Horse legs: the special catapult mechanism
A horse’s legs have special features that enable it to gallop beautifully. For example, they work like pogo sticks, storing energy between gallops. And a team led by Alan Wilson of the Royal Veterinary College, Hatfield, UK, showed that a feature previously thought to be useless has an important function. That is, certain small muscles thought to be vestigial, or useless remnants of evolution, are now known to have a vital dampening function.1,2
Dr Wilson’s further research with different colleagues3 shows that horse legs have a catapult mechanism as well. This is ‘where energy is stored slowly with a large force but is released quickly to accelerate a small mass. This mechanism, however, requires a more sophisticated lever, or cam system, to exert sufficient force on the spring and then release it.’ Fleas and grasshoppers also have a catapult system, but this is the first time it has been found in a large animal.
When the horse lands, the carpus (commonly called the ‘knee’) locks straight, while the shoulder bends forwards. This stretches the biceps muscle, which is very elastic. Eventually, the carpus buckles forward, releasing the biceps ‘spring’. This flicks the leg forward (protraction) and off the ground, so it’s ready to land on the ground for the next gallop. ‘[T]his muscle’s catapult action has an output [of power] that is comparable to over 100 times its mass of non-elastic muscle.’3
This highly efficient mechanism would not work at all without a locking and release system and the springy muscle fully in place. This is a problem for evolution, because the hypothetical small intermediate steps would have no advantage by themselves, therefore natural selection would not favour them.
- Wilson, A.M., McGuigan, M.P., Su, A. and van den Bogert, A.J., Horses damp the spring in their step, Nature 414(6866):895–899, 20/27 December 2001; comment by Alexander, R.McN., Damper for bad vibrations, same issue, pp. 855–857. Return to text.
- Sarfati, J., Useless horse body parts? No way! Creation 24(3):24–25, 2002; after ref. 1. Return to text.
- Wilson, A.M., Watson, J.C. and Lichtwark, G.A., A catapult action for rapid limb protraction, Nature 421(6918):35–36, 2 January 2003. Return to text.