Watching a pack of racehorses thundering towards a finishing line, you're probably concentrating on which animal gets its nose across the line first. But the real action is going on closer to the turf. As the horse's hoof hits the ground and transfers the weight onto the leg, it stores energy in the springy flexor tendons in the lower half of the animal's leg, ready to launch the animal forward when it takes the weight off its hoof at the end of the stride; just like a like a `child's pogo stick' explains Alan Wilson. And its little wonder that damage to the flexor tendons in the lower limb accounts for 50% of the injuries sustained by race horses; strain in the long tendons can be three times greater than the strain on a running human's tendon! But even if we can't `pogo' along on our less elastic limbs, we can change the relative stiffness of our legs to bounce us from one step into the next as we speed up from a walk to a run. Could horses vary their limb stiffness too? Wilson, and his student Polly McGuigan, began putting horses through their paces(p. 1325).

Although there was little question that the horse's lower leg was powered by elastic energy, any doubts the team might have had were dismissed when they began testing the way the limb responded as they compressed the leg and stretched the tendons. They applied a force to the top of an animal's leg and amazingly, the relaxed leg suddenly sprung into shape as the tendons stretched! And as they varied the force on the limb, the tendons lengthened,just like a spring.

Wilson explains that there are three springy tendons in the lower half of the horse's limb. But only one of the tendons has enough associated muscle to possibly adjust the tendon's stiffness, the deep digital flexor, which stretches down the back of the leg. McGuigan and Wilson wanted to know if the small muscle was powerful enough to change the leg's stiffness as the horse gears up from a walk to a gallop. First they monitored the animals' leg movements as they walked, trotted and cantered on a treadmill, measuring how the lower leg's length varied during a stride. Amazingly, when the horse galloped, it compressed the leg by 12 cm, tensioning the legs' tendons just like a pogo stick's springs.

McGuigan then videoed horses as they walked, trotted and cantered across a force plate, recording the compression in the leg and correlating it with the force on the limb. If horses could adjust the stiffness of the deep digital flexor, then the relationship would vary as the animal changed gait. But no matter what speed the horse hit the force plate, the leg's springiness remained unchanged; the horse couldn't alter the deep digital flexor's stiffness. Which doesn't bode well for horses trotting on the 21st Century's hard road surfaces.

So what the horse has gained in efficiency from its incredibly elastic lower leg, it has lost in flexibility to roam over different surfaces. But Wilson is optimistic that his work will eventually pay off, if he can design a cushioned racing surface that will make getting to the finishing line a less jarring experience.

References

McGuigan, M. P. and Wilson, A. M. (
2003
). The effect of gait and digital flexor muscle activation on limb compliance in the forelimb of the horse Equus caballus.
J. Exp. Biol.
206
,
1325
-1336.