HrPwr
HrPwr is the relationship between heart rate and power output. It can be used to evaluate Running Economy as well as Heart Rate Drift. HrPwr is easy to implement, and I have published a Connect IQ data field called HrPwr that's available here.
1 Background
My initial approach to measuring Running Economy outside of a lab was to use heart rate to estimate the effort and distance covered for the effort. This works okay on the flat, as distance covered is a good measure of effort. However, as anyone who has run hills knows, slope dramatically change effort. The Stryd footpod estimates the power output based on pace and elevation changes, which solves this problem nicely. It also gives the possibility of comparing data from running with cycling, though I've found the Stryd power estimate to be much higher for a given perceived effort than cycling.
2 Formula
The formula I use is below, and is the power output in milliwatts per Kg per heartbeat.
HR-Pwr = (Power / Weight) * 1000 / (Heart Rate – Zero Power Heart Rate)
I've used Zero Power Heart Rate rather than Resting Heart Rate as I've found that heart rate when standing is a little higher than the resting heart rate.
3 Connect IQ field
The Connect IQ data field is a simple numeric display, so it should work on most Garmin watches that support Connect IQ 1.3 or later. It will record a number of data streams into the FIT file, which can be displayed in the Garmin app or web site. (A future version might allow you to select which to record.)
- Raw Power is simply the power data stream for devices that don't record this natively.
- Smoothed Power is both smoothed and delayed.
- Smoothed HR is the smoothed heart rate.
- Smoothed HrPwr is the formula above using smoothed power and smoothed heart rate.
- Cardiac Cost is for comparison, and is based on distance rather than power. See Running Economy for details.
4 Smoothing and Delay
While HrPwr can be calculated as an instantaneous value, I've found it more useful to smooth the data and to delay the power slightly. The connect IQ field uses a simple weighted smoothing where SmoothHrPwrCounter is set to the number of seconds of smoothing required.
smoothhr = (smoothhr*(SmoothHrPwrCounter-1)/SmoothHrPwrCounter) + CurrentHR/SmoothHrPwrCounter;
