Highlights
- A biomechanical model for calculating maximum legged running speed is formulated.
- In the model, air drag equilibrates with horizontal ground reaction force.
- Ground reaction force is a muscle's force transformed through leg gearing.
- Maximum legged running speeds across all sizes of animals on earth are predicted.
- The overall speed maximum is caused by leg muscle inertia rather than fatigue.
- The impact of body design parameters on running speed is demonstrated.
- Insight into the emergence of gaits in fast running is provided.