Individual and summed joint torque (A) and power (B) curves during the stance phases of stair descent (broken lines) and ascent (solid lines) walking averaged across all subjects. Individual and summed joint torque (A) and power (B) curves during the stance phases of stair descent (broken lines) and ascent (solid lines) walking averaged across all subjects. Positive torques are extensors and positive powers are energy generation. Summed torques were similar in shape and showed that both gaits were produced by net extensor torques across all joints. Hip and ankle torques were similar in the stair gaits, whereas knee torque had one larger extensor phase in ascent and two smaller extensor phases in descent. As in ramp gait, summed powers in stair descent and ascent were nearly entirely negative and positive, respectively. In contrast to ramp gait, there was minimal power and work at the hip on the stairs. Energy dissipation in stair descent was done at the ankle joint in early stance and at the knee joint in later stance. Stair ascent was produced by positive power and work at the knee joint in early stance and at the ankle joint in later stance. Stair descent had an 8% shorter stance phase, partially leading to reduced area under the joint power curves and reduced muscle work compared to stair ascent. Paul DeVita et al. J Exp Biol 2007;210:3361-3373 © The Company of Biologists Limited 2007