Although residual force enhancement (RFE), i.e. enhanced force after active muscle stretch, is shown to be present in voluntarily activated human muscles, its relevance for everyday human movement is still elusive. Natural human motion is mainly composed of voluntarily submaximally activated muscle contractions driving coordinated multi-joint movements. Up to now there has been no study that directly investigated the presence of RFE following stretch when performing a submaximal multi-joint movement. For this purpose, n=13 subjects performed feedback controlled bilateral leg extensions at the level of 30% maximum voluntary activation in a motor-driven leg press dynamometer. Isometric-eccentric-isometric and purely isometric contractions were arranged in a randomized experimental protocol. Kinematics, forces and muscular activity were measured using optical motion tracking, 3d force plates and EMG of 9 lower extremity muscles. ANOVA identified significant RFE of external reaction force, and knee extension and plantar flexion torque (calculated by inverse dynamics). Enhanced force and torque ranged between 3% and 22% and was present for up to 22s post-stretch. In spite of motor redundancy for solving a given task, no differences between contraction conditions were observed for any of the analyzed muscles, except for tibialis anterior. On the basis of our results, RFE is present in everyday alike human movement and might be an evolutionary optimization mechanism to enhance muscular performance at a given amount of energetic effort.
Keywords: EMG; Energetic effort; Feedback control; Inverse dynamics; Motor redundancy; Multi-joint; Muscle contraction; Submaximal muscle action.
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