Walk, trot, gallop

A horse crossing a field does not move its legs the same way at all speeds. It switches between distinct patterns called gaits. Each gait is a repeating order in which the feet touch the ground. You can learn to read them from a footfall diagram, a chart with one row per foot and a bar for every moment that foot is on the ground.

Stride and duty factor. One stride is one full cycle of a limb, from a foot leaving the ground, through the swing, to the same foot leaving the ground again. The duty factor (β) of a foot is the fraction of the stride it spends on the ground:

β = time on the ground / stride time

• Walking: β is greater than 0.5. Each foot is down for more than half the stride, so at least one foot is always on the ground.
• Running: β is less than 0.5. Each foot is down for less than half the stride, so there are moments with no feet on the ground. These are flight phases.

Example (assumed numbers). A person walks with a stride time of 1.0 s and each foot on the ground for 0.6 s: β = 0.6, a walk. Running with a stride time of 0.7 s and 0.25 s of contact gives β ≈ 0.36, a run.

The gaits of a horse. Each gait is named by how many separate 'beats' (foot landings) you hear in one stride.

• Walk (four-beat): left hind, left fore, right hind, right fore, evenly spaced. There is no flight phase.
• Trot (two-beat): diagonal pairs move together, such as left fore with right hind. There is a brief moment with all feet off the ground between the beats.
• Canter (three-beat): one diagonal pair lands together while the other two feet land separately, then comes a moment of flight.
• Gallop (four-beat): the fastest. The diagonal pair of the canter splits into two separate landings, and there is a clear flight phase.

Gaits differ among animals (camels, for example, pace by moving both legs on one side together), but the rule of β < 0.5 for running stays the same.

Why change gait at all? Horses were studied on a treadmill, with their oxygen use measured at different speeds (Hoyt and Taylor, 1981). At each speed the horse chose the gait that used the least energy per metre travelled. Walking is cheapest at low speed, trotting in the middle and galloping at high speed. When the horse was forced into a different gait, its energy cost rose. So the switch is not a random whim: the body picks the pattern that is most economical for the job.