As cursorial (adapted for running) locomotion goes, bottom of the food chain is really the lizards. They’re ahead of the snake, they’ve got limbs and even elbows to lift themselves off the ground, further increasing efficiency. However their shoulder design is a little primitive and they resort to side-to-side (medio-lateral) wiggling to move forward, wasting a lot of energy in the process.
At the other end of the scale is the cheetah. The wiggle has gone and the spine dorsi-flexes (up-down) like a caterpillar. This increases stride length and hence speed by so much that the cheetah could do six mph even if it didn’t use its legs (Hildebrand, 1959). The cheetah is a sprinter, fast over short distances.
The horse however, is not just adapted for speed but efficiency.
– It has a fairly rigid spine, handy if you want to carry a rider.
The stride length is increased by running on the toenails (unguligrade), with the heels (hock) and wrist (knee halfway up the leg), lengthening the legs.
– Extra bones have been lost, leaving just the middle toe, and all the muscles are at the proximal (top) end of the leg, with long tendons running down the limb. This makes the distal (bottom) ends light, increasing stride frequency.
– Elastic energy is stored and released by the tendons, via the “extra” shock-absorbing joints that the horse has gained by running on its toes, particularly the fetlock, with the small proximal muscles acting as dampers (Wilson et al., 2001, Lawson et al., 2007). This makes the horse efficient.
– Joint constraints keep the limb motion parasagittal. Stable and efficient.
– The horse has no clavicle (collar bone). The scapula (shoulder blade) is held on by a musclar sling and hence can slide along the thorax increasing stride length and efficiency (Lawson and Marlin, in press).
In fact the horse is an engineering marvel. If you want to understand the musculo-skeletal system, study the horse.