Fascicle Lengthening During Eccentric Exercise Determines The Magnitude Of Muscle Damage

Abstract

The characteristics of the muscle-tendon interaction during eccentric contractions suggests that tendon properties and the magnitude of fascicle lengthening contribute to the extent and the variability of exercise-induced muscle damage (EIMD). The mechanical processes which predispose the severity of EIMD remains unclear. PURPOSE: To determine whether; 1) patella tendon stiffness, at rest and during eccentric contractions, 2) the magnitude of vastus lateralis (VL) fascicle lengthening during eccentric contractions, and/or 3) eccentric torque, are determinants of EIMD. METHODS: Combining dynamometry, electromyography and ultrasonography, patella tendon properties and VL architectural properties were measured pre and during the first of 6 sets of 12 maximal voluntary eccentric knee extensions (range of motion 20 - 90°, 0° = full extension). The eccentric phase of the contraction was performed at an isokinetic angular velocity of 30°s-1 and the concentric phase was performed passively at an angular velocity of 60°s-1. Maximal isometric torque loss and creatine kinase (CK) activity were measured pre-damage (-48 h), 48, 96 and 168 h post-damage as markers of EIMD. RESULTS: Compared to pre-damage, a significant reduction in maximal isometric torque (264 ± 35 Nm, 221.0 ± 48.4 Nm, p = 0.004) and a significant increase in CK (136 ± 114, 796 ± 723 UL, p = 0.014) was reported 48 and 96 hrs post EIMD respectively. Relative changes in CK correlated with the relative change in fascicle length during eccentric contractions (r = 0.53, p = 0.02) and eccentric torque (r = 0.50, p = 0.02). Additionally, the relative change in CK tended to correlate with estimated patella tendon lengthening during eccentric contractions (r = -0.41, p < 0.10). However, relative change in CK did not correlate with resting measures of patella tendon properties or VL properties. Similarly, torque loss did not correlate with any patella tendon or VL properties at rest or during eccentric contractions. CONCLUSION: The extent of fascicle lengthening during eccentric contractions is related to the magnitude of the CK response. Furthermore, at rest, patella tendon properties are not determinants of indices of EIMD. During eccentric contractions of the VL, the patella tendon might play a modulatory role during exercise, to the susceptibility of EIMD.