The Cost of Assistance: High Prosthetic Knee Torque Reduces Biological Joint Work but Induces Symmetry Trade-Offs in Knee Power and Center-of-Mass Velocity During Stair Ascent.

Stair ascent is a biomechanically demanding activity for individuals with transfemoral amputation, often requiring significant overreliance on the residual hip and sound biological joints to compensate for the lack of positive power in conventional passive prostheses. Although powered prostheses can generate positive power imitating biological legs, the optimal torque levels required to balance biological joint effort reduction with interlimb symmetry remain unknown. This study evaluated the effects of systematically increasing prosthetic knee torque on the biomechanics of stair ascent in eight individuals with transfemoral amputation. Using a powered knee-ankle prosthesis, participants performed stair ascent at six body-mass-normalized torque levels ranging from 0.2 to 1.2 Nm/kg. The results demonstrated that increasing assistive torque linearly reduced the positive mechanical work required from biological joints, effectively offloading the biological lower limbs. However, this improvement introduced a fundamental trade-off. As prosthetic knee torque approached biological levels, torque symmetry improved, but peak knee power symmetry exhibited a nonlinear relationship, reaching an optimum at low-to-medium torque levels (0.6-0.8 Nm/kg) before deteriorating at higher torques. Meanwhile, vertical center-of-mass velocity symmetry decreased linearly across the torque spectrum. These findings indicate that maximizing prosthesis assistance to minimize biological joint effort may negatively impact other critical aspects of gait. This research provides a data-driven foundation for optimizing active prosthesis control, suggesting that a balanced approach to assistance is necessary to enhance functional mobility for individuals with transfemoral amputations.