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Context: The evaluation of quadriceps muscle inhibition with the interpolated twitch technique is usually performed by stimulating the femoral nerve (FN). However, there are some problems related to the use of this stimulation site, which may be partially overcome by delivering the stimulation over the motor point (MP). This study sought to compare MP to FN stimulation at different joint angles for the evaluation of quadriceps muscle inhibition, resting peak torque, and discomfort in healthy women., Design: Cross-sectional study., Methods: Sixteen healthy women (age: 28 [4] y; body mass: 60 [5] kg; height: 162 [5] cm) participated in this study. Supramaximal paired stimuli were delivered to the FN and to the rectus femoris MP before and during maximal voluntary contractions at different knee angles (15°, 30°, 45°, 60°, and 90° of knee flexion) to assess muscle inhibition and resting peak torque. Discomfort was also recorded for each stimulation site and knee angle., Results: Muscle inhibition was similar between the 2 stimulation sites (P > .05) and was higher at 45° than at 90° (P = .03). MP stimulation evoked lower resting peak torque at 30° (P = .004), 60° (P = .006), and 90° (P = .006) and higher discomfort at 30° (P = .008) and 90° (P = .027) compared to FN stimulation., Conclusions: Despite lower resting peak torque and higher discomfort at some angles, MP stimulation provided similar muscle inhibition to FN stimulation at all knee angles and is therefore a valid method to evaluate quadriceps muscle inhibition in healthy women. MP stimulation can be used as an alternative to FN stimulation for the evaluation of quadriceps muscle inhibition with no added discomfort at the angles where muscle inhibition is the highest.

Purpose: The effectiveness of a neuromuscular electrical stimulation (NMES) program is proportional to the level of evoked torque, which can be achieved with either conventional or wide-pulse stimulations. The aim of this study was to compare evoked torque, objective fatigability, and related peripheral and central alterations, as well as changes in central nervous system (CNS) excitability induced by an acute session of conventional versus wide-pulse NMES., Methods: Seventeen young men underwent three 20-min NMES sessions: conventional (0.2 ms/50 Hz), wide-pulse at 50 Hz (1 ms/50 Hz), and wide-pulse at 100 Hz (1 ms/100 Hz). Neuromuscular measurements (i.e., maximal voluntary contraction, voluntary activation, evoked responses to femoral nerve stimulation, and CNS excitability) were performed on the right quadriceps femoris muscle before and after each NMES session. CNS excitability was measured using transcranial magnetic, thoracic, and transcutaneous spinal cord stimulations., Results: The level of evoked torque was not significantly different between conventional and wide-pulse protocols applied at the maximal tolerable current intensity. All NMES protocols induced objective fatigability (~14% decrease in maximal voluntary contraction torque, p < 0.001) associated with peripheral (decrease in doublet torque and potentiated M-wave amplitude, p = 0.002 and p < 0.001, respectively) but not central (unchanged voluntary activation, p = 0.79) alterations. However, these acute changes did not differ between NMES protocols and none of the NMES protocols modified markers of CNS excitability., Conclusion: These results may allow to conjecture that chronic effects and treatment effectiveness could be comparable between conventional and wide-pulse NMES., (© 2023 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

Purpose: The effectiveness of a neuromuscular electrical stimulation (NMES) program has been shown to be proportional to the maximal evocable torque (MET), which is potentially influenced by pulse characteristics such as duration and frequency. The aim of this study was to compare MET between conventional and wide-pulse NMES at two different frequencies., Methods: MET-expressed as a percentage of maximal voluntary contraction (MVC) torque-and maximal tolerable current intensity were quantified on 71 healthy subjects. The right quadriceps was stimulated with three NMES protocols using different pulse duration/frequency combinations: conventional NMES (0.2 ms/50 Hz; CONV), wide-pulse NMES at 50 Hz (1 ms/50 Hz; WP50) and wide-pulse NMES at 100 Hz (1 ms/100 Hz; WP100). The proportion of subjects reaching the maximal stimulator output (100 mA) before attaining maximal tolerable current intensity was also quantified., Results: The proportion of subjects attaining maximal stimulator output was higher for CONV than WP50 and WP100 (p < 0.001). In subjects who did not attain maximal stimulator output in any protocol, MET was higher for both WP50 and WP100 than for CONV (p < 0.001). Maximal tolerable current intensity was lower for both WP50 and WP100 than for CONV and was also lower for WP100 than for WP50 (p < 0.001)., Conclusion: When compared to conventional NMES, wide-pulse protocols resulted in greater MET and lower maximal tolerable current intensity. Overall, this may lead to better NMES training/rehabilitation effectiveness and less practical issues associated with maximal stimulator output limitations., (© 2023. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

This study aimed to determine which contractile properties measured by tensiomyography (TMG) could better differentiate athletes with high- and low-power values, as well as to analyse the relationship between contractile properties and power production capacity. The contractile properties of the vastus medialis (VM), rectus femoris (RF) and vastus lateralis (VL) of an Olympic women's Rugby Sevens team (n = 14) were analysed before a Wingate test in which their peak power output (PPO) was determined. Athletes were then divided into a high-power (HP) and a low-power (LP) group. HP presented an almost certainly higher PPO (9.8 ± 0.3 vs. 8.9 ± 0.4 W kg -1 , ES = 3.00) than LP, as well as a very likely lower radial displacement (3.39 ± 1.16 vs. 5.65 ± 1.50 mm, ES = 1.68) and velocity of deformation (0.08 ± 0.02 vs. 0.13 ± 0.03 mm ms -1 , ES = 1.87) of the VL. A likely lower time of delay was observed in HP for all analysed muscles (ES > 0.60). PPO was very largely related to the radial displacement (r = -0.75, 90% CI = -0.90 to -0.44) and velocity of deformation (r = -0.70, 90% CI = -0.87 to -0.34) of the VL. A large correlation was found between PPO and the time of delay of the VL (r = -0.61, 90% CI = -0.84 to -0.22). No correlations were found for the contractile properties of RF or VM. These results highlight the importance of VL contractile properties (but not so much those of RF and VM) for maximal power production and suggest TMG as a practical technique for its evaluation.

Because maximal voluntary strength of the knee extensor muscles can be facilitated by the concomitant application of neuromuscular electrical stimulation (NMES) to the contralateral homologous muscle, we aimed to determine whether this was associated with an enhanced neural drive to the agonist muscles. A secondary goal of this study was to examine the potential dose-response relationship between NMES intensity and contralateral facilitation. Twelve healthy men (age: 30 ± 7 yr) completed several maximal voluntary contractions (MVC) of the left knee extensors while the right quadriceps received low-intensity NMES (10% MVC), high-intensity NMES (30% MVC) or no NMES. Supramaximal paired stimuli were delivered to the left quadriceps muscle during and immediately after the MVCs to assess voluntary activation. The EMG activity of vastus lateralis (VL) and biceps femoris (BF) was recorded. MVC torque, voluntary activation and VL EMG activity were higher for both low-intensity and high-intensity NMES compared to no NMES (P < .05), with no difference between the two NMES conditions. The acute application of NMES to the right quadriceps facilitated MVC strength of the contralateral homonymous muscle by enhancing its efferent neural drive. No evident dose-response relationship between NMES intensity and contralateral facilitation was observed., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

Introduction: The purpose of this study was to investigate the relative activation of vastus medialis (VM) and vastus lateralis (VL) muscles during voluntary and stimulated isometric contractions at different joint angles., Methods: Sixteen healthy men (mean age: 26 years) completed maximal voluntary and stimulated contractions of the knee extensor muscles at 30°, 65°, and 100° of knee flexion. VM/VL ratios were calculated from voluntary electromyographic (EMG) and evoked torque recordings., Results: Both EMG and VM/VL torque ratios were significantly lower at 30° than at 100° of knee flexion (P < 0.05)., Conclusions: These results can be explained by the relatively small contribution of the VM muscle to knee extension torque at short muscle length. Such disadvantage of the VM muscle at extended knee positions does not seem to be compensated by an increased neural drive. Muscle Nerve 56: 968-974, 2017., (© 2016 Wiley Periodicals, Inc.)

Background: Subcutaneous adipose tissue may influence the transmission of electrical stimuli through to the skin, thus affecting both evoked torque and comfort perception associated with neuromuscular electrical stimulation (NMES). This could seriously affect the effectiveness of NMES for either rehabilitation or sports purposes., Objective: To investigate the effects of skinfold thickness (SFT) on maximal NMES current intensity, NMES-evoked torque, and NMES-induced discomfort., Method: First, we compared NMES current intensity, NMES-induced discomfort, and NMES-evoked torque between two subgroups of subjects with thicker (n=10; 20.7 mm) vs. thinner (n=10; 29.4 mm) SFT. Second, we correlated SFT to NMES current intensity, NMES-induced discomfort, and NMES-evoked knee extension torque in 20 healthy women. The NMES-evoked torque was normalized to the maximal voluntary contraction (MVC) torque. The discomfort induced by NMES was assessed with a visual analog scale (VAS)., Results: NMES-evoked torque was 27.5% lower in subjects with thicker SFT (p=0.01) while maximal current intensity was 24.2% lower in subjects with thinner SFT (p=0.01). A positive correlation was found between current intensity and SFT (r=0.540, p=0.017). A negative correlation was found between NMES-evoked torque and SFT (r=-0.563, p=0.012). No significant correlation was observed between discomfort scores and SFT (rs=0.15, p=0.53)., Conclusion: These results suggest that the amount of subcutaneous adipose tissue (as reflected by skinfold thickness) affected NMES current intensity and NMES-evoked torque, but had no effect on discomfort perception. Our findings may help physical therapists to better understand the impact of SFT on NMES and to design more rational stimulation strategies.

Introduction: In this study we evaluated the validity of garment-based quadriceps stimulation (GQS) for assessment of muscle inactivation in comparison with femoral nerve stimulation (FNS)., Methods: Inactivation estimates (superimposed doublet torque), self-reported discomfort, and twitch and doublet contractile properties were compared between GQS and FNS in 15 healthy subjects., Results: Superimposed doublet torque was significantly lower for GQS than for FNS at 20% and 40% maximum voluntary contraction (MVC) (P < 0.01), but not at 60%, 80%, and 100% MVC. Discomfort scores were systematically lower for GQS than for FNS (P < 0.05). Resting twitch and doublet peak torque were lower for GQS, and time to peak torque was shorter for GQS than for FNS (P < 0.01)., Conclusions: GQS can be used with confidence for straightforward evaluation of quadriceps muscle inactivation, whereas its validity for assessment of contractile properties remains to be determined., (© 2014 Wiley Periodicals, Inc.)

Purpose: The effects of β2-agonists on human skeletal muscle contractile function are unclear, more particularly in the presence of fatigue. Moreover, it remains to be ascertained whether central motor drive during voluntary contractions could counter for eventual contractile alterations induced by β2-agonists. This study explored central and peripheral neuromuscular adjustments of the quadriceps induced by β2-agonist terbutaline after fatiguing exercise., Methods: On 2 d, nine men performed 120 intermittent (5-s contraction/5-s relaxation) submaximal (50% maximal voluntary contraction [MVC]) voluntary contractions, 2 h after having ingested a terbutaline (8 mg) or placebo capsule, in a double-blind randomized order. The isometric knee extension peak torque of single twitch and tetanic (10 and 100 Hz) contractions and of MVC was recorded before (PRE) and after (POST) the fatiguing exercise. Twitch maximal rate of torque relaxation (-dT/dt) was also calculated. Central motor drive was estimated via quadriceps electromyogram recorded during the submaximal voluntary contractions., Results: In PRE and POST conditions, MVC and twitch peak torque were not modified by terbutaline. PRE twitch -dT/dt was 22% higher in terbutaline compared with placebo (P < 0.005), but it did not differ significantly between treatments at POST. The evoked torque of 10- and 100-Hz tetanus were significantly lower in terbutaline compared with placebo, regardless of time (-16% and -17% respectively, P < 0.05). Central motor drive during voluntary exercise did not differ between treatments., Conclusions: These results demonstrate that terbutaline affected the contractile function of the quadriceps muscle during electrically evoked contractions both before and after fatiguing exercise. However, this did not impair the maximal voluntary force-generating capacity of the knee extensors and did not result in any compensatory adjustments of the central nervous system.

Introduction: The aim of this study was to investigate differences in twitch and M-wave potentiation in the quadriceps femoris when electrical stimulation is applied over the quadriceps muscle belly versus the femoral nerve trunk., Methods: M-waves and mechanical twitches were evoked using direct quadriceps muscle and femoral nerve stimulation between 48 successive isometric maximal voluntary contractions (MVC) from 10 young, healthy subjects. Potentiation was investigated by analyzing the changes in M-wave amplitude recorded from the vastus medialis (VM) and vastus lateralis (VL) muscles and in quadriceps peak twitch force., Results: Potentiation of twitch, VM M-wave, and VL M-wave were greater for femoral nerve than for direct quadriceps stimulation (P < 0.05). Despite a 50% decrease in MVC force, the amplitude of the M-waves increased significantly during exercise., Conclusions: In addition to enhanced electrogenic Na(+) -K(+) pumping, other factors (such as synchronization in activation of muscle fibers and muscle architectural properties) may significantly influence the magnitude of M-wave enlargement., (Copyright © 2013 Wiley Periodicals, Inc.)

Introduction: In this study we investigated differences in the spatial recruitment of motor units (MUs) in the quadriceps when electrical stimulation is applied over the quadriceps belly versus the femoral nerve., Methods: M-waves and mechanical twitches were evoked using over-the-quadriceps and femoral nerve stimulation of gradually increasing intensity from 22 young, healthy subjects. Spatial recruitment was investigated using recruitment curves of M-waves recorded from the vastus medialis (VM) and vastus lateralis (VL) and of twitches recorded from the quadriceps., Results: At maximal stimulation intensity (Imax), no differences were found between nerve and over-the-quadriceps stimulation. At submaximal intensities, VL M-wave amplitude was higher for over-the-quadriceps stimulation at 40% Imax, and peak twitch force was greater for nerve stimulation at 60% and 80% Imax., Conclusions: For the VM, MU spatial recruitment during nerve and over-the-quadriceps stimulation of increasing intensity occurred in a similar manner, whereas significant differences were observed for the VL., (Copyright © 2013 Wiley Periodicals, Inc.)

The inclusion of relatively long bouts of stretching (repeated static stretches of ∼30 seconds) in the warm-up is usually associated with a drop in muscle performance. The purpose of this study was to assess the effect of a novel self-administered proprioceptive neuromuscular facilitation (PNF) paradigm with short periods of stretching and contraction on quadriceps neuromuscular function, vertical jump performance, and articular range of motion (ROM). Twelve healthy men (age: 27.7 ± 7.3 years, height: 178.4 ± 10.4 cm, weight: 73.8 ± 16.9 kg) volunteered to participate in a PNF session and a control session separated by 2-7 days. The PNF stretching lasted 2 minutes and consisted of 4 sets of 5-second isometric hamstring contraction immediately followed by 5 seconds of passive static stretch of the quadriceps immediately followed by 5 seconds isometric quadriceps contraction for each leg. For the control session, the participants were asked to walk at a comfortable speed for 2 minutes. Active ROM of knee flexion, vertical jump performance, and quadriceps neuromuscular function were tested before, immediately after, and 15 minutes after the intervention. The PNF stretching procedure did not affect ROM, squat jump, and countermovement jump performances. Accordingly, we did not observe any change in maximal voluntary contraction force, voluntary activation level, M-wave and twitch contractile properties that could be attributed to PNF stretching. The present self-administered PNF stretching of the quadriceps with short (5-second) stretches is not recommended before sports where flexibility is mandatory for performance.

Introduction: In this study we investigated the influence of gender and obesity on electrical current thresholds in an attempt to optimize the application of skeletal muscle electrical stimulation (ES) in clinical practice., Methods: Thirty-two obese and 35 age-matched, non-obese men and women received graded ES to the quadriceps muscle for sensory (detection) and motor (contraction) threshold assessment. Concomitant pain and tolerance to ES were recorded., Results: Sensory threshold was lower in women than in men (P < 0.001), both obese and non-obese. Sensory and motor thresholds were higher in obese than in non-obese subjects (P < 0.05), and body mass index was a strong predictor of motor excitability (r(2) = 0.56-0.61). Current tolerance to motor stimulation was reduced in obese individuals, particularly in women, whereas pain was not influenced by gender or obesity., Conclusions: We suggest that both gender and obesity factors should be carefully considered in the design of rational ES treatments., (Copyright © 2011 Wiley Periodicals, Inc.)

Quadriceps muscle weakness and the underlying neuromuscular deficits have been increasingly studied over the last few years in patients with knee osteoarthritis, but the applied methodologies have never been validated for this specific population. The aim of this study was to investigate test-retest reliability of several quadriceps muscle function outcomes in patients with knee osteoarthritis both before and after knee arthroplasty surgery. Ten preoperative and 20 postoperative patients participated in two identical testing sessions. A series of voluntary and/or electrically stimulated contractions of the involved quadriceps with concomitant torque and electromyographic recordings were used to characterize muscle strength, muscle activation and muscle contraction properties. Vastus lateralis morphology (thickness and fascicle pennation angle) was also assessed using ultrasonography. Overall, good reliability scores were observed for the majority of the 13 assessed variables (nine variables with intraclass correlation coefficients >0.75, 12 variables with coefficients of variation <15%). The most reliable testing protocol for patients with knee osteoarthritis would entail the assessment of (1) isometric maximal voluntary torque for evaluating muscle strength, with (2) simultaneous vastus lateralis electromyographic activity for evaluating muscle activation, (3) potentiated (resting) doublet peak torque for evaluating muscle contractility, and (4) vastus lateralis thickness for evaluating muscle size., (Copyright © 2010 Elsevier Ltd. All rights reserved.)

We evaluated the use of direct muscle stimulation for quantifying quadriceps inactivation at different contraction levels as opposed to conventional twitch interpolation using nerve stimulation. Fourteen healthy volunteers were tested. Paired stimuli were delivered to the femoral nerve or to the quadriceps muscle belly during voluntary contractions ranging from 20% to 100% of maximum, and the amplitude of the superimposed doublet was quantified to investigate inactivation. Superimposed doublet for muscle and nerve stimulation, respectively between the range of 60% to 100% of maximum (e.g., at 100%, muscle stimulation was 14 ± 5 Nm and nerve stimulation was 15 ± 6 Nm). Despite higher current doses, muscle stimulation was associated with less discomfort than nerve stimulation (P < 0.05). Collectively, our data suggest that direct muscle stimulation could be used to assess quadriceps inactivation at maximal and quasi-maximal contraction levels as a valid alternative to motor nerve stimulation., (Copyright © 2010 Wiley Periodicals, Inc.)

Tensiomyography (TMG) is a relatively novel technique to assess muscle mechanical response based on radial muscle belly displacement consecutive to a single electrical stimulus. Although intra-session reliability has been found to be good, inter-rater reliability and the influence of sensor repositioning and electrodes placement on TMG measurements is unknown. The purpose of this study was to analyze the inter-rater reliability of vastus medialis muscle contractile property measurements obtained with TMG as well as the effect of inter-electrode distance (IED). Five contractile parameters were analyzed from vastus medialis muscle belly displacement-time curves: maximal displacement (Dm), contraction time (Tc), sustain time (Ts), delay time (Td), and half-relaxation time (Tr). The inter-rater reliability and IED effect on these measurements were evaluated in 18 subjects. Intra-class correlation coefficients, standard errors of measurement, Bland and Altman systematic bias and random error as well as coefficient of variations were used as measures of reliability. Overall, a good to excellent inter-rater reliability was found for all contractile parameters, except Tr, which showed insufficient reliability. Alterations in IED significantly affected Dm with a trend for all the other parameters. The present results legitimate the use of TMG for the assessment of vastus medialis muscle contractile properties, particularly for Dm and Tc. It is recommended to avoid Tr quantification and IED modifications during multiple TMG measurements., (Copyright (c) 2010 Elsevier Ltd. All rights reserved.)

Background: Quadriceps muscle strength, which is essential for the function and stability of the knee, has been found to be impaired even years after arthroscopic partial meniscectomy. However, the neuromuscular alterations that could account for such muscle weakness remain unclear., Questions/purposes: We investigated (1) the side-to-side asymmetries in quadriceps muscle strength 6 months after arthroscopic partial meniscectomy, (2) the physiologic mechanisms (neural versus muscular) underlying muscle weakness, and (3) the impact of quadriceps weakness on muscle control at submaximal force levels., Patients and Methods: We tested 14 volunteers (10 men, four women) with an average age of 44 +/- 9 years (range, 24-59 years) at 6 +/- 1 months after unilateral medial arthroscopic partial meniscectomy. We measured maximal voluntary strength and muscle activation during isometric, concentric, and eccentric contractions using isokinetic dynamometry and surface EMG, respectively. We assessed vastus lateralis muscle size and architecture using ultrasonography. We measured muscle control at submaximal force levels with a repositioning test (knee proprioception) and a low-force target-tracking task (steadiness, accuracy)., Results: Isometric and concentric quadriceps strength and vastus lateralis EMG activity were lower on the involved than on the uninvolved side. Muscle architecture and muscle control did not differ between the involved and uninvolved sides., Conclusions: Our results showed quadriceps weakness exists 6 months after arthroscopic partial meniscectomy. As suggested by the EMG results, this is likely attributable to neural impairments (activation failure) that affect muscle control at maximal but not submaximal force outputs., Level of Evidence: Level IV, therapeutic study. See Guidelines for Authors for a complete description of levels of evidence.

The purpose of this study was to investigate the neural and muscular changes associated with electrical stimulation (ES) training and subsequent detraining. Twenty healthy active men were randomized to receive (intervention group) or not (control group) 4 weeks of ES strength training followed by 4 weeks of detraining. Quadriceps ES training sessions (20 minutes per session, 4 sessions per week) were completed under isometric loading conditions. Quadriceps maximal voluntary contraction (MVC) strength, activation level, maximal electromyographic (EMG) activity, and excitation-contraction coupling properties were assessed before training, after training, and after detraining. Maximal voluntary contraction strength did not change after training but significantly increased after detraining (+21.5%; p < 0.05). Activation level (+7.3%) and maximal EMG activity (+27.9%) increased significantly after training and remained elevated after detraining (p < 0.05). Vastus lateralis M-wave amplitude did not change during the study period, whereas quadriceps contractile properties were significantly impaired after training but then recovered to pre-training values after detraining. We conclude that the maximal force-generating capacity of the quadriceps was unchanged after 4 weeks of ES strength training because of the interplay between neural (increased activation) and muscular (contractile impairment) changes. On the other hand, recovered contractile function and preserved activation after 4 weeks of detraining resulted in significant MVC strength increases. Quadriceps strength training via ES may induce overreaching and delayed adaptations and therefore should be used with caution. These findings may help in conceiving effective ES strength training programs for physically active subjects.

Quadriceps muscle strength is an important predictor of functional abilities in patients having TKA. However, because several daily activities are characterized by a limited time to generate force, it has been suggested that rate of force development (RFD) could better predict functional difficulties than maximal strength. We therefore hypothesized the side-to-side asymmetry would be larger for RFD than for maximal strength, and RFD asymmetry relates to subjective symptoms and/or functional daily living activities. We studied 31 subjects (17 women, 14 men) 6 +/- 1 months after undergoing TKA for unilateral osteoarthritis. Symptoms and limitations during activities of daily living were quantified using the knee outcome survey-activities of daily living scale (KOS-ADLS). Quadriceps maximal strength and RFD at different times (50 to 200 ms from contraction onset) were quantified during unilateral maximal voluntary isometric actions. Side-to-side asymmetries (involved versus uninvolved side) were larger for RFD (approximately 36%) than for maximal strength (approximately 24%). Subjective knee function related to all RFD asymmetry variables, but not to maximal strength asymmetry. In addition to maximal strength, quadriceps RFD in the first 100 to 200 ms from contraction onset provides an alternative functional outcome measure for individuals undergoing TKA.

This study aimed to 1) compare electrical and magnetic stimulations for quadriceps muscle function assessment, and 2) ascertain whether the ratios of the second twitch elicited by supramaximal electrical and magnetic femoral nerve stimulation at 10 and 100 Hz (T2(10:100)) and the total twitch force elicited by the same types of stimulations (Fpaired(10:100)) are equivalent to the standard low- to high-frequency force ratio associated with submaximal electrical tetanic stimulations (Ftet(10:100)). Quadriceps force and vastus lateralis EMG were recorded at rest (n = 21 subjects), immediately after, and 30 min after a 30-min downhill run (n = 10) when 1) supramaximal electrical nerve stimulation (ENS), 2) magnetic nerve stimulation (MNS) and 3) submaximal electrical muscle stimulation (EMS) were delivered in random order at 1 (single stimulation), 10, and 100 Hz (paired stimulations). Ten- and 100-Hz 500-ms tetani were also evoked with EMS to determine Ftet(10:100). Before exercise, contractile properties with single and paired stimulations were similar for ENS and MNS (all intraclass correlation coefficients k > 0.90), but smaller for EMS (P < 0.001). M-wave characteristics were also similar for ENS and MNS (all k > 0.90). After exercise, changes in all parameters did not differ between methods. With fatigue, the changes in Ftet(10:100) were inconsistently correlated with the changes in T2(10:100) (r(2) = 0.24-0.73, P = 0.002-0.15) but better correlated with the changes in Fpaired(10:100) (immediately after exercise: r(2) = 0.80-0.83, P < 0.001; 30 min after exercise: r(2) = 0.46-0.82, P = 0.001-0.03). We conclude that ENS and MNS provide similar quadriceps muscle function assessment, while Fpaired(10:100) is a better index than T2(10:100) of low- to high-frequency fatigue of the quadriceps in vivo.

The purpose of this cross-sectional study was to compare quadriceps muscle strength and fatigue between severely obese (body mass index 34 kg/m2) and nonobese adolescents. Maximal isokinetic torque and angle of peak torque as well as isometric torque at short (40 degrees of knee flexion) and long (80 degrees of knee flexion) muscle length were measured using an isokinetic dynamometer. Muscle fatigue was quantified as the percent torque loss during an isokinetic voluntary protocol and an electrical stimulation isometric protocol. Obese adolescents produced greater absolute isokinetic (+16%; P < 0.05) and isometric torque at short (+25%; P < 0.01) but not at long muscle length (P > 0.05) compared to their lean counterparts. The angle of peak torque was significantly lower in obese than in nonobese subjects (-11%; P < 0.05), i.e., obese produced their maximal strength at shorter muscle length. Isokinetic and isometric torque normalized to the fat-free mass were not significantly different between the two groups. No significant difference in voluntary and stimulated torque loss was observed between groups. Muscle strength per unit of fat-free mass and muscle fatigue were similar in the obese and nonobese adolescents tested in this study, therefore suggesting that obesity has little or no effect on quadriceps muscle function characteristics. On the other hand, it remains to be confirmed whether the observed quadriceps muscle length specificity contributes to the reduced functional capacity of obese adolescents during complex motor tasks involving deep knee flexion (squatting, kneeling).

This study aimed to compare voluntary and stimulated exercise for changes in muscle strength, growth hormone (GH), blood lactate, and markers of muscle damage. Nine healthy men had two leg press exercise bouts separated by 2 wk. In the first bout, the quadriceps muscles were stimulated by biphasic rectangular pulses (75 Hz, duration 400 mus, on-off ratio 6.25-20 s) with current amplitude being consistently increased throughout 40 contractions at maximal tolerable level. In the second bout, 40 voluntary isometric contractions were performed at the same leg press force output as the first bout. Maximal voluntary isometric strength was measured before and after the bouts, and serum GH and blood lactate concentrations were measured before, during, and after exercise. Serum creatine kinase (CK) activity and muscle soreness were assessed before, immediately after, and 24, 48, and 72 h after exercise. Maximal voluntary strength decreased significantly (P < 0.05) after both bouts, but the magnitude of the decrease was significantly (P < 0.05) greater for the stimulated contractions (-22%) compared with the voluntary contractions (-9%). Increases in serum GH and lactate concentrations were significantly (P < 0.05) larger after the stimulation compared with the voluntary exercise. Increases in serum CK activity and muscle soreness were also significantly (P < 0.05) greater for the stimulation than voluntary exercise. It was concluded that a single bout of electrical stimulation exercise resulted in greater GH response and muscle damage than voluntary exercise.

The present study aimed to compare quadriceps femoris muscle strength and fatigue between obese (grade II and III) and nonobese adults. Ten obese (mean age: 25 years; mean BMI: 41 kg/m(2)) and ten lean (mean age: 27 years; mean BMI: 23 kg/m(2)) men were tested. Quadriceps muscle fatigue was quantified as the (percent) torque loss during a voluntary isokinetic (50 maximal contractions at 180 degrees /s) and an electrostimulated (40 Hz) isometric protocol (5 min, 10% of the maximal torque). Maximal voluntary isometric and isokinetic torque and power were also measured. Voluntary torque loss was significantly higher (P < 0.05) in obese (-63.5%) than in lean subjects (-50.6%). Stimulated torque decreased significantly (P < 0.05) but equally in the two subject groups. Obese subjects displayed higher absolute (+20%; P < 0.01) but lower relative (i.e., normalized to body mass) (-32%; P < 0.001) muscle torque and power than their lean counterparts. Obese individuals demonstrated lower fatigue resistance during voluntary but not during stimulated knee extensions compared to their nonobese counterparts. Peripheral mechanisms of muscle fatigue -- at least those associated to the present stimulated test -- were not influenced by obesity. The observed quadriceps muscle function impairments (voluntary fatigue and relative strength) probably contribute to the reduced functional capacity of obese subjects during daily living activities.

The aim of this study was to compare respiratory gas exchange variables and muscle fatigue between equal-intensity (i.e., same force output) electrostimulated and voluntary contractions of the quadriceps muscle (46+/-10% of maximal voluntary force). Twelve healthy men served as volunteers. Oxygen consumption, ventilation and respiratory exchange ratio were recorded during the exercise bouts. Muscle fatigue was quantified as the exercise-induced reduction in maximal voluntary force. The average oxygen consumption (11+/-3 versus 8+/-2 mL min(-1)kg(-1)), ventilation (23+/-4 versus 16+/-2 L min(-1)) and respiratory exchange ratio (0.96+/-0.02 versus 0.85+/-0.01) were significantly higher during electrostimulation compared with voluntary exercise (P<0.05-0.001). Maximal voluntary force decreased significantly after electrostimulation (-21+/-10%; P<0.001), while no changes were observed following voluntary exercise. Electrostimulation-resistance exercise of the quadriceps muscle elicited greater cardiorespiratory demand and muscle fatigue compared with voluntary contractions of the same intensity. These findings probably reflect differences in the patterns of motor unit recruitment between stimulated and voluntary contractions, despite equal force productions.

The aim of the present study was to further confirm the validity of measurements for characterizing neuromuscular alterations by establishing their reliability both before and after fatigue. Thirteen men (28 +/- 5 years) volunteered to participate in two separate identical sessions requiring the performance of a sustained maximal voluntary contraction (MVC) with the quadriceps muscle for 2 min. MVC and transcutaneous electrical stimulations were used before and immediately after the fatiguing contraction to investigate the reliability of MVC torque, central activation, and peripheral variables (M-wave properties, peak twitch, peak doublet) within and between sessions. Based on previous and present results, we advise the use of (1) voluntary activation level with potentiated doublet as a reference to describe central fatigue, (2) electromyographic activity of vastus lateralis muscle as a surrogate for quadriceps for both voluntary and evoked contraction, and (3) potentiated peak doublet amplitude to investigate contractile fatigue. These findings can be useful in the choice of the parameters describing central and peripheral fatigue of the quadriceps muscle in future studies.

Objective: Hamstring and patellar tendon autografts are the most frequently-used graft types for anterior cruciate ligament (ACL) reconstruction, with no consensus on their respective effects on thigh muscle strength. The objective of this study was to re-examine isokinetic knee extensor and flexor strength before and after ACL reconstruction with patellar and hamstring tendon grafts using a single-center and a relatively large database, where surgical, rehabilitation and testing procedures were strictly standardized for all patients. Methods: A total of 464 patients with a unilateral ACL rupture underwent arthroscopic ACL reconstruction with either patellar or hamstring tendon grafts. Isokinetic concentric strength was evaluated prior to surgery and at 5- and 9-month postoperative follow-ups in different patient subgroups (n = 140, 464 and 215, respectively). Results: Knee extensor strength was lower in patients operated with the patellar tendon graft at the 5-month (p < 0.05) but not at the 9-month follow-up. Knee flexor strength was lower in patients operated with the hamstring tendon graft at both postoperative time points (p < 0.05). The prevalence of quadriceps weakness was high (66-91%) in both patient groups at both follow-ups. Conclusion: Overall, postoperative recovery of thigh muscle function seems to be better with the patellar than with the hamstring tendon graft due to the fact that both, knee extensor and flexor strength, were more impaired after ACL reconstruction with the hamstring tendon autograft. [ABSTRACT FROM AUTHOR]

The clinical success of neuromuscular electrical stimulation (NMES) for neuromuscular rehabilitation is greatly compromised by the poor consideration of different physiological and methodological issues that are not always obvious to the clinicians. Therefore, the aim of this narrative review is to reexamine some of these fundamental aspects of NMES using a tripartite model perspective. First, we contend that NMES does not actually bypass the central nervous system but results in a multitude of neurally mediated responses that contribute substantially to force generation and may engender neural adaptations. Second, we argue that too much emphasis is generally placed on externally controllable stimulation parameters while the major determinant of NMES effectiveness is the intrinsically determined muscle tension generated by the current (ie, evoked force). Third, we believe that a more systematic approach to NMES therapy is required in the clinic and this implies a better identification of the patient-specific impairment and of the potential “responders” to NMES therapy. On the basis of these considerations, we suggest that the crucial steps to ensure the clinical effectiveness of NMES treatment should consist of (1) identifying the neuromuscular impairment with clinical assessment and (2) implementing algorithm-based NMES therapy while (3) properly dosing the treatment with tension-controlled NMES and eventually amplifying its neural effects. [ABSTRACT FROM AUTHOR]

Purpose: The present study aimed at comparing knee extensor neuromuscular properties determined with transcutaneous electrical stimulation using two pulse durations before and after a standardized fatigue protocol.Methods: In the first sub-study, 19 healthy participants (ten women and nine men; 28 ± 5 years) took part to two separate testing sessions involving the characterization of voluntary activation (twitch interpolation technique), muscle contractility (evoked forces by single and paired stimuli), and neuromuscular propagation (M-wave amplitude from vastus lateralis and vastus medialis muscles) obtained at supramaximal intensity with a pulse duration of either 0.2 or 1 ms. The procedures were identical in the second sub-study (N = 11), except that neuromuscular properties were also evaluated after a standardized fatiguing exercise. Electrical stimulation was delivered through large surface electrodes positioned over the quadriceps muscle and a visual analog scale was used to evaluate the discomfort to paired stimuli evoked at rest.Results: There was no difference between pulse durations in the estimates of voluntary activation, neuromuscular propagation, and muscle contractility both in the non-fatigued and fatigued states. The discomfort associated with supramaximal paired electrical stimuli was also comparable between the two pulse durations.Conclusions: It appears that 0.2- and 1-ms-long pulses provide a comparable evaluation of knee extensor neuromuscular properties. [ABSTRACT FROM AUTHOR]

Objective. Obesity and obstructive sleep apnea (OSA) are closely interconnected conditions both leading to high cardiovascular risk. Inactivity is frequent and physical activity programs remain difficult in these patients. We investigated the acute feasibility of two neuromuscular electrical stimulation (NMES) modalities in extremely inactive obese patients with OSA. Design. A randomized cross-over study, with two experimental sessions (one per condition: multipath NMES versus conventional NMES). Setting. Outpatient research hospital. Subjects. Twelve patients with obesity, already treated for OSA. Interventions. No intervention. Measures. Feasibility outcomes included NMES current intensity, knee extension force evoked by NMES, and self-reported discomfort. Results. We found higher current intensity, a trend to significantly higher evoked force and lower discomfort during multipath NMES versus conventional NMES, suggesting better tolerance to the former NMES modality. However, patients were rapidly limited in the potential of increasing current intensity of multipath NMES. Conclusion. Both NMES modalities were feasible and relatively well tolerated by obese patients with OSA, even if multipath NMES showed a better muscle response/discomfort ratio than conventional NMES. There is an urgent need for a proof-of-concept study and interventional randomized controlled trials comparing NMES therapy versus current care to justify its utilization in obese and apneic patients with low physical activity levels. [ABSTRACT FROM AUTHOR]

This study examined the validity of the twitch interpolation technique for evaluating side-to-side asymmetries in quadriceps neuromuscular function. Fifty-six subjects with a wide range of asymmetries (19 healthy, 24 with unilateral and 13 with bilateral anterior cruciate ligament reconstruction) took part in the study. Supramaximal electrical paired stimuli were delivered to the quadriceps muscle during and immediately after a maximal voluntary contraction (MVC) of the knee extensors (twitch interpolation technique). MVC torque, voluntary activation and resting doublet-evoked torque were measured separately for the two sides, and percent side-to-side asymmetries were calculated for each parameter. MVC torque asymmetry was plotted against voluntary activation asymmetry and doublet-evoked torque asymmetry, and a multiple regression analysis was also conducted. Significant positive correlations were observed between MVC torque asymmetry and both voluntary activation asymmetry (r=0.40; p=0.002) and doublet-evoked torque asymmetry (r=0.53; p<0.001), and their relative contribution to MVC torque asymmetry was comparable (r=0.64; p<0.001). These results establish the validity of the twitch interpolation technique for the assessment of neuromuscular asymmetries. This methodology could provide useful insights into the contribution of some neural and muscular mechanisms that underlie quadriceps strength deficits. [ABSTRACT FROM AUTHOR]

Transcutaneous neuromuscular electrical stimulation applied in clinical settings is currently characterized by a wide heterogeneity of stimulation protocols and modalities. Practitioners usually refer to anatomic charts (often provided with the user manuals of commercially available stimulators) for electrode positioning, which may lead to inconsistent outcomes, poor tolerance by the patients, and adverse reactions. Recent evidence has highlighted the crucial importance of stimulating over the muscle motor points to improve the effectiveness of neuromuscular electrical stimulation. Nevertheless, the correct electrophysiological definition of muscle motor point and its practical significance are not always fully comprehended by therapists and researchers in the field. The commentary describes a straightforward and quick electrophysiological procedure for muscle motor point identification. It consists in muscle surface mapping by using a stimulation pen-electrode and it is aimed at identifying the skin area above the muscle where the motor threshold is the lowest for a given electrical input, that is the skin area most responsive to electrical stimulation. After the motor point mapping procedure, a proper placement of the stimulation electrode(s) allows neuromuscular electrical stimulation to maximize the evoked tension, while minimizing the dose of the injected current and the level of discomfort. If routinely applied, we expect this procedure to improve both stimulation effectiveness and patient adherence to the treatment. The aims of this clinical commentary are to present an optimized procedure for the application of neuromuscular electrical stimulation and to highlight the clinical implications related to its use. [ABSTRACT FROM AUTHOR]

We compared the effectiveness of a home-based neuromuscular electrical stimulation (NMES) program applied to the quadriceps of the nonoperative side against sham-NMES as a complement to standard rehabilitation on knee extensor neuromuscular function in patients following anterior cruciate ligament (ACL) reconstruction. Twenty-four patients completed the 6 week NMES (n = 12) and sham-NMES (n = 12) post-operative interventions and were tested at different time points for neuromuscular function and self-reported knee function. Isometric, concentric, and eccentric strength deficits (muscle weakness) increased significantly from pre-surgery to 24 weeks post-surgery in the sham-NMES group (p < 0.05), while no significant changes were observed in the NMES group. On the stimulated (nonoperative) side, quadriceps voluntary activation and muscle thickness were respectively maintained (p > 0.05) and increased (p < 0.001) as a result of the NMES intervention, contrary to sham-NMES. Self-reported knee function improved progressively during the post-operative phase (p < 0.05), with no difference between the two groups. Compared to a sham-NMES intervention, a 6 week home-based NMES program applied to the quadriceps of the nonoperative side early after ACL reconstruction prevented the occurrence of knee extensor muscle weakness 6 months after surgery. We conclude that nonoperative-side NMES may help counteract muscle weakness after ACL reconstruction. [ABSTRACT FROM AUTHOR]

A combination of in vivo and in vitro analyses was performed to investigate muscular and neural adaptations of the weaker (nondominant) quadriceps femoris muscle of one healthy individual to short-term electrostimulation resistance training. The increase in maximal voluntary strength (+ 12%) was accompanied by neural (cross-education effect and increased muscle activation) and muscle adaptations (impairment of whole-muscle contractile properties). Significant changes in myosin heavy chain (MHC) isoforms relative content (+22% for MHC-2A and -28% for MHC-2X), single-fiber cross-sectional area (+ 27% for type 1 and +6% for type 2A muscle fibers), and specific tension of type 1 (+ 67%) but not type 2Afibers were also observed after training. Plastic changes in neural control confirm the possible involvement of both spinal and supraspinal structures to electrically evoked contractions. Changes at the single muscle fiber level induced by electrostimulation resistance training were significant and pref- erentially affected slow, type 1 fibers. [ABSTRACT FROM AUTHOR]

Endurance time of a submaximal sustained contraction is longer when the muscle is fatigued in a shortened position. The aim of the present study was to compare central and peripheral mechanisms of fatigue after an isometric contraction of the knee extensor muscles performed at 20% maximal voluntary contraction (MVC) at two knee angles (35°, short length vs. 75°, long length; 0° = full extension) until exhaustion. Eleven men (24 ± 4 yr) attended two experimental randomized sessions. Endurance time was greater at 350 compared with 75° (974 ± 457 vs. 398 ± 144 s: P < 0.001) despite a similar reduction in knee extensor MVC (-28.4 ± 16.0%, P < 0.001 vs. -27.6 ± 18.8%, P < 0.001, respectively). Voluntary activation level was similarly depressed after the fatiguing contraction performed at the two muscle lengths (-19 ± 16.7% at 35°, P < 0.01 vs. -13.7 ± 14.5% at 75°, P < 0.01). After the fatiguing contraction, peak twitch potentiation was observed only at the short length (+31.8 ± 17.6% at 35°, P < 0.01 vs. +6.4 ± 21.3% at 75°, P > 0.05), whereas M-wave properties were similarly altered for the two angles. These results suggest that 1) central fatigue at task failure for a sustained isometric contraction was not dependent on the muscle length, and 2) the longer endurance time of a sustained isometric contraction performed at a shortened length is related to potentiation. It is suggested that the greater endurance time of a sustained isometric contraction observed at 35° is related to the occurrence of potentiation at this short length, because central fatigue is similar at task failure for both tasks. [ABSTRACT FROM AUTHOR]

Quadriceps neuromuscular function remains impaired in the short- and long-term following knee arthroscopy for meniscal surgery and/or anterior cruciate ligament (ACL) reconstruction. The aim of this study was to compare quadriceps neuromuscular impairments in patients following meniscal surgery with and without ACL reconstruction. Thirty patients were tested six months after meniscal surgery with (n = 15) and without (n = 15) ACL reconstruction. We bilaterally assessed knee extension maximal voluntary contraction (MVC) torque using dynamometry, vastus lateralis thickness using ultrasound, quadriceps voluntary activation and evoked knee extension torque with transcutaneous electrical stimulation. Patient-reported outcomes were evaluated with the Knee Injury and Osteoarthritis Outcome Score (KOOS). Compared with meniscus patients, ACL patients demonstrated larger asymmetries in MVC torque (15% vs. 5%, p = 0.049) and vastus lateralis thickness (6% vs. 0%, p = 0.021). In ACL patients, asymmetries in MVC torque correlated with asymmetries in evoked torque (r = 0.622, p = 0.013). In meniscus patients, asymmetries in muscle activation correlated with KOOS quality of life (r = 0.619, p = 0.018). Patients demonstrated persistent quadriceps muscle weakness six months after ACL reconstruction, but not after isolated meniscal surgery. Quantitative and/or qualitative muscular changes likely underlie quadriceps muscle weakness in ACL patients, whereas activation failure is associated with poor quality of life in some meniscus patients. [ABSTRACT FROM AUTHOR]

Objective: The aim of the study was to determine the influence of neuromuscular electrical stimulation pulse waveform and frequency on evoked torque, stimulation efficiency, and discomfort at two neuro-muscular electrical stimulation levels. Design: This is a repeated measures study. The quadriceps muscle of 24 healthy men was stimulated at submaximal (neuromuscular electrical stimulationmax) and maximal (neuromuscular electrical stimulationsub levels using two pulse waveforms (symmetrical, asymmetrical) and three pulse frequencies (60, 80, 100 Hz). Repeated measures analysis of variance and effect sizes were used to verify the effect of pulse waveform and pulse frequency on stimulation efficiency (evoked torque/ current intensity) and discomfort and to assess the magnitude of the differences, respectively. Results: Stimulation efficiency was higher for symmetrical (neuromuscular electrical stimulationsub = 0.88 ± 0.21 Nm/mA; neuromuscular electrical stimulationmax = 1.27 ± 0.46 Nm/mA) compared with asymmetrical (neuromuscular electrical stimulationsub = 0.77 ± 0.21 Nm/mA; neuromuscular electrical stimulationmax = 1.02 ± 0.34 Nm/mA; P ≤ 0.001; effect size = 0.56-0.66) but did not significantly differ between frequencies (P = 0.17). At both neuromuscular electrical stimulation levels, there were no statistically significant differences in discomfort between pulse waveforms or frequencies. Conclusions: The higher stimulation efficiency of symmetrical pulses suggests that this waveform would be preferred to asymmetrical pulses in clinical practice. Stimulation frequencies between 60 and 100 Hz can be used interchangeably because of similar efficiency and discomfort. [ABSTRACT FROM AUTHOR]

Abstract: Objective: We investigate the influence that inguinal fat has on maximal quadriceps response to femoral nerve magnetic stimulation. Methods: Vastus lateralis M-wave amplitude and quadriceps twitch peak torque were measured (i) in lean and overweight subjects and (ii) when slices of pig fat ranging from 2 to 18mm in thickness were placed between the coil and the nerve. Results: In overweight group, the maximal response could not be elicited when intensity was⩽90% and⩽85% of maximal power for twitch torque and M-wave amplitude, respectively. The maximal response was obtained at 80% of maximal power in the lean group. Negative correlations between relative twitch torque changes and inguinal thickness were observed. Fat thickness altered quadriceps response at 14mm and 8mm in the lean and overweight groups, respectively. Conclusions: The capacity of femoral nerve magnetic stimulation to deliver supramaximal stimulation is altered when fat thickness below the coil increases. Significance: Special caution should be taken by clinicians when overweight or obese subjects are tested using femoral nerve magnetic stimulation. [Copyright &y& Elsevier]