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Force Per Active Area and Muscle Injury during Electrically Stimulated Contractions


Medicine & Science in Sports & Exercise: September 2008 - Volume 40 - Issue 9 - p 1596-1604
doi: 10.1249/MSS.0b013e3181757182
BASIC SCIENCES: Original Investigations

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.

Purpose: 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° arc at ≈45°·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.

Department of Kinesiology, The University of Georgia, Athens, GA

Address for Correspondence: Kevin K. McCully, Ph.D., Department of Kinesiology, The University of Georgia, 330 River Rd, Athens, GA 30302; E-mail:

Submitted for publication December 2007.

Accepted for publication March 2008.

©2008The American College of Sports Medicine