You’ve just finished a long, hard bike ride. But how hard was it, exactly? It depends on what you measure: how far did you ride and how fast? How hilly was the route? What heart rate zone was it in?
In the 1970s, exercise physiologists came up with the concept of training load, which they believed could provide a more detailed description of training difficulty. Training load combines training intensity and duration into a single number, with the aim of estimating the stress that exercise puts on the body. By comparing the difficulty of different training sessions, the idea was that athletes could better structure their training routines with the right mix of easy and hard days.
“It’s a great concept,” says Carl Foster, professor emeritus of exercise and sports science at the University of Wisconsin-La Crosse. For many competitive athletes, the idea is intuitive, he says, but it could also be useful for people who just exercise occasionally to get healthier.
Many fitness trackers and GPS watches now, including products from Garmin and Coros, and even the Apple Watch with its latest software update, use algorithms to estimate your training load and display that data alongside other stats like steps and heart rate.
While some fitness experts believe training load is a useful way to gauge the effectiveness of your exercise routine, others warn that paying too much attention to complex metrics generated by an algorithm can distract you from listening to your own body.
Here’s how to critically interpret your training load data:
What does Training Load mean?
Simply put, training load combines the length and intensity of your training into one metric.
The harder you train, the higher your training load will be. For example, a long, easy run will have a higher training load than a short, easy run. If you increase the intensity of that short run enough, it can potentially have a higher training load than a longer run.
But defining the intensity of exercise can be surprisingly complicated, Dr. Foster said. Today, most fitness trackers rely heavily on heart rate data to determine intensity, and each has its own proprietary algorithm for calculating training load from that. (In the mid-1990s, Dr. Foster said, Suggested “Perceived exertion,” or how subjectively hard a workout feels, could be an easier way to measure intensity and determine training load, allowing people to understand how hard they’re working “in a way that’s similar to how you communicate with your grandma,” he said.
Because there is no standard way to calculate training load and different trackers use different scales to represent load, the statistics shown by different trackers can vary widely.
How do wearable devices measure training load?
Garmin uses heart-rate data to estimate “post-exercise oxygen consumption,” or EPOC, a measurement that’s meant to be a proxy for the question: “How much of an impact did that training session have on my body?” said Joe Hykes, principal product manager at Garmin.
Coros calculates intensity based on heart rate or pace data, depending on the form of exercise, and rates your training load on a scale of low, medium or high.
Using inputs like height, weight, exercise history, and heart rate, Apple Watch ranks the difficulty of an activity on a scale of 1 to 10. However, users can manually change the difficulty rating if it doesn’t match how they felt during the activity.
Most wearables measure your cumulative training load over the course of a week, in addition to your single training load. These numbers can help you know if you’re increasing your training load too quickly. Darian Albury, head of user engagement at Coros, says that weekly and monthly training loads should be increased gradually, not abruptly.
How useful is tracking your training load?
For data believers and some athletes training for specific events, more is better.
However, there are many factors that aren’t calculated into training load, so one number can’t tell the whole story about your training.
Some coaches believe measurement is unnecessary, such as James McCurdy, founder and head coach of McCurdy Training, a coaching service in Flagstaff, Ariz. Mr. McCurdy said blindly following fitness-tracker data can get in the way of assessing what matters most: how players are feeling and what they think they’re capable of.
Philadelphia running coach Chelsea Weiss Passmore agrees: “A fitness tracker’s algorithms can’t tell you if you’re in bad shape or having an easy day,” she adds.
Both Passmore and McCurdy stressed that some data, such as weekly mileage and pace, is useful, as long as it is taken into account in the context of the situation.
While Dr. Foster supports measurement conceptually, he says you probably already base your fitness plan around your training load, probably unconsciously: If you have a goal, you know you can’t go all out every day, and you can’t take it easy every day.
Dr. Foster said you could tell by looking at a clock, but added, “You’re smart enough to do it without a clock.”
