The Role of Force in Skeletal Muscle Injury

Abstract

INTRODCUTION: Evidence from animal models suggests that multiple mechanical factors may play a causative role in skeletal muscle injury. While skeletal muscle injury has been widely studied in humans, little experimental data exist regarding the underlying cause of muscle injury. PURPOSE: The aim of the present study was to examine the role of muscle force production during eccentric exercise in skeletal muscle injury. METHODS: Eight male subjects performed 8 sets of 10 eccentric contractions evoked by surface neuromuscular electrical stimulation (SNMES) in both the right and left quadriceps femoris (QF) muscle group. Contractions were performed using an isokinetic dynamometer (Kin/Com) with contraction velocity and range-of-motion held constant at 120°/s and 80° respectively. Muscle force production was varied by randomly assigning each thigh one of two frequencies (100 Hz or 25 Hz) of SNMES. Maximal voluntary isometric contraction (MVIC), SNMES evoked isometric torque, and perceived soreness were assessed prior to the bout of eccentric exercise and for up to 14 days post exercise. T2 magnetic resonance imaging (MRI) of the QF was collected prior to and 3 days post exercise. RESULTS: Eccentric torque during SNMES induced eccentric contractions was greater for 100 vs. 25 Hz (237 ± 81 Nmvs. 135 ± 24 Nm, p = 0.02, mean ± SD). The reduction in MVIC and SNMES evoked isometric torque was greater in the 100 vs. 25 Hz (p <0.007). Additionally, subjects reported greater soreness in the 100 Hz thigh (p = 0.001). The percentage of the QF considered injured, assessed via MRI, was greater for the 100 vs. 25 Hz thigh (48 ± 23% vs. 17 ± 10%, p = 0.007). The 100 Hz thigh exhibited a greater change in the mean T2 signal intensity of the QF (5.6 ± 7.3 ms vs. 0.0 ± 1.3 ms, p <0.05) and greater increase in muscle volume (9% ± 10 vs. 1% ± 1, p <0.05). The percentage of the QF considered injured was found to differ when the QF was divided into four equal quadrants along the length of the femur (p <0.01). CONCLUSION: The decline in voluntary and SNMES torque production as well as an increase in soreness, swelling, and elevations in T2 MRI indicate that our exercise protocol did induce skeletal muscle injury. The fact that the thigh receiving 100 Hz SNMES exhibited larger reductions in torque, greater soreness and greater portion of the QF with an elevated T2 suggest that muscle force production during eccentric contractions play a significant role in contributing to the magnitude of muscle injury.