Face Mask and Heart Rate - An Experiment
By: Tim Smith
I'll admit, I don't enjoy wearing a mask while running, but I understand that in these times of pandemic, it is the right thing to do. I know there is now a lot of data which says that transmission of the virus in open air is exceedingly rare. Still, when I meet someone out on the road, I'll pull up my mask; in part to be safe, and in part to demonstrate that I care about my community.
But this article is not about why we wear a mask, but rather how does a mask affect our running.
Ever since I have been running with a mask I have wondered if wearing a mask has a measurable effect. But then after writing about Heart Rate for the December newsletter, I knew I had the tools (my running watch measures heart rate) to do some experimentation. The question is, if I exert the same effort with and without a mask, how will that affect my pace.
So this is how my reasoning went; heart rate is a good measurement of effort. Ideally, I would hop on a treadmill, set the speed, and run a mile with and without a mask, and compare the heart rates from the two trials. Done.
The first problem is, I don't have a treadmill. If I just go out and run I know that my pace will tend to go up and down and my heart rate will too. But that is not too bad. I imagined a graph (see figure 2) with speed or pace on one axis, and heart rate on the other axis. As you go faster/run harder, your heart rate goes up. My watch records heart rate and speed every second. So if I plotted those data points I expect a scatter of points, clustered around a rising line.
But other factors can change your heart rate besides how fast you are running. Heart rate is tied to effort, and around here effort often means am I running up or down a hill. Long, straight level places are hard to find in the Upper Valley. After a bit of exploring I settled on a section of sidewalk next to Rt 10, north of Hanover. From the Fire Hall to Kendall is about a kilometer, and the elevation varies by only 8 feet.
When my plans had evolved to this stage I heard that UVRC was going to be giving out masks when you renew your membership, which solved another one of my problems. Through the fall I had been running with two different masks. One is pretty flimsy, the other an impenetrable canvas. I expect that the measured effect would vary widely, and I wouldn't be able to translate it into anybody else's experience. But now I will use the UVRC mask as my standard - and you can too.
So one fine day I went out and ran a quarter mile slow, then fast, without a mask. And then repeated that trial with a mask - and then went home. That evening when I plotted the data I was disappointed to find it was a mess and there was no clear trend. However upon closer examination I realized that when I did the fast legs, my heart rate had never settled to its final steady state.
A week later I tried again, this time 1000 meters for each trial. Things looked better, but I had probably rushed from one phase to the next. Heart rate lags behind speed.
One final effort. This time with 2000 meters trials, and just trying to keep the pace steady. It will naturally vary, but that I could deal with.
In figure 1 is my final test run, as plotted by Strava. You can see four clear legs. The first and third were with mask on, the others with mask off. I did meet 2 people while in the "no mask" trial, and so slipped on the mask for a short time, but I don't think it affected ("contaminated") the results too much.
In figure 2 I have plotted speed [meters per second] vs heart rate [beats per minute]. The red points are with the mask and the blue points are without the mask. First observation; there is a lot of overlap. In fact a prudent statistician would probably either stop at this stage - or send the experimentalist back out to collect more data. But I am going to press on my analysis - even if I am delving into the statistically insignificant.
One other thing to note in this plot is that heart rate and speed are correlated, as expected. That line, which is a fit to the data, says that with every 10 heart beats a minute, I also gain 10 cm per second. I'll actually use this in a minute to help my analysis.
From this figure the blue (mask-less) tends to be more to the right (faster) than the red. But how much?
For my final plot I am only considering the data points between 143 and 154 beats-per-minute, which is where all the trials had samples. I then take each data point and, using the slope of the line from figure 2, project it to the same heart rate (143 bpm), and plot a histogram for number of samples vs speed (figure 3). The left peak (red) is with a mask, the right peak (blue) is without a mask. The difference is 0.08 meters a second, or 3 inches a second. That is a bit over a 2% effect.
This is the equivalent of always running up a 0.4% grade, gaining 4 meters every kilometer, or wearing a weight vest with 2% of your body weight.
I'll be the first to admit that this analysis is a bit dodgy, but I think that it does tell us that the effect of the mask on our energy consumption is pretty small.
But wearing a mask feels like it should be a 10% effect or more!
I think the answer here is that only the lungs are working harder. Our legs, which are the greatest consumer of oxygen, consume the same amount of oxygen at a given pace independent of whether or not you have a mask on. The diaphragm works a lot harder, but it is still a small part of our total energy consumption.
Finally, I had this idea that I was going to be working significantly harder with a mask on. It might be like training at high altitude. But that was just me trying to make lemonade. The best thing for my training will be when I again can run next to (or behind) fast teammates. I am looking forward to that day when I can see your face.
In the meantime it is only 2%, it is not that bad!