From Fellrnr.com, Running tips
Power meters have helped cyclists for a number of years, providing a valuable metric around how hard the cyclist is exercising. Stryd is attempting to provide a power meter for runners, which superficially sounds like a good idea. Certainly, there are many problems with using Heart Rate to determine training intensity, and measuring VO2 is only practical in a laboratory, so and a power meter could be a better option. However Stryd actually measures movement and then calculates power. The details are a little unclear, as their website does not explain their approach or a well, nor do they seem to be any validation studies that I could find. It seems that Stryd measures Vertical Oscillation as the primary metric, along with Ground Contact Time, from a chest strap, so it's a lot like Garmin's [[Running Dynamics]]. It seems likely that you could perform similar calculations using the Garmin system, but I find myself unconvinced by the approach.