Slipping, sliding and stability: locomotor strategies for overcoming low-friction surfaces.

Legged terrestrial animals must avoid falling while negotiating unexpected perturbations inherent to their structurally complex environments. Among humans, fatal and nonfatal injuries frequently result from slip-induced falls precipitated by sudden unexpected encounters with low-friction surfaces. Although studies using walking human models have identified some causes of falls and mechanisms underlying slip prevention, it is unclear whether these apply to various locomotor speeds and other species. We used high-speed video and inverse dynamics to investigate the locomotor biomechanics of helmeted guinea fowl traversing slippery surfaces at variable running speeds (1.3-3.6 m s(-1)). Falls were circumvented when limb contact angles exceeded 70 deg, though lower angles were tolerated at faster running speeds (>3.0 m s(-1)). These prerequisites permitted a forward shift of the body's center of mass over the limb's base of support, which kept slip distances below 10 cm (the threshold distance for falls) and maximized the vertical ground reaction forces, thus facilitating limb retraction and the conclusion of the stance phase. These postural control strategies for slip avoidance parallel those in humans, demonstrating the applicability of these strategies across locomotor gaits and the potential for guinea fowl as an insightful model for invasive approaches to understanding limb neuromuscular control on slippery surfaces.