The Magnitude of Muscle Strain Does Not Influence Serial Sarcomere Number Adaptations Following Eccentric Exercise

Abstract

Eccentric exercise, when performed by a muscle that is unaccustomed to that type of contraction, results in a delayed onset of muscle soreness (DOMS). A prolonged exposure to eccentric exercise leads to the disappearance of the signs and symptoms associated with DOMS, which has been referred to as the repeated bout effect (RBE). Although the mechanisms underlying the RBE remain unclear, several mechanisms have been proposed, including the idea of an increase in serial sarcomere number following exercise induced muscle damage. However, in the traditional DOMS and RBE protocols, muscle injury has been treated as a global parameter, with muscle force and strain assumed to be uniform throughout the muscle. PURPOSE: To assess the effects of muscle-tendon unit strain, fiber strain, joint torque and injury on serial sarcomere number adaptations in four regions of the tibialis anterior muscle (TA) following long-term eccentric exercise training. METHODS: Three groups of New Zealand White (NZW) rabbits were subjected to chronic repetitive eccentric exercise bouts (3x/week) of the ankle dorsiflexors for 6 weeks. These eccentric exercise protocols (P1, P2, P3) consisted of identical muscle tendon unit (MTU) strain, but muscle length and activation timing were systematically altered resulting in changes of joint torque and fiber strain. RESULTS: Following chronic eccentric exercise, serial sarcomere number adaptations in the central superficial region of the TA differed significantly between the three eccentric exercise protocols (+2.7±1.4%[P1],+7.6±0.9%[P2],+6.4±1.1%[P3], p<.05), even though MTU strains were not significantly different (+5.0±.0.2%[P1], +5.0±.0.5%[P2], +5.2±.0.7%[P3], p=.973). Peak torque (0.6±0.02Nm[P1], 0.7±0.03Nm[P2], 0.7±0.03Nm[P3]), and relaxation fiber strain (+2.5±0.2%[P1], +7.7±0.2%[P2], +7.8±0.2%[P3]), during eccentric exercise were the best predictors of serial sarcomere number (r2=.99 and r2=.94, respectively) across all three protocols. In addition, serial sarcomere number adaptations were not uniform across the four regions of the muscle following all three exercise protocols. CONCLUSION: MTU strain does not appear to be the primary cause for sarcomerogenesis (the RBE), as fiber strain and torque were the best predictors for cellular adaptations following chronic eccentric exercise. Ultimately, the differential adaptations within the muscle may be explained by the non-uniform architecture between regions of the muscle, resulting in differential local fiber strains, injury and adaptation. This work was supported by the CIHR Bone and Joint Training Program, the CIHR Research Chair Program, and NSERC of Canada