Eccentric exercise ≠ eccentric contraction

Abstract

Whether eccentric exercise involves active fascicle stretch is unclear due to muscle-tendon unit (MTU) series elasticity. Therefore, this study investigated the impact of changing the activation timing and level (i.e., pre-activation) of the contraction on muscle fascicle kinematics and kinetics of the human tibialis anterior during dynamometer-controlled maximal voluntary MTU-stretch-hold contractions. B-mode ultrasound and surface electromyography were employed to assess muscle fascicle kinematics and muscle activity levels, respectively. While joint kinematics were similar among MTU-stretch-hold contractions (~40° rotation amplitude), increasing pre-activation increased fascicle shortening and stretch amplitudes (9.9-23.2 mm, p ≤ 0.015). This led to increasing positive and negative fascicle work with increasing pre-activation. Despite significantly different fascicle kinematics, similar peak fascicle forces during stretch occurred at similar fascicle lengths and joint angles regardless of pre-activation. Similarly, residual force enhancement (rFE) following MTU stretch was not significantly affected (6.5-7.6%, p = 0.559) by pre-activation, but rFE was strongly correlated with peak fascicle force during stretch ( rrm = 0.62, p = 0.003). These findings highlight that apparent eccentric exercise causes shortening-stretch contractions at the fascicle level rather than isolated eccentric contractions. The constant rFE despite different fascicle kinematics and kinetics suggests that a passive element was engaged at a common muscle length among conditions (e.g., optimal fascicle length). Although it remains unclear whether different fascicle mechanics trigger different adaptations to eccentric exercise, this study emphasizes the need to consider MTU series elasticity to better understand the mechanical drivers of adaptation to exercise.

Residual force enhancement decreases when scaling from the single muscle fibre to joint level in humans

“…These lower fixed-end torque values likely occurred because, in the fascicle-matching method, some of the fixed-end contractions used to calculate rFE were at knee angles of 72.5° or 75°, which are closer to the optimal angle (∼100°) than 70°, the only knee angle used in the joint-matching method. During eccentric contractions of the vastus lateralis, much of the stretch is taken up by the tendon, limiting the amount of stretch that the contractile tissue (i.e., the fascicles) can undergo 21,46 . Thus, it is understandable that vastus lateralis fascicle length in the isometric steady state following active lengthening would be shorter than during a fixed-end isometric contraction at the same joint angle, like in the present study.…”

“…Although rFE is characterized by enhanced force in the isometric steady-state following a stretch-hold contraction compared to a fixed-end isometric contraction at the 'same muscle length' 2 , current in-vivo assessments of rFE are limited in that only joint angle is matched in the isometric steady state, not muscle fascicle length 10,15,16,20 . Assessing isometric force between a stretch-hold and fixed-end contraction at the same joint angle may not necessarily equate to assessing the same muscle fascicle length due to inter-individual variability in the in-series compliance of the muscle-tendon-unit 10,21 . Therefore, to translate in-vivo joint-level rFE Residual Force Enhancement in vivo and in vitro 3 experiments to the single fibre level, one could match fascicle length, rather than joint angle, between the isometric steady-states of the stretch-hold and fixed-end contractions.…”