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Purpose: To investigate muscle deoxygenation and neural drive-related changes during repeated cycling sprints in a fatiguing context., Methods: Nine healthy male subjects performed a repeated-sprint test (consisting of 10 x 6-s maximal sprints interspaced by 30 s of recovery). Oxygen uptake was measured breath-by-breath; muscle deoxygenation of the vastus lateralis was assessed continuously using the near-infrared spectroscopy technique. Surface electromyograms (RMS) of both vastus lateralis and biceps femoris were also recorded. Furthermore, before and after the repeated-sprint test, the percentage of muscle activation by voluntary drive (twitch-interpolated method) was measured during a maximal voluntary contraction., Results and Discussion: Consistent with previous research, our data showed a significant power decrement during repeated-sprint exercise. There was also a progressive muscle deoxygenation, but our data showed that the ability of the subjects to use available O2 throughout the entire repeated-sprint test was well preserved. Our data displayed a significant decrement in the RMS activity during the acceleration phase of each sprint across the repeated-sprint exercise. Moreover, decrement in motor drive was confirmed after exercise by a significant decrease in both percentage of voluntary activation and RMS/M-wave ratio during a maximal voluntary contraction., Conclusion: In this experimental design, our findings suggest that the ability to repeat short-duration (6 s) sprints was associated with the occurrence of both peripheral and central fatigue.

Objectives: To examine changes in exercise characteristics, maximal voluntary contraction, and explosive strength during prolonged tennis playing., Methods: Maximal isometric voluntary contraction (MVC), leg stiffness (hopping), and peak power in squat (SJ) and countermovement (CMJ) jumps were measured before, every 30 minutes during, and 30 minutes after a three hour tennis match in 12 well trained players. Heart rate (HR), the effective playing time (EPT), rating of perceived exertion (RPE), and muscle soreness of knee extensors were also measured., Results: Decreases in MVC (-9%; p < 0.05) and leg stiffness (-9%; p = 0.17) were observed after the match and were significantly correlated (r = 0.66; p = 0.05). Peak power in SJ and CMJ tests was maintained during the match but was lower (p < 0.001) 30 minutes after. Average HR and EPT were 144 (8) beats/min and 21 (4)% respectively. A strong correlation was found between EPT and HR (r = 0.93; p < 0.05). RPE and muscle soreness increased linearly during the exercise and were significantly correlated (r = 0.99; p < 0.001)., Conclusions: Progressive reductions in maximal voluntary strength and leg stiffness highly correlated with increases in perceived exertion and muscle soreness were observed throughout a three hour tennis match, whereas explosive strength was maintained and decreased only after the match. These alterations may result in less efficient on-court movement and stroke production. They are, however, lower than those reported during continuous exercise of the same duration. The intermittent pattern of tennis and the numerous stretch-shortening cycle movements partly explain these results.

Purpose: This study aimed to evaluate the benefit of using low-intensity running or electromyostimulation (EMS) to hasten the recovery process from eccentric-contraction-induced injury., Methods: Before and 30 min, 24 h, 48 h, and 96 h after a one-legged downhill run, electrical stimulations were applied to the femoral nerve of healthy volunteers. Superimposed twitches were delivered during isometric maximal voluntary contraction (MVC) to determine the voluntary activation level (%VA). For 4 d after the exercise, each subject performed either (i) 30 min of running at 50% VO2max, (ii) 30 min of low-frequency EMS on the lower limb extensor muscles, or (iii) passive recovery., Results: Recovery time courses of the different variables did not differ significantly among the three experimental conditions. MVC decreased 30 min after the exercise and did not recover thereafter (P < 0.001). Percent VA was depressed after the exercise (P < 0.05) but did not contribute to MVC decrement thereafter. Mechanical responses to 80- and 20-Hz stimulation (P80 and P20, respectively) were significantly reduced over time (P < 0.01 and P < 0.001, respectively). Interestingly, MVC, P20, and P80 decrements were not statistically different (-9.6 +/- 14.5%, -13.2 +/- 14.2%, and -12.3 +/- 11.3%, respectively) at 48 h, and the P20.P80(-1) ratio showed complete recovery at this time., Conclusions: The different recovery modes had no significant effect on the recovery time course of contractile properties. The prolonged torque loss is mainly due to peripheral alterations. Our results suggest that an alteration of the excitation-contraction coupling might be involved during the first 2 d after the eccentric exercise. From 2 to 4 d, damage to force-generating structures could account for the remaining torque deficit.

The contribution of central and peripheral factors to muscle fatigue were quantified following a high-intensity uphill running exercise. Eight male volunteers performed an intermittent exercise at 120 % of maximal aerobic speed on a treadmill with an 18 % grade. Electrically evoked and voluntary contractions of the knee extensors and EMG of the two vastii were analyzed before and immediately after the high-intensity exercise. Isometric maximal voluntary contraction decreased slightly (-7+/-8 %; p < 0.05) after exercise but no changes were found in the level of maximal activation or in the torque produced by a 80 Hz maximal stimulation applied to the femoral nerve. Following exercise, the single twitch was characterized by lower peak torque, maximal rate of force development, and relaxation (-28+/-11%, -25+/-12%, -31+/-15% respectively, p < 0.001), and higher surface of the M-wave for both vastii. The ratio between the torques evoked by 20 Hz and 80 Hz stimulation declined significantly (-22+/-10%, p < 0.01) after exercise. These findings indicate that muscle fatigue after high-intensity running exercise is due to significant alteration in excitation-contraction coupling and that this type of exercise does not induce significant central fatigue or changes at the crossbridge level.