People with a unilateral transtibial amputation (TTA) complete functional tasks asymmetrically, using compensatory strategies to accommodate for the lost ankle muscle function. These strategies may contribute the greater intact limb joint pain, low-back pain, and greater risk of falling commonly reported in this population. Prosthetists attempt to reduce asymmetries during the prosthetic alignment process. However, this process, which focuses on straight-line walking, may not capture the effect of prosthetic alignment on other functional tasks. The purpose of this dissertation was to determine how people with TTA maintain dynamic balance during turning and seat transfers and to quantify the effects of prosthetic alignment during seat transfers.

The first aim was to determine how balance regulation during turning is affected by the side the prosthesis is on and quantify how people with TTA maintain dynamic balance during a 90-degree turn. Participants with TTA had greater range of whole-body angular momentum when turning with the prosthesis on the inside compared to outside of the turn. There were altered head/trunk and legs interactions between turns and groups. The observed differences when turning with the prosthesis on the inside of a turn may suggest people with TTA have a greater risk of balance loss during turning.

The second aim was to quantify the effect of prosthetic alignment on dynamic balance during functional tasks. We compared the range and number of adjustments of whole-body angular momentum during walking, sit-to-stand, stand-to-sit, sit-to-walk, and walk-to-sit between different alignments. Sit-to-stand was the only task where alignment significantly affected angular momentum, although differences in magnitudes were small. Participants with TTA had less balance control compared to non-amputees, across alignments. These results suggest that acute changes in prosthetic alignment likely do not affect balance control during seat transfers.

The third aim was to determine the effects of anterior-posterior alignment shifts on movement strategies during sit-to-stand. We compared 3D ground reaction force impulses, sagittal-plane knee moments, anterior/posterior center of pressure position, and 3D trunk range of motion between alignments. The posterior alignment reduced braking impulse asymmetry and axial trunk range of motion compared to other alignments. These results suggest that prosthetic alignment may affect the movement strategies used during sit-to-stand which may have implications for asymmetric and altered movement patterns found in people with TTA.

The fourth aim was to determine the effect of prosthetic alignment on hip and low-back joint contact forces during sit-to-stand in people with a unilateral transtibial amputation. Using a musculoskeletal simulation framework, there were no differences in hip and L4-L5 joint contact forces between alignments. Participants with TTA had a greater peak hip joint contact force on the intact side hip compared to the amputated side across all alignments. This result may have important implications as greater cumulative intact hip loading throughout daily life may increase the risk of hip joint pain and degeneration in people with TTA.

Together, these studies support the idea that even highly functional individuals with a lower limb amputation have decreased balance control and altered joint loading across a range of functional tasks. Results from these studies also suggest that people with TTA develop compensatory strategies in response to acute changes in prosthetic alignment do not affect balance or joint loading during seat transfers. Future work should explore whether these findings extend to long-term changes in alignment or to lower functioning individuals.