Running Economy and Related Metrics
As we log those winter challenge miles (it’s now week 6!) we are gaining fitness: our hearts and running muscles are getting stronger. Our improved fitness, if measured by VO2max, should mean improved potential to run faster, and/or go for longer . But our training can also improve our Running Economy (RE). RE is like a golf score, lower is better. RE quantifies the rate of oxygen consumption for a given speed of running. If VO2max is the size of your car’s engine, RE is the fuel efficiency. A big engine is great but if not economical it will run out of fuel fast, and be passed by your Auntie Gertrude in her new hybrid.
World Champion marathoner Paula Radcliffe’s improvements in Running Economy are well documented: over a 10+ year period of training, her VO2max changed little, but her RE improved 15%. You can improve yours too, with training.
Running economy is the result of many physiological and biomechanical factors, so many that measurement of it requires a lab controlled environment. However, there is research that shows RE is strongly correlated with easier to reach metrics such as Leg Spring Stiffness, Ground Contact Time, and Vertical Oscillation. With today’s running technology, these metrics are measurable in the field. Your training platform may report these figures to you now, especially if you are also using a paired waist/chest or foot pod, which can measure vertical displacements and forces during your stride.
Leg Spring Stiffness (LSS) quantifies how well your legs store and return energy as your stride, much like a compressed spring stores energy when squeezed and releases that energy when let go. Stiffness here refers to the leg-as-a-spring model of a runner, where a stiffer spring stores more energy for the same amount of compression. Higher LSS is better, since it means a greater amount of energy is returned after foot strike to the runner’s propulsion, meaning less metabolic energy is needed by the running muscles to sustain speed. LSS can be “high” because there is greater ground force in the stride (strength), or because the leg-spring length doesn’t change much during the stride, e.g. one is not “over-striding”.
Ground Contact Time (GCT) measures how long your feet are in contact with the ground during your stride. Lower is better. Measured in milliseconds, elite runners will have GCT’s of 200 ms or less. GCT is greatly affected by cadence and also stride mechanics: a heel striker will generally have higher GCT than a forefoot striker. GCT and LSS are related as well, high LSS tends to correlate with low GCT.
Vertical Oscillation (VO) refers to the amount of up and down body movement in your stride. Lower is better. High Vertical Oscillation means you are spending energy moving your body vertically vs. horizontally and therefore spending energy that doesn’t translate to speed. Think of “running in place” as an extreme example.
All of these metrics, as proxies for RE, can help us to see if our RE is improving along with our mileage. These metrics also reinforce conventional wisdom about training: “don’t overstride”, “higher cadence is better”, etc.
Good old speedwork, like 200-400m interval training, is a great way to actively improve all of these metrics, and improve your Running Economy as a result. Strength training of the running muscles, especially hill work, is also a good way to improve on LSS. Improvements in RE will help with any running you do.
Happy Training!
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