Fig. 9. The left panel schematically illustrates the hypothesized interplay between feed-forward muscle activation and intrinsic mechanical effects during running over a terrain drop (solid line) perturbation. Arrow color and direction conventions are the same as in Fig. 4. (A) Activation of muscles in anticipation of stance results in extension of the hip, ankle and TMP joints upon tissue paper (dotted line) contact. Depending on the balance among multiarticular muscles at the knee joint, the knee either flexes (A) or remains relatively extended (B). This alters limb posture and limb loading at ground contact. When the knee is flexed and _o close to vertical (A), limb loading is low, and the distal joints act as springs (purple and blue cumulative work curves for the ankle and TMP joints absorb with low net work output). When the knee is relatively extended and _o is lower (B), limb loading is greater, and the distal muscles undergo stretch, resulting in net energy absorption (blue arrows for distal muscles, and negative cumulative work for the ankle and TMP joints). In contrast the hip behaves uniformly, producing energy, as if the hip extensors are under feed-forward control and insensitive to perturbations. The knee does little net work under either condition. We hypothesize that variation in the breaking force of the tissue paper results in altered distal muscle contraction dynamics during the perturbation (on left, greater and lower distal muscle work production in A and B, respectively), leading to altered stance phase limb posture and dynamics