Search

Purpose: Muscle pain can impair exercise performance but the mechanisms for this are unknown. This study examined the effects of muscle pain on neuromuscular fatigue during an endurance task., Methods: On separate visits, twelve participants completed an isometric time-to-task failure (TTF) exercise of the right knee extensors at ~ 20% of maximum force following an intramuscular injection of isotonic saline (CTRL) or hypertonic saline (HYP) into the vastus lateralis. Measures of neuromuscular fatigue were taken before, during and after the TTF using transcranial magnetic stimulation (TMS) and peripheral nerve stimulation., Results: The mean pain intensity was 57 ± 10 in HYP compared to 38 ± 18 in CTRL (P < 0.001). TTF was reduced in HYP (4.36 ± 0.88 min) compared to CTRL (5.20 ± 0.39 min) (P = 0.003). Maximum voluntary force was 12% lower at minute 1 (P = 0.003) and 11% lower at minute 2 in HYP (P = 0.013) compared to CTRL. Voluntary activation was 4% lower at minute 1 in HYP compared to CTRL (P = 0.006) but not at any other time point (all P > 0.05). The TMS silent period was 9% longer at 100 s during the TTF in HYP compared to CTRL (P = 0.026)., Conclusion: Muscle pain reduces exercise performance through the excacerbation of neuromuscular fatigue that is central in origin. This appears to be from inhibitory feedback from group III/IV nociceptors which acts to reduce central motor output., (© 2021. The Author(s).)

Purpose: Joint angle is a significant determinant of neuromuscular and metabolic function. We tested the hypothesis that previously reported correlations between knee-extensor torque complexity and metabolic rate ([Formula: see text]) would be conserved at reduced joint angles (i.e. shorter muscle lengths)., Methods: Eleven participants performed intermittent isometric knee-extensor contractions at 50% maximum voluntary torque for 30 min or until task failure (whichever occurred sooner) at joint angles of 30º, 60º and 90º of flexion (0º = extension). Torque and surface EMG were sampled continuously. Complexity and fractal scaling of torque were quantified using approximate entropy (ApEn) and detrended fluctuation analysis (DFA) α. [Formula: see text] was determined using near-infrared spectroscopy., Results: Time to task failure/end increased as joint angle decreased (P < 0.001). Over time, complexity decreased at 90º and 60º (decreased ApEn, increased DFA α, both P < 0.001), but not 30º. [Formula: see text] increased at all joint angles (P < 0.001), though the magnitude of this increase was lower at 30º compared to 60º and 90º (both P < 0.01). There were significant correlations between torque complexity and [Formula: see text] at 90º (ApEn, r =  - 0.60, P = 0.049) and 60º (ApEn, r =  - 0.64, P = 0.035; DFA α, ρ = 0.68, P = 0.015)., Conclusion: The lack of correlation between [Formula: see text] and complexity at 30º was likely due to low relative task demands, given the similar kinetics of [Formula: see text] and torque complexity. An inverse correlation between [Formula: see text] and knee-extensor torque complexity occurs during high-intensity contractions at intermediate, but not short, muscle lengths., (© 2021. The Author(s).)

Purpose: Altered movement complexity, indicative of system dysfunction, has been demonstrated with increased running velocity and neuromuscular fatigue. The critical velocity (CV) denotes a metabolic and neuromuscular fatigue threshold. It remains unclear whether changes to complexity during running are coupled with the exercise intensity domain in which it is performed. The purpose of this study was to examine whether movement variability and complexity differ exclusively above the CV intensity during running., Methods: Ten endurance-trained participants ran at 95%, 100%, 105% and 115% CV for 20 min or to task failure, whichever occurred first. Movement at the hip, knee, and ankle were sampled throughout using 3D motion analysis. Complexity of kinematics in the first and last 30 s were quantified using sample entropy (SampEn) and detrended fluctuation analysis (DFA-α). Variability was determined using standard deviation (SD)., Results: SampEn decreased during all trials in knee flexion/extension and it increased in hip internal/external rotation, whilst DFA-α increased in knee internal/external rotation. SD of ankle plantar/dorsiflexion and inversion/eversion, knee internal/external rotation, and hip flexion/extension and abduction/adduction increased during trials. Hip flexion/extension SampEn values were lowest below CV. DFA-α was lower at higher velocities compared to velocities below CV in ankle plantar/dorsiflexion, hip flexion/extension, hip adduction/abduction, hip internal/external rotation. In hip flexion/extension SD was highest at 115% CV., Conclusions: Changes to kinematic complexity over time are consistent between heavy and severe intensity domains. The findings suggest running above CV results in increased movement complexity and variability, particularly at the hip, during treadmill running.

Prior heavy exercise (above the lactate threshold, Th(la)) increases the amplitude of the primary oxygen uptake (VVO(2)) response and reduces the amplitude of the VO(2) slow component during subsequent heavy exercise. The purpose of this study was to determine whether these effects required the prior performance of an identical bout of heavy exercise, or if prior short-duration sprint exercise could cause similar effects. A secondary purpose of this study was to determine the effect of elevating muscle temperature (through passive warming) on VO(2) kinetics during heavy exercise. Nine male subjects performed a 6-min bout of heavy exercise on a cycle ergometer 6 min after: (1) an identical bout of heavy exercise; (2) a 30-s bout of maximal sprint cycling; (3) a 40-min period of leg warming in a hot water bath at 42 degrees C. Prior sprint exercise elevated blood [lactate] prior to the onset of heavy exercise (by aproximately 5.6 mM) with only a minor increase in muscle temperature (of approximately 0.7 degrees C). In contrast, prior warming had no effect on baseline blood lactate concentration, but elevated muscle temperature by approximately 2.6 degrees C. Both prior heavy exercise and prior sprint exercise significantly increased the absolute primary VO(2) amplitude (by approximately 230 ml x min(-1) and 260 ml x min(-1), respectively) and reduced the amplitude of the VO(2) slow component (by approximately 280 ml x min(-1) and 200 ml x min(-1), respectively) during heavy exercise, whereas prior warming had no significant effect on the VO(2) response. We conclude that the VO(2) response to heavy exercise can be markedly altered by both sustained heavy-intensity submaximal exercise and by short-duration sprint exercise that induces a residual acidosis. In contrast, passive warming elevated muscle temperature but had no effect on the VO(2) response.