Why humans switch from walking to running at a predictable speed

Settled in 1984 by R. McNeill Alexander, with experimental confirmation by D. Kram and collaborators.

It has long been obvious that everyone breaks into a run at roughly the same relative speed, but explaining why required treating legs as physical pendulums rather than just muscles. R. McNeill Alexander proposed that gait transitions are governed by dynamic similarity: animals of different leg length switch gaits at the same dimensionless Froude number (v^2 / gL), around Fr ~ 0.5 for the human walk-run transition. This tied the transition to the point where, in an inverted-pendulum walk, required centripetal force would exceed what gravity can supply to keep a foot on the ground, forcing a switch to a bouncing running gait. The Froude-number criterion was subsequently tested and largely confirmed across species and even under simulated reduced gravity, cementing it as the standard explanation.

The same simple dimensionless number that governs when a ship starts to make waves also tells a mouse, a human, and a rhinoceros exactly when to stop walking and start running.

Explanations that circulated

• People switch to running purely to go faster, with no deeper mechanical reason for the specific transition speed
• The transition speed is set mainly by muscle fatigue rather than by leg-length-scaled dynamics

References

• A dynamic similarity hypothesis for the gaits of quadrupedal mammals, R. McN. Alexander, A. S. Jayes, Journal of Zoology, 1983