Determinants of exercise induced skeletal muscle injury following eccentric exercise
Black, Christopher Durham
MetadataShow full item record
High specific force during eccentric contractions is thought to play a role in muscle injury in animals, but its role in humans is unclear. Additionally, it is unclear whether a single bout of electrically stimulated (EMS) contractions limits injury in subsequent bouts. The purposes of this study were to determine the role specific force, independent of external force production, plays in muscle injury and if a repeated-bout effect occurs with EMS. Sixteen subjects were divided into 2 groups: electrical stimulation (EMS) and voluntary (VOL). Each group performed 2 bouts of 80 eccentric contractions of the quadriceps femoris (QF) through a 90º arc at §45º/s. In the initial bout, specific force was altered in the EMS group by applying 25 Hz stimulation to one thigh and 100 Hz stimulation to the other. Force production was matched by lowering the stimulation amplitude in the 100 Hz thigh. A second bout of exercise was performed 7 weeks after the first, however only the 100 Hz thigh was used in the EMS group. Muscle injury was assessed by changes in isometric force, ratings of soreness, and T2 MRI. 100 Hz EMS induced greater force loss (P < 0. 05), soreness (P < 0.05), change in muscle volume (P = 0.03), and the amount of muscle demonstrating an increase in T2 (P = 0.005) compared to 25 Hz EMS. Following the repeated-bout, significantly smaller changes were noted in soreness ratings (32% and 47%; P < 0.05), mean change in T2 (51% and 75%; P < 0.05) and percentage of the QF demonstrating an increase in T2 (32% and 62%; P < 0.05) compared to the initial bout in the EMS and VOL groups, respectively. Our findings suggest that in humans, high specific force, rather than external force, plays an important role in the initiation of exercise induced muscle injury. Additionally, our finding that injury was reduced following a second bout of EMS evoked eccentric exercise (although not to the same extent with voluntary exercise) indicated a repeated-bout effect did occur. This finding suggests that neural adaptations were not exclusively responsible for the reduction in injury.