Changes in HRV can be caused by any stressor so it’s difficult to definitively link any changes directly to these four metrics of running strain. Similarly, a single data point is not representative of potential impacts from running strain so I wanted to explore whether there were any notable trends across many data points (i.e. hundreds of runs in some categories) that could point to potential insights.

Overall, my expectation was that an increased quantity of each strain metric would result in a lower HRV potentially indicating insufficient recovery. However, this was generally not the case which was definitely surprising! While there wasn’t significant variation in the median HRV (generally 4-5 ms), the interquartile range of HRV measurements actually generally increased slightly across the most common quantities of each metric as strain increased with only the consecutive days metric generally decreasing.

However, the most notable decreases in HRV seemed to occur with the greatest, least frequent quantities of each metric, specifically 4 consecutive days, 500-600 feet of elevation gain and greater than 10 miles of daily mileage. These greater quantities have noticeably fewer data points than the other quantities so I’m not as confident in ascribing potential significance compared to the quantities with hundreds of data points. However, it seems that these greater quantities might result in a lower HRV because I’m not as acclimated to the strain compared to the more common quantities of each strain metric. For example, my weekly running schedule is almost always 3 days, off day, 2 days, off day so I usually don’t run 4 consecutive days which might explain why those specific intervals result in a notable decline in HRV.

These results might be explained by research that shows that there is a “pronounced effect of intensity on HRV… [so] any potential influence of duration is minimized.” As a result, most studies of HRV rely on intensity as measured by heart rate as found in the WHOOP Strain Score. For example, in a study of sedentary participants, an “8-week constant load aerobic training did not change vagal related HRV indices but 4-week high-intensity endurance training after 4-week constant load aerobic endurance training was sufficient enough to induce changes in nocturnal HR and HRV.” Most of these runs were within consistent ranges of both duration and intensity (usually 6 or 8 miles x 5 times a week, intervals of 2 and 3 consecutive runs per week and 200-400 ft of elevation gain per run) so it makes sense that there wouldn’t be significant variation in HRV.

Overall, the data seems to show that different quantities of these four metrics of running strain do not dramatically affect HRV when acclimated to these overall ranges but a less frequent and more “intense” quantity, such as 4 consecutive days of running, might be “intense” enough to notably lower HRV. Instead, it seems to show a positive, steady response to strain when intensity is relatively stable or, as stated by Runner’s World, “[HRV] trending higher or staying stable typically means that there is a positive response to the stressors you're facing.”

There are several improvements I would make for any further HRV analysis. Specifically, more data points would definitely yield more accurate results, particularly including HRV measurements taken during sleep. In fact, WHOOP calculates HRV “using a dynamic average during sleep… weighted towards your last slow wave sleep stage each night, the time when you’re in the deepest period of sleep.” Instead, my analysis uses only daytime measurements taken during days with a run (i.e. most likely my lowest HRV readings) which likely results in a baseline HRV that is significantly lower than is actually the case. Additionally, adding heart rate zone data would probably show greater variation in HRV between different running intensities due to the pronounced effect of intensity on HRV as previously mentioned.