Mediolateral and anteroposterior balance control during Sit-to-Stand in individuals with transfemoral amputation

Lower limb amputation alters postural control by eliminating anatomical structures involved in weight transfer and somatosensory feedback. This study investigated sit-to-stand (STS) biomechanics in transfemoral amputees (TFAs), focusing on directional and phase-specific balance-control alterations compared with able-bodied individuals. Eleven unilateral TFAs and six able-bodied controls were recruited. Data from seven TFAs and six controls were included in the primary fixed-foot balance analysis after exclusion of four TFA participants who exhibited repeated foot-relocation behaviours during STS. The STS task was segmented into initiation, rising, and stabilization phases. Centre of pressure (COP) displacement, velocity, and path length were quantified in the anteroposterior (AP) and mediolateral (ML) directions together with limb-specific ground reaction forces (GRFs). Compared with controls, TFAs exhibited significantly greater COP displacements and velocities (p < 0.05), particularly during the initiation and stabilization phases. AP COP velocity was significantly elevated during stabilization (p < 0.05), suggesting altered sagittal-plane balance-control behaviour while achieving an upright posture. ML COP measures were consistently larger across all phases that should be interpreted in conjunction with the observed asymmetrical limb-loading patterns. Limb-specific analyses revealed that the prosthetic limb demonstrated significantly reduced COP excursions during initiation (p < 0.05) and bore a lower proportion of body weight during the rising and stabilization phases (p < 0.05), indicating persistent asymmetries in limb-specific balance-control behaviour during STS. These findings provide preliminary insight into direction-, phase-, and limb-specific adaptations during STS that may help inform rehabilitation and prosthetic strategies targeting directional stability and bilateral limb engagement.