Dynamic balance is controlled by lower-limb muscles and more difficult to maintain during stair ascent compared to level walking. As a result, individuals with lower-limb amputations often have difficulty ascending stairs and are more susceptible to falls. The purpose of this study was to identify the biomechanical mechanisms used by individuals with and without amputation to maintain dynamic balance during stair ascent. Three-dimensional muscle-actuated forward dynamics simulations of amputee and non-amputee stair ascent were developed and contributions of individual muscles, the passive prosthesis and gravity to the time rate of change of angular momentum were determined. The prosthesis replicated the role of non-amputee plantarflexors in the sagittal plane by contributing to forward angular momentum. The prosthesis largely replicated the role of non-amputee plantarflexors in the transverse plane but resulted in a greater change of angular momentum. In the frontal plane, the prosthesis and non-amputee plantarflexors contributed oppositely during the first half of stance while during the second half of stance, the prosthesis contributed to a much smaller extent. This resulted in altered contributions from the intact leg plantarflexors, vastii and hamstrings and the intact and residual leg hip abductors. Therefore, prosthetic devices with altered contributions to frontal-plane angular momentum could improve dynamic balance control during amputee stair ascent and minimize necessary muscle compensations. In addition, targeted training could improve the force production magnitude and timing of muscles that regulate angular momentum to improve dynamic balance.