Force per active area and muscle injury during electrically stimulated contractions.
Multiple mechanical factors have been implicated in the initiation of exercise-induced muscle injury. Although high absolute force levels are associated with greater injury, the importance of high force per active area independent of absolute force remains to be determined, especially in humans.


OBJECTIVE
This study sought to examine the role of specific force in muscle injury when peak eccentric force and total eccentric force were matched between two exercise bouts.


METHODS
Ten subjects (six men, four women) performed 80 electrically stimulated (EMS) eccentric contractions of the right and left quadriceps femoris (QF) through a 90 degree arc at approximately 45 degrees x s(-1). Specific force was manipulated by applying 25-Hz EMS to one thigh and 100-Hz EMS to the contralateral thigh, whereas force production was matched by lowering the EMS amplitude during the 100-Hz bout. T2 magnetic resonance images of the QF were collected before and 3 d after the eccentric exercise bouts. Injury was assessed via changes in isometric force and ratings of soreness in the QF over the course of 28 d after exercise and by determining changes in T2 relaxation time and muscle volume 3 d after exercise.


RESULTS
The 100-Hz EMS induced a greater force loss (P < 0. 05), soreness (P < 0.05), change in muscle volume (P = 0.03), and volume of muscle demonstrating an increase in T2 (P = 0.005) compared with 25-Hz EMS.


CONCLUSIONS
Our findings suggest that in humans, high specific force potentially plays an important role in the initiation of exercise-induced muscle injury.
Black
Christopher D
CD
Department of Kinesiology, The University of Georgia, Athens, GA 30302, USA.
McCully
Kevin K
KK
D000328:Adult; D004558:Electric Stimulation / Q000009:adverse effects*; D005260:Female; D006801:Humans; D007537:Isometric Contraction / Q000528:radiation effects*; D008279:Magnetic Resonance Imaging; D008297:Male; D018482:Muscle, Skeletal / Q000293:injuries* / Q000528:radiation effects; D011846:Radio Waves