The contribution of lower-limb joint quasi-stiffness to theoretical leg stiffness during level, uphill and downhill running at different speeds.

Humans change joint quasi-stiffness ( k joint ) and leg stiffness (k leg ) when running at different speeds on level ground and during uphill and downhill running. These mechanical properties can inform device designs for running such as footwear, exoskeletons and prostheses. We measured kinetics and kinematics from 17 runners (10 M; 7 F) at three speeds on 0°, ±2°, ±4° and ±6° slopes. We calculated ankle and knee k joint , the quotient of change in joint moment and angular displacement, and theoretical leg stiffness (k legT ) based on the joint external moment arms and k joint . Runners increased k ankle at faster speeds ( p < 0.01). Runners increased and decreased the ankle and knee contributions to k legT , respectively, by 2.89% per 1° steeper uphill slope ( p < 0.01) during the first half of stance. Runners decreased and increased ankle and knee joint contributions to k legT , respectively, by 3.68% during the first half and 0.86% during the second half of stance per 1° steeper downhill slope ( p < 0.01). Thus, biomimetic devices require stiffer k ankle for faster speeds, and greater ankle contributions and greater knee contributions to k legT during the first half of stance for steeper uphill and downhill slopes, respectively.