The concurrent effects of strike pattern and ground-contact time on running economy
Rocco Di Michele and Franco Merni
Journal of Science and Medicine in Sport (in press)
Running economy is a key determinant of endurance performance, and understanding the biomechanical factors that affect it is of great theoretical and applied interest. This study aimed to analyse how the ground-contact time and strike pattern used by competitive runners concurrently affect running economy.
Fourteen sub-elite male competitive distance runners completed a 6-min submaximal running trial at 14 km h−1 on an outdoor track using their habitual strike pattern (n = 7 rearfoot strikers: average age, 25.3 years old (SD = 2.4); average weight, 64.7 kg (SD = 5.6); average height, 175.3 cm (SD = 5.2); n = 7 midfoot strikers: average age, 25.0 years old (SD = 2.8); average weight, 69.6 kg (SD = 4.0); average height, 180.1 cm (SD = 5.1). During the run, the oxygen uptake and ground-contact time were measured.
Midfoot strikers showed a significantly shorter (p = 0.015) mean contact time (0.228 s (SD = 0.009)) compared with rearfoot strikers (0.242 s (SD = 0.010)). Conversely, there was no significant difference (p > 0.05) between the groups with respect to mean oxygen uptake (midfoot strikers: 48.4 ml min−1 kg−1 (SD = 5.3); rearfoot strikers: 49.8 ml min−1 kg−1 (SD = 6.4)). Linear modelling analysis showed that the effect of contact time on running economy was very similar in the two groups, with a 1 ms longer contact time involving an approximately 0.51 ml min−1 kg−1 lower oxygen uptake. In contrast, when controlling for contact time, midfoot striking involved an approximately 8.7 ml min−1 kg−1 lower oxygen uptake compared with rearfoot striking.
When adjusting the foot–ground contact biomechanics of a runner with the aim of maximising running economy, a trade-off between a midfoot strike and a long contact time must be pursued.
Previous research have found mixed results in comparing running economy to contact time, so this study set out to add to that body of literature. Essentially this study showed:
- running economy was the same between the foot strike patterns and is consistent with the most recent study on this.
- rearfoot strikers had a 6% longer contact time (0.242s vs 0.228s; p=0.015)
- pooling the data of the two groups; a linear regression showed higher or longer total contact times were associated with lower VO2 values (R²=0.29; p=0.046). This occurred in both the rearfoot and midfoot groups
- from this, on average, a 1 ms longer tc involved an approximately 0.51 ml min−1 kg−1 lower VO2 in both the groups. In contrast, at any given tc, midfoot striking involved an approximately 8.7 ml min−1 kg−1 lower VO2 than rearfoot striking.
Based on their data, the authors suggest that:
Associating a long contact time with a midfoot strike should represent the best strategy for economical running, at least at relatively low speeds. However, given that those conditions are somewhat conflicting, the best possible trade-off between the two must be pursued.
This is interesting from the point of view of increasing the cadence more toward that mythical goal of 180 steps per minute requires a shorter contact time, which this study would suggest is less economical.
Some of the issues I have with the study are that the rearfoot strikers were ~5kg heavier than the midfoot strikers, but that difference was not statistically significant. Obviously, body weight can be an issue in VO2 studies.
I have commented many times on the sample size of study’s and usually do not have an issue with them (the fanboi‘s do if they don’t like a study!). The issue with this study is that it is a between groups comparison (which generally needs a larger sample size) compared to a within subjects type study (in which you can generally get away with a smaller sample size as each subject is their control). As this study has only 7 in each group for a between groups comparison, it is getting down to the lower end of being acceptable. Having said that, the results were statistically significant and I calculated the effect sizes and they were good (which for some reason the authors did not report, nor the journal reviewers required).
As always, I go where the evidence takes me until convinced otherwise, and I not sure where the evidence has taken me in this study, except for it obviously being one of trade-off’s.
Di Michele, R., & Merni, F. (2013). The concurrent effects of strike pattern and ground-contact time on running economy Journal of Science and Medicine in Sport DOI: 10.1016/j.jsams.2013.05.012