The Mechanics of Horse Legs

1. What they did

Horses are large animals that run fast, run far, turn quickly and carry or pull heavy loads. Their legs must be long, yet swing very fast; light, yet strong enough to deliver great thrust and take great loads; and all this with enough economy of effort to give endurance. In this review article, written for a general science readership, Milton Hildebrand brings together about 20 years of research in functional morphology, the branch of biology that relates the form of animals to how they move and eat.

He explains how the horse’s legs work under four headings: long, fast strides; combining light weight with strength; inertia, energetics and endurance; and maneuverability and stability. He draws on other researchers’ studies and on his own work, including film analysis of running horses and cheetahs and geometric models of legs as pendulums.

2. What they found

Long, fast strides

• Some horses can sprint at nearly 70 kilometres per hour (km/hr). Harness pacers have run 0.8 km at 52.0 km/hr and trotters the same distance at 49.7 km/hr. One horse covered 80 km at 18.2 km/hr, and a wild ass was chased by car for 26 km at 48 km/hr.
• A racing horse completes about 2½ strides per second. The rate at which muscle fibres can shorten seems to limit stride rate. Beyond moderate speeds, horses go faster mainly by lengthening the stride, which commonly measures 6.4 to 7.5 m in a racing horse.
• The horse’s legs are long relative to its body: longer than those of its closest relatives (rhinoceroses and tapirs), longer even than a cheetah’s, and similar to a greyhound’s. Longer legs might cause the fore and hind legs to interfere; horses sometimes strike a forefoot with a hind foot at a fast trot and must straddle the fore with the hind at the gallop.
• As the horse family evolved, the foot lengthened most, and its posture changed so that the wrist and heel (the hock) are held high and the animal stands on the tip of its toe.
• The shoulder blade is upright and has no collar bone, so it can turn and add its speed to the other leg segments.
• Horses also lengthen the stride by being off the ground. A fast-trotting or pacing horse has two unsupported intervals per cycle, totalling 35% to 43% of the stride; at a fast gallop it is off the ground once, for 20% to 30% of the stride.

Light weight with strength

• The cannon bone has a safety factor of 3 to 4 for a steady gallop and 2 to 3 for a more demanding jump. Under unnatural conditions imposed by people, the lower leg sometimes fails.
• Legs swing mainly front and back. Muscles that spread and close the legs or rotate the lower leg have been reduced or lost, saving weight, especially in the lower leg. Each foot has one digit, with vestiges of two others.
• Joints are hinges, strengthened by a flange on one bone turning in a groove in the other. These passive mechanisms save the weight and fuel cost of stabilising muscles.
• At rest, a locking mechanism (a parallelogram of the shin bone, a tendon, the femur and the heel bone, held by the kneecap) keeps the stifle and hock joints from collapsing. Cloven-footed mammals have a similar mechanism. “Snap joints” at the hock and elbow and the suspensory ligaments at the fetlock also give passive support.

Inertia, energetics and endurance

• If body size doubles with no change of shape, muscle and tendon strength rises four times but mass rises eight times. Small animals such as foxes can run fast and far without special adaptations; the large horse cannot.
• The horse’s spine is held relatively rigid, unlike the flexing backs of cheetahs, greyhounds and rabbits. Horse legs follow lower, more conservative paths than a cheetah’s. A galloping horse is off the ground about a quarter of each cycle; a dashing cheetah, half.
• The fetlock flexes most when the leg is vertical, shortening the leg just when this reduces lifting of the body.
• The swinging leg as a pendulum. From his geometric models, the author found that mammals tend to walk at close to the natural period of their legs. If the leg’s actual period is between ⅔ and 1¼ times its natural period (most walks, easy trots and slow gallops), the energy saving should be substantial, though this was not measured directly.
• The supporting leg. Like a vaulter, the body rises and falls over each supporting leg, trading kinetic and potential energy. Other researchers found 35% to 50% energy recovery at walking speeds in dogs, sheep and monkeys; horses probably benefit similarly, but this has not been shown.
• Springs. The suspensory ligament (which evolved from a foot muscle, as shown by scars on fossil horse bones) and the flexor tendons stretch as the fetlock bends under load, nearly to the ground in fast travel, then snap back. This cushions impact, lets the bones be lighter, supports the fetlock, and recycles energy. In donkeys, energy recovery at the trot is close to optimum. In the hind leg, the tendinous core of a shin muscle (peroneus tertius) snaps back to flex the hock as the foot lifts.
• Angular momentum. A mechanism not reported before for horses: like a cracking whip, the heavy upper arm slows at mid-swing and passes its rotation to the lighter forearm, which then rises quickly.
• Light lower legs. The heavy, fleshy parts of the leg are near the body; the lower leg is only bone, tendon and hoof. The radius of gyration of the hind leg is about 40% of leg length in a horse, against about 49% in a bear (Figure 6).
• Choosing gaits. Ponies have, for each gait, a most economical speed, and they change gait where that saves energy. The author’s three-segment model showed more energy in the cycling leg when a horse walks fast than when it trots at the same speed; his results were ambiguous for the change from trot to gallop.

Maneuverability and stability

• Long legs keep the centre of mass high, and small footprints reduce the area of support. Walking and trotting horses place their feet so that the track along the line of travel is narrow, and fast-running horses have only one or two feet down at a time. These features favour maneuverability.
• Horses use the transverse gallop (same lead fore and hind), not the rotary gallop of cheetahs and dogs, which probably favours stability over maneuverability.
• On a turn, a galloping animal must lead with its inside forefoot, so the footprints of each pair fall closer to the curving line of travel. Lead changes happen when all feet are off the ground. Horses have one unsupported interval per stride, while fast carnivores have two, so foxes, wolves and big cats can dodge at shorter notice.

3. What they concluded

• The legs work together as oscillating levers, powered struts, springs and cushions, giving horses remarkable speed, endurance and maneuverability.
• Nearly all this understanding was unknown 20 years earlier, and most was learned in the previous 8 years, with tools such as high-speed filming, treadmills, force plates, strain gauges, accelerometers and computers.
• Horses are “still far ahead of us”: the estimates need testing and refining, and many factors are likely yet to be discovered.

6. Other technologies, skills or innovations

• The author estimated the natural swinging periods of legs from geometric models of their centres of oscillation, because measuring a leg pendulum directly is very difficult, and compared them with filmed walking cycles.
• A three-segment model of the leg, tracing the centre of mass of each segment through the cycle, to compare the mechanical energy of the leg at different gaits.
• Tools named for this field: high-speed cinephotography, stop-motion film analysers, large treadmills, force plates, strain gauges, accelerometers, computers and digitisers.