It's a long time since humans had to rely on two feet for a speedy get-away, but for most creatures, a good sprint take off can make all the difference to life and death. Tom Roberts explains that acceleration during an escape is an essential aspect of natural selection. Yet the biomechanics of acceleration is barely understood, probably for good reason; getting an animal to accelerate reliably is notoriously difficult. But this didn't deter Roberts and his research assistant, Jeffrey Scales. They boldly set about startling turkeys into a trot to investigate which leg joints contribute to the bird's accelerating sprint, and found that the hip and ankle are key players in the bird's acceleration (p. 4165).

According to Roberts, working with running turkeys over the years had its moments. The males just have a `bad attitude'; you can never be sure when they'll turn on you! Fortunately the females are relatively cooperative, so when Roberts needed to film the surly birds accelerating across a force plate,Scales landed the job of startling the female sprinters. But even the females'psychology was contrary, and despite adjusting their startle tactics to each bird's temperament, Scales and Roberts could only use 10% of the 500 sprint sequences they filmed to investigate the bird's acceleration biomechanics.

Digitising the position of the bird's hip, knee, ankle and tarsometatarsal-phalangeal joints as they strode across the force plate,Scales and Roberts were able to calculate the work done at each joint as the birds accelerated, and found that surprisingly, the ankle and hip were the only joints that produced the work required to accelerate the birds. The knee and tarsometatarsal-phalangeal joints didn't contribute at all to the bird's acceleration.

Intrigued, Roberts and Scales wondered how the muscles attached to the working joints powered the bird's acceleration. Were they contracting with more force, or were they contracting over a longer distance? Roberts knew that if he analysed the distance swung by each joint as the bird's accelerated, he could distinguish between both types of contraction to find out how the birds power a get-away. Analysing each joint's trajectory, Roberts and Scales realised that the muscles in the hip and ankle increased the length they shortened by as the birds sped up.

But Roberts was puzzled. How could the ankle produce so much mechanical work, when the large muscles joined to the joint were attached through springy tendons? Surely the muscle's work would simply be soaked up by the elastic tendon, rather than contributing to the bird's acceleration.

Roberts decided to `think about the problem in another way'. He explains that when an animal trots at a constant speed, the limb muscles do no net mechanical work; the leg muscles store work in tendons during the first half of the step, recovering the stored work when the muscle stops contracting in the second half of the step as the body moves forward. Roberts suspects that the accelerating birds also use energy stored in the tendon by muscular contraction. In this case, he suggests that the muscle continues contracting,after storing elastic energy in the tendon during the first half of the step. He believes that by combining the work stored in the tendon with the contracting muscle's work in the later stage of the step cycle, sprinters get off to a flying start.

References

Roberts, T. J. and Scales, J. A. (
2004
). Adjusting muscle function to demand: joint work during acceleration in wild turkeys.
J. Exp. Biol.
207
,
4165
-4174.