Most of us can’t escape the ultra-shuffle as we reach the later stages of races. As we fatigue our biomechanics change in many ways, including changes in stride length and frequency. In this article I will shy away from the nitty gritty details of biomechanics and focus on the relationship between stride length and frequency and how they impact running economy.
At the same running speed, stride length and frequency are reciprocal with one increasing and the other decreasing. Running economy (or running efficiency) is defined as the oxygen consumption (or energy) required for a given running speed. Along with maximal oxygen consumption and speed at lactate threshold, running economy is very important for performance and can vary as much as 30% between runners with similar maximal oxygen consumptions. Previously, I have discussed how strength training and speed work enhance running economy, this article will focus on the role of stride length and frequency on running economy. Perhaps the ultra-shuffle is actually a mechanism to optimize efficiency, or maybe it’s just a consequence of fatigue causing us to change our stride length and frequency for the worse?
both novice and experienced runners self-select a stride frequency below optimal, but experienced runners are significantly closer to optimal stride frequency.
A number of studies have looked at the relationship between self-selected (i.e., natural) stride frequency and an experimentally imposed stride frequency. Researchers examined stride frequency at three different speeds; self-selected, 90% and 110% of self-selected. At each speed heart rate was measured. The lowest heart rate was found at 83 strides/minute, whereas the self-selected stride frequency was close, but suboptimal at 78-80 strides/minute.
Researchers had novice and experienced runners run at a single speed, but using seven different stride frequencies all while measuring both heart rate and oxygen consumption to assess running economy. Novice runners’ self-selected stride frequency was 77 strides/min, but optimal was 84 strides/min. Experienced runners self-selected at 85 strides/min, but their optimal stride frequency was 87 strides/min. Thus, both novice and experienced runners self-select a stride frequency below optimal, but experienced runners are significantly closer to optimal stride frequency.
Researchers also found that both heart rate and oxygen consumption can be used to measure running economy, which allows us to determine our own most economical stride frequency by measuring heart rate after running for 3-5 minutes at different stride frequencies. The data indicate that as we gain running experience we increase our self-selected stride frequency, but we are still likely running at a slightly lower stride frequency than is optimal.
The two above studies were done on runners who were not fatigued, but as we know the ultra-shuffle comes on after hours of running and fatigue has set in. Far less research has examined this question of whether self-selected stride frequency changes during fatigue and how that might impact running economy, but a couple of studies provide some insight.
First, runners who ran for an hour at near maximal intensity show a slight decrease in stride frequency from the beginning of the run to the end. But they managed to run at an optimal stride frequency for almost the entire time, suggesting that faster running results in self-selection of a more optimal stride frequency, further supporting the use of speed work in training, but not providing much insight into what happens during an ultra.
One research group focused a number of studies on how running economy and stride characteristics change in response to mountain ultras. Runners in the challenging Tour De Geant 330k race did not change stride frequency in post-race testing compared to pre-, but they did improve running economy on uphill (10 or 15% grade) treadmill tests, which suggests that such a long race may elicit other beneficial changes in running economy.
To summarize, in non-fatigued situations we tend to run at slightly too slow of a stride frequency, but we can improve our stride frequency with training. As we fatigue in an ultra we do increase stride frequency (on the flats) and adopt a stride to minimize damage, but whether these changes in stride characteristics also improve running economy have not been systematically tested and likely will vary depending on the type of terrain and speed of running.
Stay tuned as research in this area continues and for now keep shuffling on.
Matt Laye of Boise, Idaho, has a PhD in Medical Physiology and is an Assistant Professor of Health and Human Performance at The College of Idaho. He enjoys competing on trails and on the roads in distances from the mile to 100 miles. He has averaged under 8 min/mile for 100 miles and under 5:30/mile over a marathon.
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