Fascicle stretch during partial muscle deactivation does not enhance the human tibialis anterior's subsequent neuromechanical output

Abstract

We were interested in whether stretching the muscle via elastic tissue recoil during partial muscle deactivation could trigger stretch-induced mechanisms that subsequently enhance the muscles steady-state neuromechanical output. Two torque-controlled experiments were conducted to test this aim. In Experiment 1, fifteen participants performed fixed-end dorsiflexion contractions to a moderate or high-then-moderate level with torque-drop rates of 0, 10, 20, 40, or 80% MVC{middle dot}s-1 from 60-40% MVC, while net ankle joint torque, tibialis anterior (TA) muscle activity level, and TA ultrasound images were recorded. The same measurements were performed in Experiment 2, which tested twelve different participants who performed fixed-end contractions to three reference levels or to a higher-then-lower level (with torque-drop amplitudes of 85-45, 85-30, and 85-15% MVC at 20% MVC{middle dot}s-1). Increased fascicle shortening amplitudes (Experiment 1: 1-2 mm, p[&le;].049; Experiment 2: 5-10 mm, p<.001) to initially higher joint torques and different fascicle stretch rates (0.6 to 4.3 mm{middle dot}s-1, p[&le;].041) and amplitudes (4-9 mm; p[&le;].001) did not significantly affect TAs subsequent muscle activity level relative to the reference contractions at similar joint torques (0-1% MVC, p[&ge;].659 and p[&ge;].626). However, the torque steadiness relative to the reference conditions was significantly reduced after the 85-15% MVC (p=.036) and 85-30% MVC (p=.036) torque drops. Consequently, the history of force production affected the control of muscle force more than TAs steady-state neuromuscular output. These findings indicate that assessing fascicle kinematics concurrently with motor unit behavior during fixed-end contractions with large torque drops might provide unique insights into the neuromechanical contributors to impaired torque control. NEW & NOTEWORTHYThe history of force production affected the subsequent control of dorsiflexion torque more than tibialis anteriors (TAs) neuromuscular output following larger amounts of fascicle stretch during fixed-end contractions. Larger fascicle stretches also reduced the median frequency of TAs EMG signal, which tentatively indicates that the voluntary control of muscle force was altered in the steady state. Together, these findings indicate that voluntary force control might be impaired by stretch-induced and/or activity-level-dependent mechanisms during fixed-end contractions.