Fig. 1. Horizontal plane GRFs in upright- and sprawled-posture trotters during a step when running at a constant average speed. (A) Upright-posture quadruped running with similar leg function. Fore- and hindlegs first (t₁) both generate decelerating fore–aft forces (arrows) followed by an accelerating force later in the step (t₃). No lateral GRFs are present. (B) Sprawled-posture hexapod running with differential leg function. Fore- and middle legs first (t₁) both generate decelerating forces, while the hindleg generates an accelerating force (Full et al., 1991). All legs develop large lateral forces directed toward the midline. At midstep (t₂) forelegs continue to generate decelerating forces and the hindleg an accelerating force. The middle leg only develops a lateral force. At the end of the step (t₃), the foreleg generates a decelerating force. Hind- and middle legs both generate accelerating forces. (C) Hypothetical sprawled-posture quadruped running with similar leg function resulting from adding opposing lateral forces to the upright posture pattern in A. Fore- and hindlegs first (t₁) both generate decelerating forces followed by an accelerating force later in the step (t₃). Lateral GRFs were added assuming sprawled-posture animals tend to produce them. Horizontal forces sum to produce a clockwise yaw throughout the step. (D) Sprawled-posture quadruped running with differential leg function. GRFs approximate those measured in the present study on geckos. The foreleg first (t₁) generates the majority of fore–aft decelerating force. At midstep (t₂), fore- and hindlegs only generate lateral forces directed toward the midline. Later in the step (t₃) hindlegs generate all of the fore–aft accelerating force. The major decelerating force by the foreleg (t₁) and accelerating force by the hindleg (t₃) are directed to the animal's COM, reducing yaw, and are aligned axially along the leg, reducing joint moments.