There are a number of different ways to measure your run intensity, but which one is best? Matt Fitzgerald analyses the pros and cons of the various tools at your disposal.
Intensity is one of the most important variables in run training. Loosely defined as how hard the body is working relative to its limit, intensity is the primary determinant of the effect of a given run on a runner’s fitness. Each specific intensity level offers different benefits, so it’s essential that runners hit the right intensity in each workout. This requires that they have a reliable way to measure intensity in real time.
There are several different methods of intensity measurement. Which one is best? The answer is that each of them has pros and cons. I recommend that runners choose the method that fits their needs best or use a combination of methods. Let’s take a look at three familiar ways of measuring run intensity and then finish with a few words about a couple of newer methods.
Oldies but Goodies
I’m old enough to remember when heart rate monitors first hit the mainstream. Athletes like me welcomed the advent of heart rate-based training because it allowed us to do something that had never been possible before, which was measure a reliable physiological measure of intensity in real time as we trained.
There are limits to the method’s reliability, however. Heart rate is influenced by a myriad of factors besides intensity, including diet, caffeine, weather, altitude, stress, and hormone levels. Consequently, you can’t always trust that a higher heart rate reflects a higher intensity of exercise. Another limitation is cardiac lag, or the time it takes for the heart to catch up to changes in intensity. Because of this, heart rate monitors are virtually useless for regulating intensity in interval sessions and other workouts featuring abrupt changes in pace.
The thing I like best about heart rate is that it’s not a performance metric. The most common and costly training error in endurance sports is pushing too hard on easy days. Heart rate does a better job than other metrics of holding athletes back in these sessions because no one’s embarrassed by a low heart rate the way they are by a slow pace or low power output.
Pace is the original intensity metric. In the old days, runners had no other way to measure how hard they were working, and even pace monitoring wasn’t easy. They had to run on measured courses, take splits when they passed a distance marker, and do mental math to calculate their pace. The advent of accelerometers and GPS enabled runners to monitor pace continuously as they went.
While pace is not a direct measure of intensity, it’s a useful proxy. For example, VO2max intensity typically occurs at an intensity that a runner can sustain for six-and-a-half to seven minutes. When you want to target this intensity, all you have to do is aim for a pace you could hold for this duration.
Pace is less useful in hilly environments, as gravity skews the relationship between pace and intensity. An eight-minute mile pace is far more intense on a 10 percent grade than it is on a 0 percent grade. The other downside of pace, as I suggested in the previous section, is that, as a performance metric, it tempts runners to go too fast in easy runs. The fact that pace is a performance metric is also a strength, however. Race performances are judged by time, after all, so it makes sense to practice running by time in training.
Like pace, power is not an intensity metric but can easily be correlated with intensity. The leading manufacturer of run power meters, Stryd, does this automatically by using training data to calculate and continuously update critical power, which corresponds the highest intensity a runner can sustain before losing metabolic homeostasis. The accuracy of these calculations was validated by a recent study in the journal Biology in Sport.
There is more than one effective way to monitor and control intensity, so feel free to go with your preference.
A key advantage of power vis-a-vis pace is that hills do not skew its relationship to physiological intensity. If you’re running 8 minutes per mile on flat ground and you start running up a hill with a 10 percent grade at the same pace, your power output will increase, accurately reflecting a rise in intensity. Run power meters also account for the intensity effects of winds and different surfaces. As I like to say, “A watt is a watt is a watt.”
The Latest . . . and Greatest?
The most widely used metabolic indicator of exercise intensity is blood lactate. An intermediate product of aerobic metabolism, lactate leaks into the bloodstream from muscle cells at increasing rates as exercise intensity increases, making it a reliable marker of how hard the athlete is working. The problem with lactate measurement historically has been that it requires annoying mini blood draws that cannot be done during exercise but only during pauses in workouts.
Thankfully, a new generation of noninvasive wearable technologies allow athletes to measure blood lactate continuously (albeit indirectly, through sweat) during workouts. Not yet commercially available, these products, including the Belgian-made IDRO, offer a nice complement to heart rate, which is a measure of cardiorespiratory load, whereas lactate indicates metabolic load. Like heart rate, though, lactate levels lag behind intensity changes, making lactate measurement less useful in workouts featuring lots of variation in work rate.
Arguably the truest indicator of exercise intensity is total ventilation, or how much air is being taken in by the athlete. A product of breathing rate (measured in breaths per minute) and tidal volume (a measure of how much air is being taken in with each inhalation), total ventilation (VE) is closely correlated with both cardiorespiratory and metabolic load as well as perceived exertion (RPE), which is significant because it is the latter that ultimately limits endurance performance.
Until recently, total ventilation was only measurable in a laboratory environment, but a new wearable device called Tyme Wear tracks VE indirectly through a strain gauge that senses the expansion and contraction of the chest cavity. Because the most important intensity thresholds are defined by changes in ventilation, and not by associated changes in other physiological parameters such as heart rate and lactate, this new technology makes precise targeting of specific intensities virtually foolproof for athletes. I’ve started using Tyme Wear with some of the athletes I coach and have found ventilation-based training to be a powerful way to regulate intensity, track changes in fitness, and identify what’s working and what’s not in the plans I build for them.
Don’t Overthink It
As a matter of personal philosophy, I like to base the advice I offer to runners not just on science but also on the proven best practices of elite runners. With respect to intensity measurement, there is no single proven best practice. While a majority of pros train mainly by pace, some also pay attention to heart rate, others prefer power, and increasing numbers are experimenting with the newer technologies discussed in this article. What this tells us is that there is more than one effective way to monitor and control intensity, so feel free to go with your preference. By all means, put some thought into your selection. Just don’t overthink it.