Selles RW, Bussmann JB, Klip LM, Speet B, Van Soest AJ, Stam HJ. Adaptations to mass perturbations in transtibial amputees: kinetic or kinematic invariance? Arch Phys Med Rehabil 2004;85:2046-52. Objective To establish the adaptation strategy transtibial amputees use after mass perturbation of their prosthetic lower leg. Design We investigated whether the measured adaptations to mass perturbation of the lower leg in transtibial amputees can better be described as (1) a kinetic invariance strategy in which kinetics (joint torques) remain the same while kinematics (joint angles) change or (2) a kinematic invariance strategy in which kinematics remain the same while kinetics change. Setting A gait laboratory. Participants Ten transtibial amputees. Interventions Five different mass conditions. Main outcome measures Measured joint torques and angles during the swing phase in the different mass conditions. Results Mass perturbation induced more significant changes and larger effect sizes in joint torques than in joint angles. Conclusions Transtibial amputees adapt to mass perturbation primarily by maintaining the same kinematic pattern and adjusting their joint torques, that is, they use a kinematic invariance strategy. This implies that manipulating prosthetic inertial properties does not directly influence gait kinematics and that inertial properties should be evaluated in terms of the energetic cost of the swing phase. © 2004 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation.