The influence of mechanically and physiologically imposed stiff-knee gait patterns on the energy cost of walking

Objective: To investigate the relative roles of mechanically imposed and physiologically imposed stiff-knee gait (SKG) patterns on energy cost. Design: Repeated-measures, within-subjects design. Setting: Research laboratory. Participants: Individuals (N=20) without musculoskeletal, neuromuscular, or cardiorespiratory limitations. Interventions: Participants walked on an instrumented treadmill at their self-selected overground gait speed for 3 randomly ordered conditions: (1) control, (2) mechanically imposed stiff-knee gait (SKG-M) using a lockable knee brace, and (3) physiologically imposed stiff-knee gait (SKG-P) using electrical stimulation to the quadriceps. Each condition was performed with 0% and 20% body weight support. Indirect calorimetry determined net metabolic power, and motion capture measured lower extremity joint kinematics and kinetics. Main Outcome Measures: Net metabolic power, knee flexion angle, circumduction, hip hiking, and hip flexion and ankle plantarflexion moments. Results: Participants walked at 1.25±.09m/s. Net metabolic power was significantly increased by 17% in SKG-M and 37% in SKG-P compared with control (mean increase: .66±.60W/kg for SKG-M; 1.39±.79W/kg for SKG-P; both P<.001). Furthermore, SKG-P required greater net metabolic power than SKG-M (P<.001). Simulated SKG was associated with increased circumduction and hip hiking. Despite no change in ankle plantarflexion moments (P=.280), the hip flexion moment was increased during SKG-P (.43±.15Nm/kg·m) compared with control (.31±.08Nm/ kg·m; P<.001). Conclusions: The increase in energy cost associated with simulated SKG was due in part to abnormal mechanical compensations, and in part to an increase in quadriceps activity. Understanding the mechanisms underlying the increase in quadriceps activity will enable a reduction in the energy cost of walking with SKG.