Ostrich Running Lab: Spring-Leg Biomechanics (2D)

This 2D companion is a real side-view biomechanics model of the mechanism that makes ostriches the fastest bipedal runners: below the walk-run transition speed the stance leg behaves as a rigid inverted-pendulum strut (compass-gait walking, with an energy-losing heel-strike collision), and above it the leg becomes a damped spring (a Spring-Loaded Inverted Pendulum, or SLIP, model) that stores and returns elastic energy through a simulated Achilles-tendon spring each stride. Leg stiffness, tendon energy-return efficiency, target speed and body mass are all live parameters that drive an actual numerically integrated ground-reaction-force curve, stride length and frequency, and the Froude-number gait switch — not a decorative animation layered on top of one.