Low back pain is a debilitating and costly condition that affects the majority of people in their lifetime. Some populations, like people with a transtibial amputation, have a higher risk for developing low back pain, which may be explained by altered biomechanical loading. People with a transtibial amputation have altered movement strategies during walking that lead to changes in low back loading, but little is known about their low back biomechanics during the sit-to-stand motion. A musculoskeletal model with detail of the lumbar spine was developed and validated during trunk range of motion trials. This model was used to analyze eight people with a transtibial amputation and eight people without an amputation. People with an amputation had greater trunk angles compared to people without an amputation, which was associated with greater low back compressive loads. Identifying this movement strategy has potential to guide interventions aimed at reducing low back pain.