Optimization of foot placement for individuals with total knee replacements during sit-to-stand transfers

Sit-to-stand transfers are a fundamental activity of daily living, but it remains unknown how to systematically recommend a foot placement that matches an individual's physical capabilities. The first goal of this study was to measure biomechanical changes in sit-to-stand movements as a function of foot placement. The second goal was to develop a model that would optimize foot placement for ease of sit-to-stand and would determine challenging foot placements for strengthening. A female older adult who had undergone bilateral total knee replacement participated in the study. Video and force platform data were collected for sit-to-stand movements from foot-back, foot-neutral, right-staggered, and left-staggered placements. The optimal foot placement was determined by minimizing the highest strength percentage for any single joint torque. Strength-challenging foot placements were found by maximizing the highest strength percentages for targeted joint torques. The right-staggered placement was optimal and had a limiting factor of left hip extension torque (strength percentage 74.7%). Maximum left hip extension torque (76.7 Nm) was similar to the maximum right hip extension torque (79.5 Nm) for this foot placement. The strength-challenging foot placement with the highest strength percentage was for left ankle plantar flexion torque (89.4%) and occurred while using the left-staggered placement. The optimal right-staggered foot placement for this older adult balanced the hip extension torques between the stronger, painful right side and the weaker, less painful left side. This model-based technique has the potential to allow clinicians to select foot placements that may enhance the functional independence of older adults with osteoarthritis.