Mechanics of motion in nature
Many animals change the rhythm of their footfalls and the pattern of their steps depending on their speed. Horses are a good example of this because they move in distinct gaits. Each gait has a specific range of speed for which it is the most efficient pattern of motion.
A horse’s slowest gait is the walk. As with most four-legged mammals, each footfall happens separately, creating a four-beat rhythm. Depending on the breed and purpose of the horse, the gaits following the walk vary; however, the three most common are the trot/jog (trot for English and jog for Western), canter/lope, and gallop.
When a horse trots, its legs move in pairs. It is most common for the horse to pair its legs diagonally, meaning that the front left and back right move at the same time, as do the front right and back left. The diagonal motion is most efficient for covering long distances at low speed. However, the pattern of footfalls often creates a bouncing motion that may or may not be exaggerated, depending on the horse. In Western riding, riders stay seated on their horses’ backs during the jog (i.e. trot). To avoid the bouncing motion, which can take a toll on the rider, Western horses are kept collected during the jog, meaning that they take shorter strides and move at a slower pace.
English riders can choose to sit the trot, but usually post instead. Posting means that the rider repeatedly stands up slightly in their stirrups while the horse trots. Although the weight felt by the horse from the rider does not change, the rider’s motion affects how the horse moves. A short post causes the horse to have more collected strides, decreasing its speed. A taller post (i.e. standing up more fully in the saddle) causes the horse to lengthen its strides and move faster.
Some horses naturally pair their lateral legs together, meaning that the front and back leg on the same side move at the same time. This gait, called the pace, has less vertical motion and is faster than the traditional trot, but also requires more energy.
When a horse canters, it has a three-beat gait, meaning that its hooves pound out a regular rhythm separated into three beats. This happens because two of the legs are paired in diagonal, like in the trot, while the other two legs move on separate beats. This creates a rocking motion that is easier for riders to sit.
The canter is much more easily sustained over time than its sister gait, the gallop.
The gallop is basically a faster version of the canter, but with four beats instead of three because the paired legs no longer land at the same time. Every gait above the walk has a suspension phase in which all four hooves are off the ground and the horse is moving forward through the air, with the gallop having the longest-lasting phase. Horses perform the transverse gallop, in which the suspension phase begins after the two front hooves have struck the ground and ends when the back two hooves land. An indicator of the horse’s capacity for speed is demonstrated by its stride angle, which is the angle its lateral front and back legs make at the most extended point in its stride. Secretariat, a record-setting racehorse with one of the largest known stride angles, had an angle of 110 degrees.
In comparison, cheetahs and greyhounds demonstrate the rotary gallop, which is the fastest and most tiring of all gaits. Unlike the transverse gallop, the rotary gallop has two suspension phases. Animals performing this gallop are fully in the air when all four feet are gathered together, after both front legs have struck the ground, and again when the stride is fully extended after both hind legs have struck the ground. At full extension, the cheetah’s stride can measure in at upwards of 120 degrees.
Just for those who are curious, one analysis of Usain Bolt’s stride measured him at an angle of 116 degrees.