Modelling and biomechanical evaluation of sitting movements: implications for energy-efficient lower-limb prostheses and exoskeletons

Lower-limb joint negative mechanical powers during stand-to-sit movements were computed via inverse dynamics modelling to estimate the biomechanical energy available for regeneration with lower-limb prostheses and exoskeletons. Nine subjects performed 20 standing and sitting movements. Lower-limb kinematics and ground reaction forces were measured. System identification was used to determine optimal subject-specific body segment parameters. Joint moments and angular kinematics were computed via inverse dynamics and kinematics analyses. Joint mechanical powers were calculated from the joint moments and angular velocities. The maximum negative mechanical powers from the hip, knee, and ankle joints were 3.6 ± 0.9 W/kg, 1.6 ± 0.6 W/kg, and 0.4 ± 0.3 W/kg, respectively. Assuming conventional energy regeneration efficiencies (i.e., maximum 37%), knee joint prostheses and exoskeletons with adequate powertrains could theoretically regenerate 0.6 ± 0.2 W/kg of maximum electricity during stand-to-sit movements while providing system deceleration.