRACT FROM AUTHOR]

Purpose: Muscle stretching effect on the range of motion (ROM) and force deficit in non-stretched muscle, and the underlying mechanisms, is an ongoing issue. This study aimed to investigate crossover stretching effects and mechanisms on the plantar flexor muscles. Methods: Fourteen recreationally active females (n = 5) and males (n = 9) performed six sets of 45-s static stretching (SS) (15-s recovery) to the point of discomfort of the dominant leg (DL) plantar flexors or control (345-s rest). Participants were tested for a single 5-s pre- and post-test maximal voluntary isometric contraction (MVIC) with each plantar flexor muscle and were tested for DL and non-DL ROM. They were tested pre- and post-test (immediate, 10-s, 30-s) for the Hoffman (H)-reflex and motor-evoked potentials (MEP) from transcranial magnetic stimulation in the contralateral, non-stretched muscle. Results: Both the DL and non-DL-MVIC force had large magnitude, significant (↓10.87%, p = 0.027, pƞ2 = 0.4) and non-significant (↓9.53%, p = 0.15, pƞ2 = 0.19) decreases respectively with SS. The SS also significantly improved the DL (6.5%, p < 0.001) and non-DL (5.35%, p = 0.002) ROM. The non-DL MEP/MMax and HMax/MMax ratio did not change significantly. Conclusion: Prolonged static stretching improved the stretched muscle's ROM. However, the stretched limb's force was negatively affected following the stretching protocol. The ROM improvement and large magnitude force impairment (statistically non-significant) were transferred to the contralateral muscles. The lack of significant changes in spinal and corticospinal excitability confirms that the afferent excitability of the spinal motoneurons and corticospinal excitability may not play a substantial role in non-local muscle's ROM or force output responses. [ABSTRACT FROM AUTHOR]

Previous studies in animal models showed that exercise-induced metabolites accumulation may sensitize the mechanoreflex-induced response. The aim of this study was to assess whether the magnitude of the central hemodynamic and ventilatory adjustments evoked by isolated stimulation of the mechanoreceptors in humans are influenced by the prior accumulation of metabolic byproducts in the muscle. 10 males and 10 females performed two exercise bouts consisting of 5-min of intermittent isometric knee-extensions performed 10% above the previously determined critical force. Post-exercise, the subjects recovered for 5 min either with a suprasystolic circulatory occlusion applied to the exercised quadriceps (PECO) or under freely-perfused conditions (CON). Afterwards, 1-min of continuous passive leg movement was performed. Central hemodynamics, pulmonary data, and electromyography from exercising/passively-moved leg were recorded throughout the trial. Root mean square of successive differences (RMSSD, index of vagal tone) was also calculated. Δpeak responses of heart rate (ΔHR) and ventilation ( Δ V ˙ E ) to passive leg movement were higher in PECO compared to CON (ΔHR: 6 ± 5 vs 2 ± 4 bpm, p = 0.01; 3.9 ± 3.4 vs 1.9 ± 1.7 L min−1, p = 0.02). Δpeak of mean arterial pressure (ΔMAP) was significantly different between conditions (5 ± 3 vs − 3 ± 3 mmHg, p < 0.01). Changes in RMSSD with passive leg movement were different between PECO and CON (p < 0.01), with a decrease only in the former (39 ± 18 to 32 ± 15 ms, p = 0.04). No difference was found in all the other measured variables between conditions (p > 0.05). These findings suggest that mechanoreflex-mediated increases in HR and V ˙ E are sensitized by metabolites accumulation. These responses were not influenced by biological sex. [ABSTRACT FROM AUTHOR]

Research in movement science aims at unravelling mechanisms and designing methods for restoring and maximizing human functional capacity, and many techniques provide access to neural adjustments (acute changes) or long-term adaptations (chronic changes) underlying changes in movement capabilities. First described by Paul Hoffmann over a century ago, when an electrical stimulus is applied to a peripheral nerve, this causes action potentials in afferent axons, primarily the Ia afferents of the muscle spindles, which recruit homonymous motor neurons, thereby causing an electromyographic response known as the Hoffmann (H) reflex. This technique is a valuable tool in the study of the neuromuscular function in humans and has provided relevant information in the neural control of movement. The large use of the H reflex in motor control research on humans relies in part to its relative simplicity. However, such simplicity masks subtleties that require rigorous experimental protocols and careful data interpretation. After highlighting basic properties and methodological aspects that should be considered for the correct use of the H-reflex technique, this brief narrative review discusses the purpose of the H reflex and emphasizes its use as a tool to assess the effectiveness of Ia afferents in discharging motor neurones. The review also aims to reconsider the link between H-reflex modulation and Ia presynaptic inhibition, the use of the H-reflex technique in motor control studies, and the effects of ageing. These aspects are summarized as recommendations for the use of the H reflex in motor control research on humans. [ABSTRACT FROM AUTHOR]

Background: Vestibulospinal reflexes play a role in maintaining the upright posture of the trunk. Head orientation has been shown to modify the vestibulospinal reflexes during standing. This study investigated how vestibular signals affect paraspinal muscle activity during walking, and whether head orientation changes these effects.Sixteen participants were instructed to walk on a treadmill for 8 min at 78 steps/min and 2.8 km/h in four conditions defined by the presence of electrical vestibular stimulation (EVS) and by head orientation (facing forward and facing leftward), while bipolar electromyography (EMG) was recorded bilaterally from the paraspinal muscles from cervical to lumbar levels.In both head orientations, significant phasic EVS-EMG coherence in the paraspinal muscles was observed at ipsilateral and/or contralateral heel strikes. Compared to walking with the head forward, a significant decrease was found in EVS-evoked responses (i.e., EVS-EMG coherence and gain) when participants walked with the leftward head orientation, with which EVS induced disturbance in the sagittal plane. This overall decrease can be explained by less need of feedback control for walking stabilization in the sagittal plane compared to in the frontal plane. The decrease in coherence was only significant at the left lower vertebral levels and at the right upper vertebral levels around left heel strikes.These findings confirm the contribution of the vestibular afferent signals to the control of paraspinal muscle activity during walking and indicate that this control is changed in response to different head orientations.Methods: Vestibulospinal reflexes play a role in maintaining the upright posture of the trunk. Head orientation has been shown to modify the vestibulospinal reflexes during standing. This study investigated how vestibular signals affect paraspinal muscle activity during walking, and whether head orientation changes these effects.Sixteen participants were instructed to walk on a treadmill for 8 min at 78 steps/min and 2.8 km/h in four conditions defined by the presence of electrical vestibular stimulation (EVS) and by head orientation (facing forward and facing leftward), while bipolar electromyography (EMG) was recorded bilaterally from the paraspinal muscles from cervical to lumbar levels.In both head orientations, significant phasic EVS-EMG coherence in the paraspinal muscles was observed at ipsilateral and/or contralateral heel strikes. Compared to walking with the head forward, a significant decrease was found in EVS-evoked responses (i.e., EVS-EMG coherence and gain) when participants walked with the leftward head orientation, with which EVS induced disturbance in the sagittal plane. This overall decrease can be explained by less need of feedback control for walking stabilization in the sagittal plane compared to in the frontal plane. The decrease in coherence was only significant at the left lower vertebral levels and at the right upper vertebral levels around left heel strikes.These findings confirm the contribution of the vestibular afferent signals to the control of paraspinal muscle activity during walking and indicate that this control is changed in response to different head orientations.Results: Vestibulospinal reflexes play a role in maintaining the upright posture of the trunk. Head orientation has been shown to modify the vestibulospinal reflexes during standing. This study investigated how vestibular signals affect paraspinal muscle activity during walking, and whether head orientation changes these effects.Sixteen participants were instructed to walk on a treadmill for 8 min at 78 steps/min and 2.8 km/h in four conditions defined by the presence of electrical vestibular stimulation (EVS) and by head orientation (facing forward and facing leftward), while bipolar electromyography (EMG) was recorded bilaterally from the paraspinal muscles from cervical to lumbar levels.In both head orientations, significant phasic EVS-EMG coherence in the paraspinal muscles was observed at ipsilateral and/or contralateral heel strikes. Compared to walking with the head forward, a significant decrease was found in EVS-evoked responses (i.e., EVS-EMG coherence and gain) when participants walked with the leftward head orientation, with which EVS induced disturbance in the sagittal plane. This overall decrease can be explained by less need of feedback control for walking stabilization in the sagittal plane compared to in the frontal plane. The decrease in coherence was only significant at the left lower vertebral levels and at the right upper vertebral levels around left heel strikes.These findings confirm the contribution of the vestibular afferent signals to the control of paraspinal muscle activity during walking and indicate that this control is changed in response to different head orientations.Conclusion: Vestibulospinal reflexes play a role in maintaining the upright posture of the trunk. Head orientation has been shown to modify the vestibulospinal reflexes during standing. This study investigated how vestibular signals affect paraspinal muscle activity during walking, and whether head orientation changes these effects.Sixteen participants were instructed to walk on a treadmill for 8 min at 78 steps/min and 2.8 km/h in four conditions defined by the presence of electrical vestibular stimulation (EVS) and by head orientation (facing forward and facing leftward), while bipolar electromyography (EMG) was recorded bilaterally from the paraspinal muscles from cervical to lumbar levels.In both head orientations, significant phasic EVS-EMG coherence in the paraspinal muscles was observed at ipsilateral and/or contralateral heel strikes. Compared to walking with the head forward, a significant decrease was found in EVS-evoked responses (i.e., EVS-EMG coherence and gain) when participants walked with the leftward head orientation, with which EVS induced disturbance in the sagittal plane. This overall decrease can be explained by less need of feedback control for walking stabilization in the sagittal plane compared to in the frontal plane. The decrease in coherence was only significant at the left lower vertebral levels and at the right upper vertebral levels around left heel strikes.These findings confirm the contribution of the vestibular afferent signals to the control of paraspinal muscle activity during walking and indicate that this control is changed in response to different head orientations.Graphical abstract: Vestibulospinal reflexes play a role in maintaining the upright posture of the trunk. Head orientation has been shown to modify the vestibulospinal reflexes during standing. This study investigated how vestibular signals affect paraspinal muscle activity during walking, and whether head orientation changes these effects.Sixteen participants were instructed to walk on a treadmill for 8 min at 78 steps/min and 2.8 km/h in four conditions defined by the presence of electrical vestibular stimulation (EVS) and by head orientation (facing forward and facing leftward), while bipolar electromyography (EMG) was recorded bilaterally from the paraspinal muscles from cervical to lumbar levels.In both head orientations, significant phasic EVS-EMG coherence in the paraspinal muscles was observed at ipsilateral and/or contralateral heel strikes. Compared to walking with the head forward, a significant decrease was found in EVS-evoked responses (i.e., EVS-EMG coherence and gain) when participants walked with the leftward head orientation, with which EVS induced disturbance in the sagittal plane. This overall decrease can be explained by less need of feedback control for walking stabilization in the sagittal plane compared to in the frontal plane. The decrease in coherence was only significant at the left lower vertebral levels and at the right upper vertebral levels around left heel strikes.These findings confirm the contribution of the vestibular afferent signals to the control of paraspinal muscle activity during walking and indicate that this control is changed in response to different head orientations. [ABSTRACT FROM AUTHOR]

Purpose: This study investigated the effect of ischemic preconditioning (IP) on metaboreflex activation following dynamic leg extension exercise in a group of healthy participants. Method: Seventeen healthy participants were recruited. IP and SHAM treatments (3 × 5 min cuff occlusion at 220 mmHg or 20 mmHg, respectively) were administered in a randomized order to the upper part of exercising leg's thigh only. Muscle pain intensity (MP) and pain pressure threshold (PPT) were monitored while administrating IP and SHAM treatments. After 3 min of leg extension exercise at 70% of the maximal workload, a post-exercise muscle ischemia (PEMI) was performed to monitor the discharge group III/IV muscle afferents via metaboreflex activation. Hemodynamics were continuously recorded. MP was monitored during exercise and PEMI. Results: IP significantly reduced mean arterial pressure compared to SHAM during metaboreflex activation (mean ± SD, 109.52 ± 7.25 vs. 102.36 ± 7.89 mmHg) which was probably the consequence of a reduced end diastolic volume (mean ± SD, 113.09 ± 14.25 vs. 102.42 ± 9.38 ml). MP was significantly higher during the IP compared to SHAM treatment, while no significant differences in PPT were found. MP did not change during exercise, but it was significantly lower during the PEMI following IP (5.10 ± 1.29 vs. 4.00 ± 1.54). Conclusion: Our study demonstrated that IP reduces hemodynamic response during metaboreflex activation, while no effect on MP and PPT were found. The reduction in hemodynamic response was likely the consequence of a blunted venous return. [ABSTRACT FROM AUTHOR]

Abnormalities in the muscle metaboreflex concur to exercise intolerance and greater cardiovascular risk. Exercise training benefits neurocardiovascular function at rest and during exercise, but its role in favoring muscle metaboreflex in health and disease remains controversial. While some authors demonstrated that exercise training enhanced the sensitization of muscle metabolically afferents and improved neurocardiovascular responses to muscle metaboreflex activation, others reported unaltered responses. This narrative review aimed to: (a) highlight the current evidence on the effects of exercise training upon cardiovascular and autonomic responses to muscle metaboreflex activation; (b) analyze the role of training components and indicate potential mechanisms of metaboreflex adaptations; and (c) address key methodological features for future research. Though limited, accumulated evidence suggests that muscle metaboreflex adaptations depend on the individual clinical status, exercise modality, and training duration. In healthy populations, most trials negated the hypothesis of metaboreflex improvement due to chronic exercise, irrespective of the training duration. Favorable changes in patients with impaired metaboreflex, particularly chronic heart failure, mostly resulted from long-term interventions (> 16 weeks) including aerobic exercise of moderate to high intensity, performed in isolation or within multimodal training. Potential mechanisms of metaboreflex improvements include enhanced sensitivity of channels and receptors, greater antioxidant capacity, lower metabolite accumulation, increased functional sympatholysis, and muscle perfusion. Future research should investigate: (1) the dose-response relationship of training components within different exercise modalities to elicit improvements in individuals showing intact or impaired muscle metaboreflex; and (2) potential and specific underlying mechanisms of metaboreflex improvements in individuals with different medical conditions. [ABSTRACT FROM AUTHOR]

Purpose: The hemodynamic consequences of exercise in hypoxia have not been completely investigated. The present investigation aimed at studying the hemodynamic effects of contemporary normobaric hypoxia and metaboreflex activation. Methods: Eleven physically active, healthy males (age 32.7 ± 7.2 years) completed a cardiopulmonary test on an electromagnetically braked cycle-ergometer to determine their maximum workload (Wmax). On separate days, participants performed two randomly assigned exercise sessions (3 minutes pedalling at 30% of Wmax): (1) one in normoxia (NORMO), and (2) one in normobaric hypoxia with FiO2 set to 13.5% (HYPO). After each session, the following protocol was randomly assigned: either (1) post-exercise muscle ischemia (PEMI) to study the metaboreflex, or (2) a control exercise recovery session, i.e., without metaboreflex activation. Hemodynamics were assessed with impedance cardiography. Results: The main result was that the HYPO session impaired the ventricular filling rate (measured as stroke volume/diastolic time) response during PEMI versus control condition in comparison to the NORMO test (31.33 ± 68.03 vs. 81.52 ± 49.23 ml·s−1,respectively, p = 0.003). This caused a reduction in the stroke volume response (1.45 ± 9.49 vs. 10.68 ± 8.21 ml, p = 0.020). As a consequence, cardiac output response was impaired during the HYPO test. Conclusions: The present investigation suggests that a brief exercise bout in hypoxia is capable of impairing cardiac filling rate as well as stroke volume during the metaboreflex. These results are in good accordance with recent findings showing that among hemodynamic modulators, ventricular filling is the most sensible variable to hypoxic stimuli. [ABSTRACT FROM AUTHOR]

Purpose: Postural reactions (PR) of standing subjects have been mostly investigated in response to platform displacements or body perturbations of fixed magnitude. The objective of this study was to investigate the relationship between PR and the peak force and impulse of the perturbation.Methods: In ten healthy young men, standing balance was challenged by anteriorly directed perturbations (peak force: 20-60 N) delivered to the back, at the lumbar (L) or inter-scapular (IS) level, by means of a manual perturbator equipped with a force sensor. Postural reactions as expressed by the displacement of the center of pressure (CoP) were recorded using a force platform. Two sets of 20 randomly ordered perturbations (10 to each site) were delivered in two separate testing sessions.Results: The magnitude of CoP response (∆CoP) was better correlated with the impulse (I) than with the peak force of the perturbation. The normalized response, ∆CoPn = ∆CoP/I, exhibited good reliability (ICCs of 0.93 for IS and 0.82 for L), was higher with IS than with L perturbations (p < 0.01), and was significantly correlated with the latency of CoP response: r = 0.69 and 0.71 for IS and L, respectively.Conclusion: These preliminary findings support the concept that manually delivered perturbations can be used to reliably assess individual PR and that ∆CoPn may effectively express a relevant aspect of postural control. [ABSTRACT FROM AUTHOR]

Purpose: Physiological markers that estimate sympathetic activation may be used to infer pain and stress in humans. To date, effective reproducible methods are invasive and pose an undesired risk to participants. Previous work in animal models has used infrared thermography to measure the temperature of the lacrimal caruncle region and may be a promising method for measuring stress and pain non-invasively. The current study aimed to determine whether this method is useful in humans.Methods: Sixteen young healthy participants (age: 18-35) were recruited and underwent sympathetic activation using a cold pressor test (CPT) and a muscle chemoreflex (MCR), and completed a control trial. Throughout all trials, infrared thermographic imaging of the lacrimal caruncle, heart rate, heart rate variability, mean arterial blood pressure and pulse transit time were measured.Results: Heart rate (MCR: 4 ± 3 bpm, CPT: 17 ± 4 bpm p < 0.01) and mean arterial pressure increased (MCR: 6 ± 2, CPT: 5 ± 2 mmHg, p < 0.01) and pulse transit time decreased (p = 0.03) with both sympathetic activation interventions. However, lacrimal caruncle temperature did not vary under any condition remaining at 35.2 ± 0.2 °C which was similar to baseline.Conclusions: Our findings suggest infrared thermographic monitoring of eye temperature in humans does not reliably relate to sympathetic activation. This could be due to hemodynamic responses at the lacrimal caruncle that may be more complex than previously proposed with sympathetic activation. Alternatively, pulse transit time seems like a promising non-invasive measure of changes in sympathetic activation in humans. [ABSTRACT FROM AUTHOR]

Purpose: It has been shown that an inspiratory muscle warm-up (IMW) could enhance performance. IMW may also improve the near-infrared spectroscopy (NIRS)-derived tissue oxygen saturation index (TSI) during cycling. However, there exists contradictory data about the effect of this conditioning strategy on performance and muscle oxygenation. We examined the effect of IMW on speed skating performance and studied the underpinning physiological mechanisms related to muscle oxygenation.Methods: In a crossover, randomized, single-blind study, eight elite speed skaters performed 3000 m on-ice time trials, preceded by either IMW (2 × 30 breaths, 40% maximal inspiratory pressure) or SHAM (2 × 30 breaths, 15% maximal inspiratory pressure). Changes in TSI, oxyhemoglobin-oxymyoglobin ([O2HbMb]), deoxyhemoglobin-deoxymyoglobin ([HHbMb]), total hemoglobin-myoglobin ([THbMb]) and HHbMbdiff ([O2HbMb]-[HHbMb]) in the right vastus lateralis muscle were monitored by NIRS. All variables were compared at different time points of the race simulation with repeated-measures analysis of variance. Differences between IMW and SHAM were also analyzed using Cohen's effect size (ES) ± 90% confidence limits, and magnitude-based inferences.Results: Compared with SHAM, IMW had no clear impact on skating time (IMW 262.88 ± 17.62 s vs. SHAM 264.05 ± 21.12 s, effect size (ES) 0.05; 90% confidence limits, - 0.22, 0.32, p = 0.7366), TSI, HbMbdiff, [THbMb], [O2HbMb] and perceptual responses.Conclusions: IMW did not modify skating time during a 3000 m time trial in speed skaters, in the conditions of our study. The unchanged [THbMb] and TSI demonstrate that the mechanisms by which IMW could possibly exert an effect on performance were unaffected by this intervention. [ABSTRACT FROM AUTHOR]

Introduction: An internal perturbation of standing balance activates muscles critical for maintaining balance and is preceded by anticipatory postural adjustments (APAs). In healthy younger adults, a measure of spinal excitability in the form of the Hoffmann (H) reflex becomes depressed during APAs but how aging affects the reflex control of APAs is unknown.Methods: We compared H reflex excitability profiles in the right soleus muscle, indirectly indicating APA, between younger (n = 11, age 19-24 years), middle-aged (n = 10, age 37-56 years), and older healthy adults (n = 11, age 63-78 years). Subjects rapidly raised the right-dominant arm in response to an auditory cue. The H reflex was evoked 120 ms, 100 ms, 80 ms, 60 ms, 40 ms, 20 ms, and 0 ms before as well as 20 ms after the onset of the right anterior deltoid muscle activation. For data processing, each trial was controlled for the corresponding background EMG activity before normalizing the standing data to the data in sitting in the 8 time bins.Results: All subjects showed a silent period in the soleus background electromyographic activity, suggesting the presence of APA. We found that the stereotypical H reflex depression associated with APAs in younger adults was reduced in middle-aged adults and reversed to facilitation in older adults. The depression occurred in 10 out of 11 younger adults, whereas all 11 older adults exhibited facilitation.Conclusion: Because APAs are organized at the supraspinal level, we speculate a supraspinal origin of the age-related reflex facilitation during APAs. [ABSTRACT FROM AUTHOR]

Purpose: Delayed onset muscle soreness (DOMS) has been shown to induce changes in muscle activity during walking. The aim of this study was to elucidate whether DOMS also affects interlimb communication during walking by investigating its effect on short-latency crossed responses (SLCRs).Methods: SLCRs were elicited in two recording sessions by electrically stimulating the tibial nerve of the ipsilateral leg, and quantified in the contralateral gastrocnemius muscle. The second recording session occurred 24-36 h after the participants (n = 11) performed eccentric exercises with the ipsilateral calf.Results: DOMS caused a decreased magnitude of the spinally mediated component of the SLCR in the contralateral gastrocnemius medialis.Conclusions: The results of the current study provide insight on the relationship between pain and motor control. Muscle pain affects the spinal pathway mediating interlimb communication, which might result in a reduced ability to maintain dynamical stability during walking. [ABSTRACT FROM AUTHOR]

Purpose: We previously reported that suppressive middle latency cutaneous reflexes (MLRs) in the peroneus longus (PL) are exaggerated in subjects with chronic ankle instability, and the changes are related to functional instability. However, the time-varying history of these neurophysiological changes after an ankle sprain is yet to be elucidated. Therefore, in the present study, we investigated the time course of the changes in the PL MLR after an ankle sprain in relation to the number of sprain recurrences.Methods: Twenty-three subjects with ankle sprain were classified into 3 groups according to their history of ankle sprain: first ankle sprain, 2-3 ankle sprains, and ≥4 ankle sprains. Twenty-three age-matched control subjects also participated. The PL MLRs were elicited by stimulating the sural nerve while the subjects performed different levels of isometric ankle eversion. Gain of MLR was estimated using linear regression analysis (slope value) of the amplitude modulation of MLRs obtained from graded isometric contractions.Result: The gain of MLRs first increased 4 weeks after the injury. In subjects with their first ankle sprain, the MLRs returned to almost baseline levels after 3 months. In contrast, the increase in MLR gain persisted even after 3 months in subjects with recurrent ankle sprains. In addition, the MLR gains were closely related to functional recovery of the ankle joint.Conclusions: Our findings suggest that the recovery process of MLR gains were strongly affected by the history of ankle sprains as well as the functional recovery of the ankle joint. [ABSTRACT FROM AUTHOR]

Purpose: To determine the sex difference in the impact of central venous pressure (CVP) on the pressor response induced by ischemic handgrip exercise.Methods: Twelve young healthy individuals (six males, 25 ± 3 years) performed ischemic handgrip exercise during mild levels of lower body negative pressure (LBNP, -5 mmHg) and during a 10° head-down tilt (HDT) to lower and increase CVP, respectively. The protocol consisted of 3 min of baseline ischemia, followed by 2 min of isometric handgrip exercise at 35 % of maximal voluntary contraction force, and 2 min of post-exercise circulatory occlusion. Mean arterial pressure (MAP) was assessed continuously by finger plethysmography and CVP was estimated from the venous pressure of the non-exercising dependent arm.Results: Baseline CVP was greater during HDT than LBNP (8.4 ± 1.8 vs. 6.5 ± 1.8 mmHg, p < 0.01). MAP was greater during LBNP than HDT throughout the protocol (p = 0.05). During ischemic handgrip exercise, CVP increased in males but not in females (Group × protocol interaction: p = 0.01). A group × condition interaction was also observed for MAP, with males showing a greater MAP during LBNP than HDT (110 ± 2 vs. 103 ± 2 mmHg, p < 0.01).Conclusions: Baseline CVP inversely affected the pressor response to handgrip exercise in all individuals, with a greater MAP response observed during LBNP than HDT. Increase in CVP in males may be due to a greater splanchnic vasoconstrictor response to ischemic handgrip exercise. Therefore, combined baseline CVP and changes in CVP likely contributed to the greater MAP response observed during LBNP in males. [ABSTRACT FROM AUTHOR]

Purpose: The muscle metaboreflex activation has been shown essential to reach normal hemodynamic response during exercise. It has been demonstrated that patients with multiple sclerosis (MS) have impaired autonomic functions and cardiovascular regulation during exercise. However, to the best of our knowledge, no previous research to date has studied the metaboreflex in MS patients. The purpose of this study was to investigate the hemodynamic response to metaboreflex activation in patients with MS (n = 43) compared to an age-matched, control group (CTL, n = 21).Methods: Cardiovascular response during the metaboreflex was evaluated using the post-exercise muscle ischemia (PEMI) method and during a control exercise recovery (CER) test. The difference in hemodynamics between the PEMI and the CER test was calculated and this procedure allowed for the assessment of the metaboreflex response. Hemodynamics was estimated by impedance cardiography.Results: The MS group showed a normal mean blood pressure (MBP) response as compared to the CTL group (+6.5 ± 6.9 vs. +8 ± 6.8 mmHg, respectively), but this response was achieved with an increase in systemic vascular resistance, that was higher in the MS with respect to the CTL group (+137.6 ± 300.5 vs. -14.3 ± 240 dyne · s(-1) cm(-5), respectively). This was the main consequence of the MS group's incapacity to raise the stroke volume (-0.65 ± 10.6 vs. +6.2 ± 12.8 ml, respectively).Conclusion: It was concluded that MS patients have an impaired capacity to increase stroke volume (SV) in response to low level metaboreflex, even if they could sustain the MBP response by vasoconstriction. This was probably a consequence of their chronic physical de-conditioning. [ABSTRACT FROM AUTHOR]

Purpose: We quantified the acute and chronic effects of whole body vibration on athletic performance or its proxy measures in competitive and/or elite athletes. Methods: Systematic literature review and meta-analysis. Results: Whole body vibration combined with exercise had an overall 0.3 % acute effect on maximal voluntary leg force (−6.4 %, effect size = −0.43, 1 study), leg power (4.7 %, weighted mean effect size = 0.30, 6 studies), flexibility (4.6 %, effect size = −0.12 to 0.22, 2 studies), and athletic performance (−1.9 %, weighted mean effect size = 0.26, 6 studies) in 191 (103 male, 88 female) athletes representing eight sports (overall effect size = 0.28). Whole body vibration combined with exercise had an overall 10.2 % chronic effect on maximal voluntary leg force (14.6 %, weighted mean effect size = 0.44, 5 studies), leg power (10.7 %, weighted mean effect size = 0.42, 9 studies), flexibility (16.5 %, effect size = 0.57 to 0.61, 2 studies), and athletic performance (−1.2 %, weighted mean effect size = 0.45, 5 studies) in 437 (169 male, 268 female) athletes (overall effect size = 0.44). Conclusions: Whole body vibration has small and inconsistent acute and chronic effects on athletic performance in competitive and/or elite athletes. These findings lead to the hypothesis that neuromuscular adaptive processes following whole body vibration are not specific enough to enhance athletic performance. Thus, other types of exercise programs (e.g., resistance training) are recommended if the goal is to improve athletic performance. [ABSTRACT FROM AUTHOR]

This study investigated the effects of acute hypoxia on spinal reflexes and soleus muscle function after a sustained contraction of the plantar flexors at 40 % of maximal voluntary isometric contraction (MVC). Fifteen males (age 25.3 ± 0.9 year) performed the fatigue task at two different inspired O fractions (FiO = 0.21/0.11) in a randomized and single-blind fashion. Before, at task failure and after 6, 12 and 18 min of passive recovery, the Hoffman-reflex ( H) and M-wave ( M) were recorded at rest and voluntary activation (VA), surface electromyogram (RMS), M-wave ( M) and V-wave ( V) were recorded during MVC. Normalized H-reflex ( H/ M) was significantly depressed pre-exercise in hypoxia compared with normoxia (0.31 ± 0.08 and 0.36 ± 0.08, respectively, P < 0.05). Hypoxia did not affect time to task failure (mean time of 453.9 ± 32.0 s) and MVC decrease at task failure (−18 % in normoxia vs. −16 % in hypoxia). At task failure, VA (−8 %), RMS/ M (−11 %), H/ M (−27 %) and V/ M (−37 %) decreased ( P < 0.05), but with no FiO effect. H/ M restored significantly throughout recovery in hypoxia but not in normoxia, while V/ M restored significantly during recovery in normoxia but not in hypoxia ( P < 0.05). Collectively, these findings indicate that central adaptations resulting from sustained submaximal fatiguing contraction were not different in hypoxia and normoxia at task failure. However, the FiO-induced differences in spinal loop properties pre-exercise and throughout recovery suggest possible specific mediation by the hypoxic-sensitive group III and IV muscle afferents, supraspinal regulation mechanisms being mainly involved in hypoxia while spinal ones may be predominant in normoxia. [ABSTRACT FROM AUTHOR]

Poling is considered to make a significant contribution to cross-country skiing with the skating technique. To better understand this contribution, the current investigation compared roller ski skating on a treadmill with the so-called G3 skating technique with (G3-P) and without poling (G3-NP). Seven male elite skiers performed 5-min submaximal tests at 8, 12, and 15 km h, as well as an incremental test to exhaustion with both techniques on a 5 % incline. Ventilatory variables were assessed by open-circuit indirect calorimetry and three-dimensional kinematics analyzed using the Qualisys Pro Reflex system. G3-P was associated with approximately 15 % higher peak velocity and 10 % higher peak oxygen uptake than G3-NP in the incremental test (both P < 0.01). All ventilatory variables, as well as heart rate and blood lactate concentration were lower with G3-P as compared to G3-NP at 12 and 15 km h (all P < 0.01). Gross efficiency (i.e., the ratio of work rate to metabolic rate) at 12 km h was higher in G3-P (14.9 %) than G3-NP (13.5 %) ( P < 0.01). Moreover, with G3-P cycle time and length were both 30 % longer, with correspondingly reduced cycle rates (all P < 0.01). In addition, the ski gliding and swing phases were longer and the angle between the skis smaller with G3-P (both P < 0.01), whereas the push-off time was independent of technique and velocity. Taken together, these results indicate that poling makes an important contribution to propulsion and velocity during ski skating, specifically by enhancing peak oxygen uptake, skiing efficiency and associated biomechanical variables. [ABSTRACT FROM AUTHOR]

The purpose of this study was to determine to what extent one session of isotonic and isometric ankle dorsi and plantar flexion training induces changes in the frequency-dependent depression of the soleus H-reflex. Further, adaptation of reciprocal Ia inhibition exerted from tibialis anterior flexor group I afferents on soleus motoneurons, and presynaptic inhibition of Ia afferent terminals induced by a conditioning afferent volley following stimulation of the antagonist nerve were established with subjects seated before and after training. The soleus H-reflexes evoked at the inter-stimulus intervals of 1, 2, 3, 5, and 8 s were normalized to the mean amplitude of the H-reflex evoked every 10 s. Conditioned H-reflexes were normalized to the associated control H-reflex evoked with subjects seated before and after training. Twenty-six subjects were randomly assigned to one or more of the 4 exercise groups. Isometric ankle dorsi flexion training decreased the reciprocal and presynaptic inhibition, while isotonic ankle dorsi flexion had no significant effects. Isotonic plantar flexion training decreased only the reciprocal inhibition, whilst isometric plantar flexion had no significant effects on the reciprocal or presynaptic inhibition. None of the training exercise protocols affected the amount of homosynaptic depression of the soleus H-reflex. Our findings support the notion that plastic changes of reciprocal and presynaptic inhibition due to exercise are transferrable to a resting state, and that homosynaptic depression remains unaltered after a single session of ankle training. Further research is needed to outline the time-course of plastic changes of spinal inhibitory mechanisms in humans. [ABSTRACT FROM AUTHOR]

The objectives of this study were to (1) investigate the modulation of the H reflex immediately after and 24 h after eccentric exercise in the presence of delayed-onset muscle soreness (DOMS) and (2) test the reproducibility of the H reflex in trapezius across days. H reflexes were recorded from the dominant middle trapezius muscle by electrical stimulation of the C3/4 cervical nerve in ten healthy subjects. DOMS was induced by eccentric exercise of the dominant shoulder. H reflexes were obtained in four sessions: '24 h before', 'Pre', 'Post', and '24 h after' eccentric exercise. Ratios of maximal H reflex and M wave responses ( H/ M) were compared between sessions. In addition, a between session comparison was done for the ratios of H reflex amplitudes ( H/ M, and H/ M) obtained from the stimulus intensity needed to obtain 75 and 50 % of H at '24 h before'. No ratio changes were found when comparing '24 h before' and 'Pre' recordings. A decrease in H/ M was found at 'Post' ( P < 0.05) and decreases in both H/ M and H/ M were observed at '24 h after' ( P < 0.05). This study presented evidence that an acceptable day-to-day reproducibility of the H reflex could be obtained with the applied experimental setup. Furthermore, immediately after and 24 h after exercise a stronger stimulus intensity was needed to reach the same magnitude of the H reflex reflecting that the recruitment curve was shifted to the right. This modulation of the stimulus-response relationship could be caused by presynaptic inhibition of Ia afferent fibres' input to the motoneuron by group III and IV afferents. [ABSTRACT FROM AUTHOR]

The use of electrical stimulation (ES) can contribute to our knowledge of how our neuromuscular system can adapt to physical stress or unloading. Although it has been recently challenged, the standard technique used to explore central modifications is the twitch interpolated method which consists in superimposing single twitches or high-frequency doublets on a maximal voluntary contraction (MVC) and to compare the superimposed response to the potentiated response obtained from the relaxed muscle. Alternative methods consist in (1) superimposing a train of stimuli (central activation ratio), (2) comparing the MVC response to the force evoked by a high-frequency tetanus or (3) examining the change in maximal EMG response during voluntary contractions, if this variable is normalized to the maximal M wave, i.e. EMG response to a single stimulus. ES is less used to examine supraspinal factors but it is useful for investigating changes at the spinal level, either by using H reflexes, F waves or cervicomedullary motor-evoked potentials. Peripheral changes can be examined with ES, usually by stimulating the muscle in the relaxed state. Neuromuscular propagation of action potentials on the sarcolemma (M wave, high-frequency fatigue), excitation-contraction coupling (e.g. low-frequency fatigue) and intrinsic force (high-frequency stimulation at supramaximal intensity) can all be used to non-invasively explore muscular function with ES. As for all indirect methods, there are limitations and these are discussed in this review. Finally, (1) ES as a method to measure respiratory muscle function and (2) the comparison between electrical and magnetic stimulation will also be considered. [ABSTRACT FROM AUTHOR]

This study was undertaken to investigate the effect of low-level loading on digit tracking and the associated physiological tremors in the moving and the stationary digits. Sixteen healthy adults conducted positional tracking with the middle finger under the loaded and unloaded conditions; meanwhile, trajectory of the middle finger, electromyographic activities of the extensor digitorum (ED)/flexor digitorum superficialis, and physiological tremors of the index, middle, ring, and little fingers were recorded. The results showed that load imposition (<70 g) on the middle finger improved tracking congruency, in association with reduction of inter-digit tremor coupling and enhancement of tiny movement jerks. Principal component analysis suggested that inertial load suppressed the 8-12 Hz central rhythm but potentiated the 25-40 Hz coherence spectra of major principal components and electromyographic signals of the ED. It was concluded that low-level inertial load could facilitate corrective movement adjustments and selective digit control during manual tracking, relevant implicitly to decreased common central drive and enhanced heteronymous reflex loops. [ABSTRACT FROM AUTHOR]

Recent studies have shown that the nociceptive withdrawal reflex threshold (NWR-T) and the electrical pain threshold (EP-T) are reliable measures in pain-free populations. However, it is necessary to investigate the reliability of these measures in patients with chronic pain in order to translate these techniques from laboratory to clinic. The aims of this study were to determine the test-retest reliability of the NWR-T and EP-T after single and repeated (temporal summation) electrical stimulation in a group of patients with chronic low back pain, and to investigate the association between the NWR-T and the EP-T. To this end, 25 patients with chronic pain participated in three identical sessions, separated by 1 week in average, in which the NWR-T and the EP-T to single and repeated stimulation were measured. Test-retest reliability was assessed using intra-class correlation coefficient (ICC), coefficient of variation (CV), and Bland-Altman analysis. The association between the thresholds was assessed using the coefficient of determination (r (2)). The results showed good-to-excellent reliability for both NWR-T and EP-T in all cases, with average ICC values ranging 0.76-0.90 and average CV values ranging 12.0-17.7%. The association between thresholds was better after repeated stimulation than after single stimulation, with average r (2) values of 0.83 and 0.56, respectively. In conclusion, the NWR-T and the EP-T are reliable assessment tools for assessing the sensitivity of spinal nociceptive pathways in patients with chronic pain. [ABSTRACT FROM AUTHOR]

This study aimed to reveal the neural and muscular adjustments following a repeated-sprint (RS) running exercise. Sixteen subjects performed a series of neuromuscular tests before, immediately after and 30 min (passive recovery) post-RS exercise (12 x 40 m sprints interspaced by 30 s of passive recovery). Sprint times significantly lengthened over repetitions (+17% from the first to the last sprint; P < 0.05). After RS running exercise, maximal voluntary contraction torque of the plantar flexors (-11 +/- 7.3%), muscle activation (twitch interpolation) (-2.7 +/- 3.4%) and soleus maximal M-wave amplitude (-20 +/- 17%) were significantly (P < 0.05) reduced but returned close to baseline after 30 min. Both soleus EMG activity and maximal Hoffmann reflex normalized with respect to M-wave amplitude did not change during the whole experiment. From pre- to post-RS exercise, evoked twitch response was characterized by lower peak torque and maximal rate of torque development (-13 and -11%, respectively, P < 0.05), but was not different from baseline after recovery. Peak tetanus at 20 and 80 Hz were 17 and 8% lower (P < 0.05) in the fatigued state, respectively. Acute muscle fatigue induced by RS running exercise is mainly peripheral as the short-term (30 min) recovery pattern of plantar flexors contractile properties follows that of the voluntary force-generating capacity. [ABSTRACT FROM AUTHOR]

Different back muscle reflex assessment protocols have shown abnormally longer reflex latency responses of back muscles in chronic low back pain (CLBP). However, many confounding variables are difficult to control, such as the load magnitude and the preactivation of trunk muscles. The aims of this study were to evaluate, in 30 subjects with CLBP and 30 healthy controls, the activation levels of back muscles during pre-loading and their reflex responses to sudden loading. After subjected to six practice perturbations, 20 sudden and unexpected forward perturbations of the trunk were applied in 30 CLBP subjects (14 women) and 31 controls (17 women), while attempting to minimize the confounding effect of preactivation level and perturbation amplitude. Reflex latency and amplitudes were computed from the surface EMG signals of four back muscles (bilaterally at L5, L3, L1, T10 vertebral levels). EMG was also collected from abdominal muscles. Subjects with CLBP significantly increased the preactivation of back muscles (abdominal preactivation the same) relative to controls while no sex effect was observed. While adjusting statistically for these differences, reflex amplitude was significantly higher in subjects with CLBP and men, compared to healthy controls and women, respectively. Interestingly, contrary to most of the literature available, no between-group effects were detected for reflex latency, which could potentially be explained by an appropriate control of confounding variables, but this remains to be clarified in future research. [ABSTRACT FROM AUTHOR]

Blockade of the skeletal muscle Na(+)-K(+)-ATPase pump by digoxin could result in a more marked hyperkaliema during a forearm exercise, which in turn could stimulate the mechano- and metaboreceptors. In a randomized, double-blinded, placebo-controlled, and cross-over-design study, we measured mean blood pressure (MBP), heart rate (HR), ventilation (V(E)), oxygen saturation (SpO(2)), muscle sympathetic nerve activity (MSNA), venous plasma potassium and lactic acid during dynamic handgrip exercises, and local circulatory arrest in 11 healthy subjects. Digoxin enhanced MBP during exercise but not during the post-handgrip ischemia and had no effect on HR, V(E), SpO(2), and MSNA. Venous plasma potassium and lactic acid were also not affected by digoxin-induced skeletal muscle Na(+)-K(+)-ATPase blockade. We conclude that digoxin increased MBP during dynamic exercise in healthy humans, independently of changes in potassium and lactic acid. A modest direct sensitization of the muscle mechanoreceptors is unlikely and other mechanisms, independent of muscle reflexes and related to the inotropic effects of digoxin, might be implicated. [ABSTRACT FROM AUTHOR]

The purpose of this study was to investigate whether neural adaptations following functional multiple-joint leg press training can induce neural adaptations to the plantar flexor muscles in a single-joint contraction task. Subjects were randomised to a maximal strength training (MST) (n = 10) or a control group (n = 9). MST consisted of 24 sessions (8 weeks) of 4 x 4 repetitions of horizontal leg press using maximal intended velocity in the concentric phase with the movement ending in a plantar flexion. Neural adaptations in the soleus and gastrocnemius medialis (GM) were assessed by surface electromyographic activity and V-waves during maximum voluntary isometric contraction (MVIC), and also by H-reflexes in the soleus during rest and 20% MVIC. One repetition maximum leg press increased by 44 +/- 14% (mean +/- SD; P < 0.01). Plantar flexion MVIC increased by 20 +/- 14% (P < 0.01), accompanied by 13 +/- 19% (P < 0.05) increase in soleus, but not GM surface electromyography. Soleus V/M(SUP) increased by 53 +/- 66% and in GM by 59 +/- 64% (P < 0.05). Normalised soleus H-reflexes remained unchanged by training. No changes occurred in the control group. These results suggest that leg press MST can induce neural adaptations in a single-joint plantar flexion MVIC task. [ABSTRACT FROM AUTHOR]

The exercise pressor reflex (EPR) is composed of the mechanoreflex and the metaboreflex and has been shown to be overactive in spontaneously hypertensive rats. The aim of the present study was to isolate the metaboreflex using post-exercise ischemia (PEI) and examine the BP response in normotensive (NTN) and hypertensive (HTN) humans. We hypothesize that the post-exercise ischemia-induced maintenance of BP will be greater in HTN when compared to NTN adults. A total of 15 NTN (65 +/- 1 years) and 12 HTN (64 +/- 1 years) adults were recruited. Beat-to-beat mean arterial pressure (MAP) was measured non-invasively (Finometer). Dynamic handgrip exercise (DHE) was performed for 3 min followed by 2 min of PEI. An unpaired t test was used to examine differences between groups. As compared to resting baseline values, the change in MAP during PEI was greater in HTN than NTN subjects (HTN: Delta = 12 +/- 3 mmHg, NTN: Delta = 6 +/- 1 mmHg, P < 0.05). These data suggest that HTN humans have enhanced metaboreflex sensitivity. [ABSTRACT FROM AUTHOR]

We previously reported that ventilatory response at the onset of light leg exercise was augmented in delayed onset muscle soreness (DOMS) after eccentric exercise (ECC) utilizing the leg. In this study, we examined whether the same result would appear in light exercise in which an arm with DOMS was utilized. Eleven subjects performed ECC using one arm, and we measured ventilatory responses to a 20-s single-arm extension-flexion exercise and to passive movement (PAS) before and 2 days after ECC (D2). We found that ventilatory response to both the exercise and PAS in which all subjects perceived DOMS was augmented at D2. It was concluded that initial hyperpnea, which occurred during arm exercise in DOMS, was enhanced, and from examining the result of PAS, enhanced initial exercise hyperpnea may be due in part to an exaggerated peripheral neural reflex. [ABSTRACT FROM AUTHOR]

The purpose of this study was to assess the reliability of the RIII reflex threshold and Pain threshold in three repeated trials using electrocutaneous stimuli. Each trial was separated by a mean of 4.3 +/- 2.9 days (between-trials) and included two repeated measurements (within-trial) of the RIII reflex threshold (RIII-T) and the Pain threshold (PT) separated by 20 min. The participants were 14 healthy males (mean age +/- SD, 23.5 +/- 5.3 years). There was a significant difference between the RIII-T and PT. The reliability of the RIII-T and PT shows a between-trials coefficient of variance (CV(SEM)) of 16.1 and 16.9%, respectively. The within-trial CV(SEM) for RIII-T and PT was 5.4 and 4.3%, respectively. There was a significant correlation between the RIII-T threshold and PT. The parallel association and correlation of the RIII-T with the PT suggests that the RIII-T is valid in experimental pain studies under standardised resting conditions. [ABSTRACT FROM AUTHOR]

Ligamentomuscular and muscular stretch reflexes are known to contribute to knee joint stability. After anterior cruciate ligament (ACL) injury, a more intense and adjusted muscular response is required to maintain joint stability, but this neuromuscular control of the knee has not been clearly proved. The aim of the study is to record electromyography (EMG) signal and muscular fibre length variations in quadriceps and hamstrings of the knee with and without ACL, and to analyze and integrate the ligament strain and the muscular reaction to forced anterior tibial translation (ATT). In 17 knees from 12 cats, EMG electrodes and ultrasonomicrometry crystals were inserted into four main periarticular muscles, with strain gauges on periarticular ligament insertions. Their output signal was compared before and after ACL surgical section in series of ATT (at 90 degrees and 30 degrees knee flexion), and also during knee flexion and extension. Linear regression analysis was performed between the EMG signal and muscular fibre length variations, and between the EMG signal and the strain on ligament insertions, in the search of this reflex neuromuscular response. In the ACL deficient knees, the studied muscles showed a poor adjustment to motion of EMG firing, inversely to controls. The muscle stretch reflexes showed poorer correlation with post-peak EMG activity than the ligaments. ATT control depended mainly on hamstrings activity in control knees, whereas in unstable knees, quadriceps activity was associated with more tibial translation. Acute ACL-deficient knees showed poor neuromuscular control with weak ligamentomuscular reflexes and no muscular stretch reflexes, suggesting the ineffectiveness of acute muscular reaction to provide early mechanical knee stabilization after injury. [ABSTRACT FROM AUTHOR]

Changes of the normal gravitational acceleration are known to affect sensorimotor performance. For example, subjects exposed to three times the normal terrestrial acceleration (+3 Gz) in a centrifuge will produce exaggerated isometric force. The present study compares the effects of high-Gz to that of micro-Gz on isometric force production. Twelve right-handed human subjects were tested during the microgravity (micro-Gz), high gravity (high-Gz) and normal gravity (normal-Gz) episodes of parabolic flight. Holding an isometric joystick in their right hand, they produced forces of 5, 15, or 25 N in different directions, according to visually prescribed vectors. In some parabolas, 70 Hz vibration was applied to the flexor- and extensor-side of the wrist, in order to reduce spinal segmental activity. If compared to normal-Gz, produced forces were higher in high-Gz, and still higher in micro-Gz. Vibration reduced the magnitude of produced forces independent of the Gz-level. We confirm that isometric force increases in high-Gz, and document that it increases in micro-Gz as well. This increase is probably not related to degraded segmental excitability and proprioception, since it was not modified by vibration and manifests before proprioception becomes effective. [ABSTRACT FROM AUTHOR]

We compared the ventilatory and circulatory responses during 20 s of light dynamic leg and arm exercises performed separately using dominant and non-dominant limbs. Seventeen subjects performed a 20-s single-leg knee extension-flexion exercise with a load of 5% of maximal muscle strength attached to the ankle. Fifteen of the seventeen subjects also did a single-arm elbow flexion-extension exercise in which a load was attached to the wrist in the same way as in the leg exercise. Similar movements were passively performed on the subjects by experimenters to avoid the effects of central command. The magnitude of change from rest (gain) in minute ventilation during passive movement (PAS) was significantly smaller in the dominant limbs than in the non-dominant limbs, though a significant difference was not detected during voluntary exercise (VOL). In contrast, heart rate and blood pressure responses did not show any differences between the dominant and non-dominant limbs during either VOL or PAS. In conclusion, the initial ventilatory response to PAS in the dominant limbs was lower than that of the non-dominant limbs, though the ventilatory response to VOL was not. Circulatory responses were not different between the dominant and non-dominant limbs. These results suggest that peripheral neural reflex during exercise could be different between dominant and non-dominant limbs and that ventilatory response at the onset of exercise might be controlled by the dual neural modulation of central command and peripheral neural reflex, resulting in the same ventilatory response to both dominant and non-dominant limb exercise. [ABSTRACT FROM AUTHOR]

Parallel increases in strength and rate of force development (RFD) are well-known outcomes from the initial phase of resistance training. However, it is unknown whether neural adaptations with training contribute to improvements of both factors. The aim of this study was to examine whether changes in H-reflex amplitude with resistance training can explain the gain in strength or rather be associated with RFD. Twelve subjects carried out 3 weeks of isometric maximal plantarflexion training, whereas 12 subjects functioned as controls. H-reflexes were elicited in the soleus muscle during rest and sub-maximal contractions at 20 and 60% of maximal voluntary contraction (MVC). In addition, surface electromyography (sEMG) was recorded from the soleus, gastrocnemius and tibialis anterior muscles during MVC. The resistance training provided increases in maximal force of 18%, RFD of 28% and H-reflex amplitude during voluntary contractions of 17 and 15% while no changes occurred in the control group. In contrast, the maximal M-wave, the maximal H-reflex to maximal M-wave ratio during rest and sEMG during MVC did not change with training. There was a positive correlation between percentage changes in H-reflex amplitude and RFD with training (r = 0.59), while significant association between percentage changes in H-reflex amplitude and maximal force was not found. These findings indicate the occurrence of changed motoneuron excitability or presynaptic inhibition during the initial phase of resistance training. This is the first study to document that increased RFD with resistance training is associated with changes in reflex excitability. [ABSTRACT FROM AUTHOR]

Increased hematocrit (Hct) attributable to splenic contraction accompanies human apneic diving or apnea with face immersion. Apnea also causes heart rate reduction and peripheral vasoconstriction, i.e., a cardiovascular diving response, which is augmented by face immersion. The aim was to study the role of apnea and facial immersion in the initiation of the hematological response and to relate this to the cardiovascular diving response and its oxygen conservation during repeated apneas. Seven male volunteers performed two series of five apneas of fixed near-maximal duration: one series in air (A) and the other with facial immersion in 10 degrees C water (FIA). Apneas were spaced by 2 min and series by 20 min of rest. Venous blood samples, taken before and after each apnea, were analysed for Hct, hemoglobin concentration (Hb), lactic acid, blood gases and pH. Heart rate, skin capillary blood flow and arterial oxygen saturation were continuously measured non-invasively. A transient increase of Hct and Hb by approximately 4% developed progressively across both series. As no increase of the response resulted with face immersion, we concluded that the apnea, or its consequences, is the major stimulus evoking splenic contraction. An augmented cardiovascular diving response occurred during FIA compared to A. Arterial oxygen saturation remained higher, venous oxygen stores were more depleted and lactic acid accumulation was higher across the FIA series, indicating oxygen conservation with the more powerful diving response. This study shows that the hematological response is not involved in causing the difference in oxygen saturation between apnea and apnea with face immersion. [ABSTRACT FROM AUTHOR]

Physical exercise inhibits cardiac vagal activity. To study the relationship between heart rate (HR) and respiratory pattern, we applied the 4-s exercise test (4sET) and measured cardiac vagal index (CVI) in 30 healthy subjects who served as their own controls, using the standard plus three additional variations, essentially respiratory, of the original protocol: (a) a maximum inspiratory apnea of 16 s, of which 8 s were in the pre-exercise phase (4sET(insp)); (b) free respiratory pattern (4sET(unc)); and (c) maximum expiratory apnea of 12 s (4sET(exp)). The respective results were expressed by the following CVIs: CVI(insp), CVI(unc) and CVI(exp). CVI was determined in a continuous digital ECG recording through a specific ratio of two RR interval durations. The results [(mean +/- SEM)] for the four different maneuvers were as follows: CVI (1.56 +/- 0.05), CVI(insp) (1.55 +/- 0.05), CVI(unc) (1.63 +/- 0.05) and CVI(exp) (1.37 +/- 0.02). ANOVA-Bonferroni significant differences were only found between CVI(exp) and CVI(insp) (P = 0.009), CVI(unc) (P < 0.001) and CVI (P = 0.003). Dividing our sample in terciles according to CVI values, those with lower CVI, showed an attenuation of biphasic HR response after a 15 s maximum inspiratory apnea. We conclude that cardiac vagal reflex seems to be influencing the biphasic HR response modulation after a 12 s inspiratory apnea as described in the original protocol of 4sET, and this appears to be the option that best discriminates the cardiac vagal reflex, with less variability in the maneuvers when subjects are divided in terciles. [ABSTRACT FROM AUTHOR]

In several explosive types of sport events the leg extensor muscles are subjected to very high impact loads. Thus, extreme requirements exist for the neuromuscular system to develop sufficient muscle stiffness in the lower extremities in order to tolerate these high impact loads. Therefore, it would be challenging to measure reflex modulation during high impact activities, and with different athlete populations. In the present experiment, H-reflex and short latency reflex (M1) sensitivity was measured during drop jump exercises among high jumpers and sprinters. The changes in both reflex peak-to-peak amplitudes showed a significant (P < 0.05) reduction towards the end of the exercise for the sprinters. In addition, the same subject group showed a remarkable increase in serum creatine kinase (CK) activity 2 h after the jumps. Similar changes could not be observed for the high jumpers. These results clearly indicate different neural adaptation strategies for the two athlete groups. Reduction in H-reflex sensitivity and an increase in CK-activity in sprinters were taken as evidence for presynaptic inhibition, probably induced by substances related to muscle damage. Since high jump training includes more high impact loading, it was assumed that it could lead to some structural adaptation and, thus, prevents exercise induced reflex modification to a certain extent. [ABSTRACT FROM AUTHOR]

The purpose of this study was to investigate the effect of altered local temperature on soleus H-reflex and compound muscle action potential (M wave) in young and older women. H-reflex and M wave responses were elicited in 10 young (22.3±3.3 years) and 10 older (72.5±3.2 years) women at three muscle temperatures: control (34.2±0.3°C), cold (31.3±0.5°C) and warm (37.1±0.2°C). H-reflex output, expressed as the ratio between maximal H-reflex and maximal M wave (Hmax/Mmax), was lower in the older, compared with the younger, group, regardless of temperature. In control temperature conditions, for example, the Hmax/Mmax ratio was 36.8±24% in the young and 25.4±20% in the older ( P<0.05). Warming had no effect on the H-reflex output in either group, whilst cooling increased H-reflex output only in the younger group (+28%). In both groups, cooling increased (+5.3%), and warming decreased (−5.5%) the H-reflex latency. This study confirms that older individuals experience a reduced ability to modulate the reflex output in response to a perturbation. In a cold environment, for example, the lack of facilitation in the reflex output, along with a delayed reflex response could be critical to an older individual in responding to postural perturbations thus potentially compromising both static and dynamic balance. [ABSTRACT FROM AUTHOR]

The purpose of this study was to investigate whether acute passive stretching (APS) reduced maximal isometric voluntary contraction (MVC) of the plantar flexors (PF) and if so, by what mechanisms. The PF in 15 female volunteers were stretched for 10 min (5×120 s) by a torque motor to within 2° of maximum dorsiflexion (D) range of motion (ROM). MVC with twitch interpolation, maximal Hoffmann reflex (Hmax) and compound action potentials (Mmax) were recorded at 20° D. Stretch reflexes (SR) were mechanically induced at 200° s-1 between 0° and 10°D and SR torque and EMG amplitude were determined. All tests were assessed pre- (pre) and post-APS (post-test1). MVC, SR, andMmax were again assessed after additional stretch was applied [mean 26 (1)° D; post-test2] to test if the optimal angle had been altered. EMG was recorded from soleus (SOL), medial gastrocnemius (MG) and tibialis anterior (TA) using bipolar surface electrodes. APS resulted in a 27% decrease in mean peak passive torque (P<0.05). MVC and SR torque were 7% (P<0.05) and 13% lower at post-test1 (P<0.05), respectively. SR EMG amplitude of SOL and MG was reduced by 27% (P<0.05) and 22% (P<0.05), respectively. TheHmax/Mmax EMG andHmax/Mmax torque ratios were unchanged at post-test1. At post-test2, MVC and SR EMG recovered to pre-APS values, while the SR andMmax torque increased by 19% and 13%, respectively (P<0.05). The decrease in MVC during post-test1 was attributed to changes in the mechanical properties of PF and not to reduced muscle activation. [ABSTRACT FROM AUTHOR]

The behaviour of the soleus H-reflex is considered to be motor task-dependent. However, the speed of movements as well as the motor recruitment level is altered when motor tasks are changed. Therefore it is ambiguous to what extent the motor task-dependent changes found between walking and running, for example, are simply due to changes in these two parameters. The purpose of this study was to investigate how movement speed and motor recruitment level separately influence the soleus H-reflex behaviour when the motor task is unchanged. Soleus H-reflexes were elicited during pedalling at different cadences and crank loads, by which movement speed and muscle recruitment level were modified separately. The H-reflex gain was expressed as the slope of the linear relation between the reflex amplitudes and the background electromyelogram (EMG), and the reflex threshold was expressed by the intercept. The results showed a decrease in reflex gain by 54% (P=0.001) when the speed of movement was doubled from 40 to 80 rpm (repetitions per minute) without changes in the level of soleus EMG activity. Reflex gain decreased 40% (P=0.002) when the soleus EMG level was increased by 47% without changing the speed of movement. No significant changes were found in the reflex threshold. We conclude that as speed of movement and motor recruitment level influence the gain of the soleus H-reflex, it is significant that these two parameters are comparable before changes in H-reflexes are stated to be task-dependent. [ABSTRACT FROM AUTHOR]

Sudden loading of the spine is not only considered a risk factor for the development of low-back pain but also enables an evaluation of the stability of the spine when conducted under laboratory conditions. In the present study the upper spine was pulled in the anterior direction and the stiffness as well as activity in the erector spinae muscle was measured with different pre-tension in the erector spinae. The results showed that increased activity in the erector spinae prior to loading led to increased stiffness (stiffness coefficients from 297 Nm rad(-1) to 438 Nm rad(-1)) and a decrease in the extra neural signal input to the muscles to maintain the stability. It is therefore clear that increased tension in the erector spinae muscle will create a larger stability of the spine to anterior perturbations. However, contracting the muscles around the spine increases the load on the spinal structures. In 34% of the experiments a silent period in the electromyographic signal was present after loading in the period when the torso was moving in the anterior direction. This phenomenon is discussed. [ABSTRACT FROM AUTHOR]

The physiological differences between active and passive changes in posture have been previously established. This study determined the extent of the differences in the initial cardiovascular responses to the passive head-up tilt (HUT) and the active squat-stand test (SST). Eleven females and 13 males underwent one +75° HUT and one SST. Beat-to-beat diastolic blood pressure (DBP), systolic blood pressure (SBP), mean arterial pressure (MAP) and heart rate (HR) were determined non-invasively. Data were recorded 10 s prior to (control) and 30 s after tilt or stand (event). Blood pressure and HR responses were analysed by calculating the deviation from control at 10 s (T10), 20 s (T20) and 30 s (T30) after the onset of each test. The DBP response (reflecting changes in systemic vascular resistance) at T10 was -10 (2) mmHg [mean (SEM)] for the HUT and -25 (2) mmHg for the SST (P<0.01). DBP returned to control levels by T30 for the HUT, but remained depressed for the SST. MAP responses directly reflected these changes in DBP. HR significantly increased from control values (P<0.001) for the HUT [+14 (1) bpm] and the SST [+16 (1) bpm], and remained elevated for the entire 30-s period for both tests. This study demonstrates that although the magnitude of the initial blood pressure decrease is greater for the active SST compared with the passive HUT, the reflex compensatory response is no different, making the SST a greater challenge for the cardiovascular reflexes. [ABSTRACT FROM AUTHOR]

Reductions in tonic vagal controls of the heart and depressed baroreflex sensitivity (BRS) have been associated with a postural fall in blood pressure (BP) and the incidence of cardiac events among older people. We examined the hypothesis that BP regulation during orthostatic challenge as well as heart rate variability (HRV) at rest can be better maintained in long-term exercise-trained, healthy, older men (aged 60–70 years). Subjects were classified into two groups; long-term exercise-trained (LTET, n=14) and sedentary (SED, n=10) according to their history of physical activity. Prior to the dynamic BRS assessment, supine resting autonomic cardiac modulation was assessed by means of time domain HRV [standard deviation of ECG R–R interval (RRISD) and the coefficient of variation (CV)]. The BRS was assessed during 60° head-up tilting by simultaneously measuring beat-by-beat systolic blood pressure (SBP) and ECG R–R interval changes. The BRS gain was determined by the regression slope coefficient based on the extent of the SBP fall and the corresponding ECG R–R shortening during the orthostatic challenge. The results indicated that the LTET group manifested greater ECG R–R interval fluctuations with significantly higher resting RRISD and CV, compared with the SED group [59.5 (10.4) versus 27.7 (7.8) ms, p<0.05; 5.5 (0.8) versus 2.8 (0.7)%, p<0.05], respectively. Using dynamic BRS testing during the acute orthostatic challenge, the LTET group showed a significantly higher BRS gain than the SED [6.4 (0.8) versus 3.8 (0.6) ms·mmHg-1, p<0.017] group. These results indicate that CV and BRS are well maintained in healthy, LTET older individuals when compared with their sedentary peers. Our data suggest that this augmented autonomic cardiac modulation reflects better parasympathetic responsiveness in LTET individuals. Data provide further support for long-term exercise training as another possible cardioprotective factor that might decrease susceptibility to ventricular fibrillation as well as assist arterial BP at the onset of an orthostatic challenge in older men. [ABSTRACT FROM AUTHOR]

Previous studies have shown that prolonged vibration of the rectus femoris decreases maximal voluntary knee extension performance in the ipsilateral leg. In the present study, measurements of maximal voluntary isometric knee extension contractions with the ipsilateral (right) leg and the contralateral (left) leg were made immediately before and after vibration treatment. Significant reductions in maximal force and maximum rate of force generation occurred in both the ipsilateral and contralateral legs following 30 minutes of continuous vibration at both 30 Hz and 120 Hz, with 30 Hz causing the greatest ipsilateral effects. However, although the level of neural activation (iEMG) of the vibrated muscle (right rectus femoris) was reduced following 30 Hz vibration (P=0.026), there were no significant changes occurring in a synergistic muscle (right vastus lateralis) or in either contralateral muscle. It was concluded that muscle vibration may act through spinal reflex pathways to influence the homonymous motoneuron pool. The effects on contralateral force but not specific muscle iEMG suggest an effect on heteronymous motoneuron pools or an effect acting on central descending drive to contralateral muscles. These findings may have implications for the rehabilitation of patients with an immobilised limb. [ABSTRACT FROM AUTHOR]

Reflex inhibition of the motoneuron pool following fatiguing contractions may be mediated by the build-up of byproducts of fatigue. Endurance training is accompanied by neuromuscular adaptations that would alter the production and/or clearance of metabolic substrates. The purpose of the study was to determine the extent of reflex inhibition during and after fatigue in endurance-trained individuals compared to sedentary controls. Subjects produced isometric ankle plantarflexion contractions at 30% of maximal voluntary contraction (MVC) until their MVC torque declined by 30%. H-reflexes were measured during a brief rest period every 3 min as well as superimposed upon the contraction every minute. Both groups of subjects experienced a similar amount of reflex inhibition by the end of the fatiguing protocol, although the endurance time was twice as long for the endurance-trained subjects. The endurance-trained subjects showed a greater reduction in H-reflex amplitude early in the fatiguing protocol compared to the sedentary subjects. These experiments have demonstrated that the neuromuscular processes associated with fatigue-related reflex inhibition must be multi-faceted and cannot be explained solely by small-diameter afferents responding to the byproducts of muscle contraction. [ABSTRACT FROM AUTHOR]

There continues to be great interest in evaluating the adaptive plasticity of the human nervous system in response to exercise training or other interventions. For various reasons, researchers have been interested in estimates of spinal reflex processing in intact human subjects before and after training. A reflex pathway that has been employed in this regard is the Hoffmann (H) reflex. This brief review describes the basic procedure for evoking the H reflex in different muscles. Other sections address methodological issues that affect interpretation of the H reflex. In particular, the role that presynaptic inhibition serves in the modification of the H reflex and how this precludes its use as an unambiguous measure of alpha-motoneuron excitability will be discussed. Applications of the H reflex to study adaptive plasticity in humans is also reviewed, and methodological requirements that should be maintained for accurate interpretation of H reflexes in exercise studies are presented. [ABSTRACT FROM AUTHOR]

The purpose of this study was to elucidate the effect of raised body temperature per se during acute heat stress on the spontaneous arterial baroreflex control of heart rate (fc) in humans. To investigate whether unloading of cardiopulmonary baroreceptors during whole-body heating would alter the arterial baroreflex control of fc, we controlled loading of the cardiopulmonary baroreceptors by head-down tilt (HDT) at angles of 5°, 10°, 15°, and 30° during heat stress produced by hot-water-perfused suits. The sensitivity of the arterial baroreceptor-cardiac reflex was calculated from the spontaneous changes in beat-to-beat arterial pressure and fc. As an index of cardiopulmonary baroreceptor loading, the left atrial diameter (LAD) was measured by echocardiography. During whole-body heating, the LAD decreased with the rising body core temperature and increased with the HDT. The decreased LAD during heating almost recovered to the normothermic control level by 10° HDT. In the supine position, cardiac baroreflex sensitivity remained unchanged during heating. Arterial pressure, fc and cardiac baroreflex sensitivity were not changed by HDT ranging from 5° to 30° during heating. These results suggest that cardiac baroreflex sensitivity remain unchanged during graded loading of the cardiopulmonary baroreceptors in heat-stressed humans. Also, we conclude that the sensitivity of the spontaneous arterial baroreflex controlling the fc is not influenced by raised body temperature per se during acute heat stress. [ABSTRACT FROM AUTHOR]