When training as a cross-country runner during her college days, Kristine Snyder recalls her coach telling her to run at a specific stride frequency and wondering where the number came from. ‘The reality is that optimal stride frequency varies from person to person,’ says Snyder, who adds, ‘If you can run at a stride frequency that is going to minimise your metabolic cost – the optimal stride frequency – that is going to allow you to go farther and faster.’ But what determines an individual's optimal stride frequency? ‘There has been this idea in the literature for a while that the reason our optimal stride frequency is optimal is because our elastic energy storage is maximal at that frequency,’ explains Snyder, but no one had successfully tested this theory. Intrigued by the problem at a scientific and personal level, Snyder teamed up with Claire Farley at the University of Colorado, USA, to find out whether elastic energy storage determines optimal stride frequency (p. 2089).
‘We decided to see what would happen if we could reduce elastic energy storage, which may allow us to determine what happens when it is present,’ says Snyder. She explains that when you run on the flat you store elastic energy in your tendons as your foot strikes the ground and recover a great deal of this energy when you push off again. However, when running up an incline, the energy stored in the tendons as the foot lands is insufficient to lift you up the slope when you push off next, so muscle provides the additional energy. Conversely, when descending a hill more energy could be stored in the tendons than is needed, so the muscles dissipate the excess to maintain a constant speed. Essentially, Snyder and Farley could alter runners' maximal elastic energy usage by making them run uphill or downhill to find out if elastic energy usage sets a runner's optimal stride frequency.
The duo decided to measure the metabolic effects of running at different stride frequencies on flat and inclined surfaces. They reasoned that if elastic energy storage is a significant factor in determining a runner's optimal stride frequency then the metabolic rates of athletes running at different stride frequencies on sloped surfaces would vary less than the metabolic rates of athlete's running at different stride frequencies on a flat surface.
Recruiting outstanding runners from Boulder, Colorado, Snyder identified each runner's preferred stride frequency and set a metronome ticking to test whether they could run at 85%, 92%, 100%, 108% and 115% of their natural running frequencies. ‘They really strongly did not like using anything besides their preferred frequency,’ remembers Snyder, but having established that they could, she measured their metabolic rates as they ran at each frequency on the flat and angled treadmills.
Plotting the athlete's metabolic rates against their stride frequencies for each of three treadmill angles, the duo expected each parabola to have a different gradient if the optimal stride frequency was determined by the runner's ability to store elastic energy. However, the parabolas were all very similar: optimal stride frequency is not determined solely by elastic energy storage.
In that case, what does set a runner's optimal stride frequency? ‘I think we have a combination of things,’ says Snyder. She suspects that optimal stride frequency occurs at intermediate stride frequencies where the athlete's metabolic cost is lowest – as runners' muscles work harder at low and high stride frequencies and they also have to work harder to push off at high stride frequencies.