Residual Limb Revision Surgery Alters Velocity-Curvature Coupling during Stepping and Turning of a Transfemoral Amputee

Two-Thirds Power Law is a frequently observed relationship in human movement, relating velocity and curvature of movement trajectory. These movements span handwriting, larvae crawling, and human-robot interaction. Despite vast acceptance as a common principle of biology, it is unknown if the power law applies to interaction between amputees and prostheses, and if interventions to augment the physical connection between amputees and prostheses influence this speed-curvature coupling during demanding forms of human locomotion. The purpose of this study was to determine if individuals with transfemoral amputation exhibit a biologically-appropriate power law relationship during non-steady-state locomotion, and if a surgical intervention to reduce residual limb soft tissue would influence the observed coupling. We hypothesized that a power regression would well characterize amputee locomotion, and that limb revision surgery would result in a non-linear power coupling close to one-third and overall increased speed (i.e., higher linear coupling) in each non-steady-state movement. The subject performed repeated trials of left and right 90° turns during walking, as well as Foursquare Step Test (FSST), while whole-body kinematics were captured. After fitting center-of-mass velocity and curvature to the power law, the power coupling in FSST was similar to the Two-Thirds Power Law, while turning was not. Finally, the intervention was shown to increase linear coupling suggesting an overall improvement in movement tempo characterized by modest changes in velocity, enabling tasks to be achieved more quickly.