Biomechanics / Performance

The high cost of ankle instability

What’s your running goal? Are you looking to run faster, further or injury-free? Every runner has room for growth, and a key to developing that potential is knowing what specific factors, whether metabolic, mechanical or otherwise, might be holding you back from reaching your targets.

A strong predictor of running performance is VO2 max, or maximal oxygen uptake 1. While VO2 max is an accurate guage for endurance capacity, it does not necessarily equate with running efficiency, as it only captures one element of performance potential. Even the most “fit” runner, as approximated by VO2 max, may be hindered by physiological or biomechanical features that prevent them from reaching their full athletic potential. “Cost of transport” – or the amount of energy used to run a given distance (per unit mass) – is an alternative metric  for assessing performance potential, which takes into account several additional critical factors.

In their study recently published in the Journal of Experimental Biology, researchers characterized the metabolic and mechanical factors that most strongly influenced performance in a multi-day ultra-running event. Eleven male runners completed the three-day, 93-km Italian race “Magraid”. The researchers acquired several physiological measures before and after each race day.

Standing-on-toesTo little surprise, they found that race performance (quantified as average speed) was predicted by high VO2 max and low cost of transport. To determine why cost of transport might be lower (better) for some runners than others, they further studied how cost of transport related to other functional variables. Reduced cost of transport was associated with several characteristics, including greater leg muscle power and vertical stiffness (during the stance phase), a smaller foot-print index (a proxy for ankle stability 3) and lower step frequency.

During any endurance race – but particularly one of this magnitude – it matters less how efficient you are when the starting gun fires, than how well you can maintain a high efficiency level throughout the run. Therefore, the researchers examined what factors influenced the change in cost of transport over each race day. A larger increase in cost of transport from the start to finish was predicted by the change in foot-print index, step frequency and vertical stiffness. In other words, as the runners fatigued and became sloppy as the race progressed, they showed less ankle stability and stiffness, and took more frequent steps. A low foot-print index, the strongest predictor, was also associated with greater leg power, vertical stiffness, and calf tendon stiffness and force (which, expectedly, correlated with calf structure).

While the researchers observed a logical relationship between foot-print index and structural properties of the calf, it’s unclear if calf morphology predicted cost of transport. Calf structure may correlate with ankle stability, and ankle stability may correlate with running efficiency, but does calf structure also correlate with running efficiency? Somewhat unexpectedly, they also observed a correlation between cost of transport and step frequency, hinting that a lower cadence may be more efficient. Given the almost universally accepted doctrine that a high cadence promotes efficiency and reduces injury risk, this finding is odd indeed. The authors speculate that the effect may reflect deviation from one’s personal optimal frequency, or increased internal work with higher frequency. Regardless of the reason, they point out the possibility that a lower cadence may impart greater long-term muscle damage. As with any of the correlative findings reported here, we can’t be certain whether step frequency influences, results from, or is even directly related to, running performance, without further intervention studies.

Based on these findings, what should a runner prioritize to improve – and maintain a high level of – running efficiency? Besides strong leg muscles, this study suggests that the stiffness and stability of the ankle and lower leg critically contribute to an endurance runner’s efficiency. These features presumably allow the muscles and tendons to more optimally store and release energy.

So if you’re looking to PR at your next 10k, marathon or 100-miler, what can you take home from this study? Go ahead and keep up those speed intervals to boost your aerobic capacity and don’t slack on those squats and lunges to maintain those powerful legs. But if you’re looking to stop running like a gas-guzzling SUV and maximize your energy efficiency, don’t ignore your ankles. Devote some time each day to those single-leg squats and hops, and play-time with that wobble board, and you just might notice a rise in your MPG.


  1. Billat V et al. 2003. Training and bioenergetic characteristics in elite male and female Kenyan runners. Med Sci Sports Exerc. 35:297-304.
  2. Lazzer S et al. 2013. Factors affecting energy cost of running during an ultra-endurance race. J Exper Biol. 44:1325-44
  3. Huang PY et al. 2011. Foot pressure and center of pressure in athletes with ankle instability during lateral shuffling and running gait. Scand J Med Sci Sports. 21:e461-7
Lazzer S, Taboga P, Salvadego D, Rejc E, Simunic B, Narici M, Buglione A, Giovanelli N, Antonutto G, Grassi B, Pisot R, & di Prampero PE (2013). Factors affecting energy cost of running during an ultra-endurance race. The Journal of experimental biology PMID: 24265425

One thought on “The high cost of ankle instability

  1. Pingback: ScienceSeeker Editors’ Selections: December 8-28, 2013 | ScienceSeeker Blog

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