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Med Sci Sports Exerc. 2008 Sep;40(9):1605-15. doi: 10.1249/MSS.0b013e3181788dbe.

Muscle injury after repeated bouts of voluntary and electrically stimulated exercise.

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Department of Kinesiology, The University of Georgia, Athens, GA 30302, USA.


Repeated bouts of eccentric exercise reduce the amount of exercise-induced muscle injury.


This study sought to evaluate the importance of neural adaptations by comparing the repeated bout effect on muscle injury caused by voluntary and electrically stimulated eccentric exercise.


Sixteen subjects (nine men, seven women) were assigned into two groups; electrical stimulation (STIM) and voluntary (VOL). Each group performed 2 identical bouts of 80 eccentric contractions of the quadriceps femoris (QF) through a 90 degree arc at approximately 45 degrees x s(-1), separated by 7 wk. T2-weighted magnetic resonance images of the QF were obtained before and 3 d after each exercise bout. Injury was assessed by determining changes in T2 relaxation time and muscle volume 3 d after exercise, and changes in isometric force and ratings of soreness for 28 d after exercise.


The initial bout of exercise caused significant changes in T2 relaxation time, isometric force, and ratings of soreness in both STIM and VOL groups (P < 0.05). After the repeated bout, significantly smaller changes were noted in soreness ratings (P < 0.05), mean change in T2 (P<0.05), and percentage of the QF demonstrating an increase in T2 (P < 0.05) compared with the initial bout in both exercise groups.


A repeated-bout effect was observed after electrically stimulated exercise, and the magnitude of the effect was similar to that observed with voluntary exercise. This suggests that the primary mechanism for the reduction in muscle injury after repeated exercise bouts is not related to changes in muscle recruitment and is potentially related to structural changes within the muscles.

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