Although pretty common among cyclists, measuring power output in runners still puzzles runners and coaches. Not long time ago, measuring power output in runners was something restricted to the labs that would use expensive setups such as special treadmills with force platforms, while the power output measurement, although quite precise, was rather complex involving calculations of positive and negative mechanical works of the limbs (see Farris et al, 2012). It’s not surprising, therefore, that the term “Running Economy” became more commonly used to infer on running “efficiency” than actual gross efficiency (work input/work output) per se, like it can be done with cyclists more promptly (Burke, 2003). Recently (~2013) a company named Stryd launched a foot pod that by the means of a set of accelerometers can infer the power output provided since that the runners weight is known. Stryd represents HUGE ADVANCEMENT on the field of athletics, as power output is a variable that allows us now to assess valuable information on several aspects of running form such as efficiency and economy, and at the same time suffers less interference of external factors such as inclination. Nevertheless, like any other tool, one has to know how to use it in order to make the most of it. Here are a few tips on how to use Stryd, its advantages over using only speed/pace and HR, and a few myths I wanna go about:
1. Set your power zones: You can run a critical power test (e.g. Jones et al, 2017), a CPET or any field test such as the 1.5 mile -2.4k – (ASCM, 2013) with your power-meter on.
- Advantages – Knowing your own power zones corresponding to a “metabolic status” will help you to individualize your training intensity and guide you while you run.
- Myth – knowing my “power-zones” will replace my “Heart-Rate zones”. Although power has its own features, HR zones are directly related to your metabolic state, while power is extrinsic. Both HR and power zones should be assessed and used properly.
2. Know what power output is: it basically measures how fast “propulsion” forces are applied.
- Advantages: This is a cool one since power will add extra information beyond speed, which is largely affected by inclination and terrain. It will be easier for to know “how fast you should go up or down”.
- Myth – Speed /pace becomes useless – Not at all! Speed (and pace) is a direct measure of your performance, it tells how fast you will finish a race or workout, so don’t ditch it!
3. Calculate and track your efficiency: This one is still disregarded even by cyclists. Now with Stryd you have the “power” to objectively access your running efficiency. You can estimate VO2 based on HR, and then your energy expenditure based on VO2. Energy expenditure can be converted into “power-input” and le voilà you can infer on gross efficiency based on power output.
- Advantages: Efficiency tells you a lot about your technique and room for improvement. You can improve your technique based on an actual parameter instead of the so acclaimed – and meaningless to me – “good form”.
- Myth – Efficiency is superior to running economy. Well, this statement is tricky, since, in theory, it would be correct if we assumed perfect transference from power output to propulsion/locomotion, which is not the case. Even in bicycles, the power produced by the torque applied on the crank set is not 100% converted into movement, there are always losses. Imagine, the power output estimated by accelerometers inside a foot pod placed on your shoe? Since running economy takes speed into account it still adds extra information on your effectiveness to propel your body. Take a look at both charts extracted from my Stryd during one of my long distance runs on s trail; they display Power, Efficiency, and Running Economy (RE), against distance – Note that my efficiency and power followed a similar behaviour almost everytime, whereas RE didn’t. In other words, sometimes I was producing more power more efficiently but not translating it into more propulsion and vice –versa. The shady area of the chart is a clear instance of such a disagreement: RE increases, while efficiency and power go down.
Stryd is a brilliant and elegant solution for a problem that has been challenging coaches and athletes and even a few sports scientists for a while; it allows for measuring runners’ power output. The practical applications are numerous and promising, but as any other piece of equipment, we have to educate ourselves on the proper way to use it. Thus, it seems that Stryd is an awesome new tool that will add to the other variables runners already track such as HR and speed, but it does not replace them.
1- Farris DJ, Sawicki GS. The mechanics and energetics of human walking and running: a joint level perspective. Journal of the Royal Society Interface. 2012;9(66):110-118.
2- Burke E. High-tech cycling. 2nd ed. Champaign, Ill.: Human Kinetics; 2003. viii, 319 pp.
3- Jones AM, Vanhatalo A. The ‘Critical Power’ Concept: Applications to Sports Performance with a Focus on Intermittent High-Intensity Exercise. Sports Med. 2017 Mar;47(Suppl 1):65-78.
4-ACSM’s Guidelines for Exercise Testing and Prescription. 9th ed. Philadelphia Wolters Kluwer / Lippincott Williams & Wilks; 2013. 456 p