Phase Relationship between Spinal Motion and Limb Support Determines High-speed Running Performance in a Cheetah Model with Asymmetric Spinal Stiffness

TL;DR

This study uses a simple cheetah model to show how the phase relationship between spinal motion and limb support, influenced by asymmetric spinal stiffness, affects high-speed running performance and stability.

Contribution

It demonstrates that the phase relationship between spinal motion and limb support, modulated by asymmetric spinal stiffness, is crucial for optimizing running performance.

Findings

Cheetah-like solutions reduce ground reaction forces while maintaining velocity.

Asymmetric spinal stiffness influences the phase relationship between spinal motion and limb support.

The phase relationship is a key factor in high-speed running performance.

Abstract

Cheetahs are characterized by large spinal flexion and extension during high-speed running, yet the dynamical role of the phase relationship between spinal motion and limb support remains unclear. We aimed to clarify how this phase relationship affects running performance, focusing on the effect of asymmetric spinal stiffness. Using a simple planar cheetah model with asymmetric torsional spinal stiffness, we numerically searched for periodic bounding solutions over a range of stiffness parameters and compared their ground reaction forces, horizontal velocities, and stability. We obtained both cheetah-like solutions, in which the spine extends after hindlimb liftoff and flexes after forelimb liftoff, and non-cheetah-like solutions, in which the spine flexes after hindlimb liftoff and extends after forelimb liftoff. Under asymmetric spinal stiffness, cheetah-like solutions reduced ground reaction forces while maintaining horizontal velocity more effectively than non-cheetah-like solutions. The phase relationship between spinal motion and stance timing is a key determinant of high-speed running performance. These findings provide a dynamical understanding of cheetah locomotion and suggest design principles for spined legged robots.