Whether we totter along on two legs, or sail about on four, for all animals there comes a point when walking simply won't stay the pace at a realistic metabolic price. We must upgrade to a more economical gait. The factors that trigger a gait shift have intrigued biomechanists for decades. Tim Griffin explains that although the trot to gallop gait change in quadrupedal animals has been analysed extensively, much less was known about the transition from a walk to a trot. Griffin explains that this transition is particularly interesting, because the shift requires a switch from inverted pendular biomechanics when walking, to elastic energy storage in tendons when animals break into a trot. But what factors trigger an animal to change from a walking to a trotting gait? Would animals shift-up when they reach some mechanical trigger point, or would they reach some metabolic threshold where it's simply more efficient to upgrade to a bouncier gait? Griffin and Rodger Kram decided to analyse systematically gait transitions on animals spanning a wide range of sizes over a consistent body shape, hoping to find what triggers the walk-to-trot gait change (p. 4215).

And few species boast the remarkable size range, yet retain the same body shape, spanned by horses. Teaming up with Steven Wickler and Donald Hoyt at California State Polytechnic University's Equine Research Centre, Griffin and an army of helpers began putting a range of horses, from miniatures through to statuesque Clydesdales, through their paces to find the factors that trigger a gait change.

First Griffin measured each horse's performance on a treadmill as they stepped up from a walk to a trot. Only when they trotted comfortably for a minute did Griffin decide that the animals had reached their transition speed. And when he plotted the transition speeds against the horses' leg lengths,`they lined up great' says Griffin; the horses shifted up at speeds proportional to the square root of their leg lengths, suggesting a biomechanical transition trigger. Griffin also analysed each horse's performance in terms of the animal's Froude number, a dimensionless value representing the animals biomechanics, and found that all of the animals switched to a trot at a similar value: around 0.35. Some critical factor in the animal's mechanics was reached as it increased walking speed, triggering the trotting switch.

But would metabolic factors also play a role in the animal's decision to switch gait? Fortunately, the horses were happy wearing respirometry masks as Griffin took them up through their transition speed while he measured their metabolic rates. Analysing each animals' cost of transport relative to their speed as they switched from walking to trotting, Griffin realised that all of the animals shifted up `at the metabolically optimal transition speed' he says.

So the walk-to-trot transition state is triggered both by mechanical and by metabolic factors, and Griffin admits that he is surprised by how tightly both are linked. None of which seems to perturb the horses, which happily break into a trot once the pace is right.

Griffin, T. M., Kram, R., Wickler, S. J. and Hoyt, D. F.(
). Biomechanical and energetic determinants of the walk–trot transition in horses.
J. Exp. Biol.