Which of these two runners trains harder: Sally, who jogs 35 miles per week, or Billy who runs three 5K races and a total of 20 miles every week?
The correct answer is Billy. Why? Because training more does not always equal training harder. Intensity must also be considered. Exercise scientists refer to the combination of volume and intensity in training as "workload." Most runners track volume only in their training logs. But it's a good idea to monitor your workload as well because it gives you a more accurate picture of how hard you're really training.
There are various ways to measure workload. One of the better-known methods is called "training impulse," or TRIMP, which was developed by a Canadian exercise physiologist named Eric Banister in the 1970s. This metric calculates a score for each workout based on its duration and its average intensity as measured by heart rate. It sounds simple enough, but there are several nuances that take these calculations beyond the reach of the average runner. The actual formula for calculating a TRIMP score is: time (in minutes) x ?HR x y, where ?HR = fractional elevation in HR or HR reserve and y = a "weighting factor." It's enough to make your head explode.
Fortunately, there's a simpler way to calculate a training load that requires only basic math skills. It's known as "session RPE." This metric is based on the idea that how hard a workout feels is a reliable indicator of how physiologically stressful it really is. When runners feel exhausted, they are exhausted, and when they feel rested, they are rested. Pretty simple.
Originally, a rating of perceived exertion, or RPE, was used only to assess how hard an athlete was working at any given moment of a workout. The original RPE scale ranged from 6 to 20, but these days a 1 to 10 scale is most commonly used. Here's how each number on the scale translates into plain English:
1 — Very, very easy
2 — Very Easy
3 — Easy
4 — Somewhat easy
5 — Very slightly hard
6 — Somewhat hard
7 — Hard
8 — Very hard
9 — Very, very hard
10 — Maximal
This scale was in use for some time before Carl Foster of the University of Wisconsin-La Crosse got the idea to modify it so that it could be used to quantify the difficulty of an entire workout. The formula for this calculation couldn't be simpler: time (in minutes) x average RPE. For example, the session RPE for a run lasting 30 minutes with an average RPE of 4 for the entire workout is 120.
The only tricky part is getting comfortable with subjective ratings of workout difficulty. The key is to consider the entire workout rather than how you felt at the very end or at the most difficult point in the session. This process has been made easier by the assignment of standard RPE values to various types of workouts. Here are some of them:
Easy Run: 2-3
Long Run: 3-4
Tempo Run: 7
Interval Run: 8
To get a session RPE score for any of these types of runs, just multiply the associated RPE by the total duration of the workout in minutes. For other types of workouts not on the list, assign your own rating of perceived exertion and then multiply that number by how long it lasted.