Fig. 4. (A) Schematic representation of regional patterns of joint work over the course of stance summarizing patterns observed for muscle groups of various animals during running. Curved arrows indicate the rotational motion of the distal segment of each joint (clockwise versus counterclockwise). Arrow color shows whether net energy is produced (red) or absorbed (blue) or zero (gray) at that joint during stance. These joint patterns are shown in relation to muscle work patterns in B and C. Muscle path arrows indicate hypothesized contraction of muscle groups, undergoing net shortening and positive work (red), lengthening and negative work (blue), or no net length change (gray). In this example, the hip extends, doing positive work mainly during the second half of stance (C); the knee flexes, doing negative work (energy absorption) mainly during the first half of stance (B); the ankle initially flexes during the first half of stance (B) and then extends during the second half of stance (C), doing net positive work; and the TMP (tarsometatarsal–phalangeal) joint dorsiflexes doing negative work throughout stance, though shown in B only (overall limb work is zero, characteristic of steady level locomotion assuming no work is done by other pelvic and trunk muscles). In comparison, the hamstrings (biceps) shorten while active, performing positive work (second half of stance, C); the quadriceps (vastus) undergo net lengthening while active doing negative work (first half of stance, B); the triceps surae [gastrocnemius (soleus, not shown) and plantaris] contract isometrically doing zero net work throughout stance, whereas the digital flexors are stretched and absorb energy initially (B) and then remain isometric during the second half of stance (not shown). As a result of their biarticular organization, the gastrocnemius, plantaris and digital flexor act as force links and, although they do no significant net work as a group, act to transfer energy from the hip and knee joints (via the hamstrings and/or quadriceps) to the ankle joint. This pattern is observed during steady level locomotion as well as during jumping in several species. See text for additional details.