The main scope of this study is to analyse muscle-driven forward dynamics simulation of stair locomotion to understand the functional differences of individual muscles during the movement. A static optimization was employed to minimize a performance criterion based on the muscle energy consumption to resolve muscle redundancy during forward dynamics simulation. The proposed method was employed to simulate a musculoskeletal system with ten degrees of freedom in the sagittal plane and containing 18 Hill-type musculotendon actuators per leg. Simulation results illustrated that simulated joint kinematics closely tracked experimental quantities with root-mean-squared errors less than 1°. In addition, estimated muscle activations have a good agreement with the salient features of the electromyographic recordings of the major muscles of the lower extremity. Distribution of mechanical power for individual muscles was estimated to elucidate the muscle's contribution to body support and forward progression during stair locomotion. The accuracy and relatively high computational performance of the proposed method make it suitable to generate subject-specific simulations of various activities for individuals with movement disorders in clinical studies.
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