Mechanical catapults, famous for heaving pots of flaming oil over castle walls, also play a key role in powering the movement of horses. A new study shows that a horse's biceps stores energy in springlike connective tissue and returns it in rapid bursts to ready the leg for its next step.
To gallop at top speeds, a horse must be able to quickly bring its legs into position for the next step. But this is no easy feat for the heavy, long-limbed horse. Big muscles contract more slowly and generate less power than small muscles. Based on muscle contraction alone, a horse could only produce a 20th of the energy needed to move its legs into position fast enough, calculates physiologist Alan Wilson of the Royal Veterinary College in Hatfield, Hertfordshire, U.K., and his colleagues.
Horses have a trick, though. Unlike the muscles of most vertebrates, the horse biceps contains a lot of collagen and is highly elastic. To demonstrate how this elasticity boosts power, Wilson's group had horses run and trot on a treadmill equipped with a plate that measures the forces produced by the forelimb at each step. Working from video, they measured the limb-joint angles as the horse moved through its stance. Combining these data with measures of how much force the muscle produced at various degrees of extension, they computed how much energy could be stored in and released by the elastic muscles.
In the 2 January issue of Nature, the team reports that the biceps muscle acts like a catapult. It accumulates energy slowly as the tendons stretch while the foot is on the ground. That energy is then released like a slingshot when one joint buckles as the horse's foot leaves the ground. This elastic catapult action propels the foot forward with 100 times the power of an equivalently sized nonelastic muscle.
This study “reveals another of many ingenious mechanisms for energy conservation in the running horse,” says equine physiologist and scientific consultant Nancy Toby of Quantico Marine Corps Base in Virginia. “This is a cleverly discovered piece of the puzzle!”