Fig. 1. Model structure: (A) large horse, (B) small horse, (C) goat, (D) large dog, (E) small dog and (F) chipmunk. Joint locations, segment dimensions and mass distributions are from photographic, video and anatomical data (Muybridge, 1957; Taylor et al., 1974; Fedak et al., 1982; Alexander, 1985; Farley et al., 1993). All segments are represented as rigid bodies. Pin (rotary) joints are included on the back and neck. Each leg rotates about a pin joint at the shoulder or hip and changes length through a prismatic (telescoping) joint at the elbow or knee. Active hip and shoulder torques control the forward motion from stride to stride. Motions are restricted to the sagittal plane.