Fascicle lengthening during a large torque reduction subsequently decreases dorsiflexion torque steadiness

Here we assessed the effect of different rates and amplitudes of active torque reduction on the subsequent steady-state EMG amplitude, torque steadiness, and fascicle length during submaximal voluntary fixed-end (isometric) dorsiflexion contractions. Based on previous findings following active muscle-tendon unit lengthening, we expected a reduced steady-state EMG amplitude and torque steadiness relative to torque-matched reference contractions. In Experiment 1, thirteen participants performed reference contractions to 40% of maximum voluntary torque (MVT) and test contractions to 60 then 40% MVT over durations of 0.25, 0.5, 1 or 2 s. In Experiment 2, fourteen different participants performed reference contractions to 15, 30, and 45% MVT and test contractions to 85 then 15, 30, or 45% MVT at a fixed rate. Increasing the rate or amplitude of active torque reduction respectively increased tibialis anterior’s fascicle lengthening rate (1.6 to 6.9 mm·s − 1 , p ≤.041) or amplitude (3.6 to 7.5 mm; p <.001). However, fascicle lengthening did not subsequently change EMG amplitudes relative to the reference conditions (−1.5 to 2.7% MVC, p ≥.102) at similar steady-state torques. Fascicle lengthening amplitudes of ≥ 8 ± 3% (5.2 ± 1.7 mm) reduced torque steadiness (0.7 to 1.5%, p ≤.005) relative to reference conditions though, but the contributing mechanisms remain unclear and are worthy of further investigation.