Fig. 7. Mean joint work in stair gait across all subjects. Values are means ± s.d. Knee and ankle muscles were the primary energy dissipators and generators in stair descent and ascent. They performed 92% and 95% of the negative and net muscular work in descent, respectively and 85% and 87% of the positive and net muscular work in ascent, respectively. As in ramp walking, negative work in ramp ascent was relatively evenly distributed among the muscle groups, whereas positive work in descent was produced primarily (i.e. 73%) by the ankle muscles.