Workload and Rate of Perceived Exertion

There’s a concept in exercise science called the Perceived Rate of Exertion (PRE). It’s a 0–10 scale that measures how much exertion you feel during an exercise.

The fact that PRE is relevant when there are already objective measurements of workload (reps, weight, speed, etc) points to the fact that subjective intensity and workload do not have a simple linear relationship. (If they did, you would measure subjective intensity using workload.)

Cleveland Clinic gives a good overview of the PRE scale here. At the bottom of the page, they say,

In most cases, you should exercise at a level that feels 3 (moderate) to 4 (somewhat heavy).

This is true for exercise and I will argue this is also true for work in general. Let’s start with the assumptions:

  1. Subjective intensity and workload do not have a simple linear relationship.
  2. Subjective intensity levels out past a certain point. In other words, there’s little difference between working 100 and 110 hours a week or squatting your 1RM or five pounds shy of your 1RM.

With these two in mind, we can approximate the relationship between workload and Perceived Rate of Exertion with a modified sigmoid curve.

Perceived Rate of Exertion is bounded between 0 and 10 (0<y<10), where 0 is no effort at all and 10 is maximal exertion.

Workload does not have an upper bound (x>0). Workload = 10 represents that previous personal best at this particular task. It is always possible to exceed a personal best, so workload can exceed 10.

The green line represents a 1:1 linear relationship between PRE and workload. The parts of the curve that are below the blue line (0<x<5, x>10) are places where the Perceived Rate of Exertion is less than the corresponding workload rating — in other words, places where you get more bang for your buck. The section of the curve that is above the blue lines is where the Perceived Rate of Exertion is greater than the corresponding workload rating. This is where you get the least bang for your buck.

If we define “bang for your buck” as PRE/workload, we can graph “bang for your buck” against workload to visualize the efficiency curve.

The green line represents the same thing as in the last graph — a 1:1 linear relationship between PRE and workload. Looking at this graph, it’s much more clear that the section between 5 and 10 is the least efficient place on the curve.

If we were optimizing for efficiency, we would target Zone 1 and Zone 3 exclusively. In practice, you can’t always exceed your personal best, so we can’t always avoid the right portion of Zone 2. Adjusting Zone 3 leftward, we get something like this:

Mapping these zones onto the original Perceived Rate of Exertion vs Workload curve, we get:

Now let’s go back to Cleveland Clinic’s recommendation:

In most cases, you should exercise at a level that feels 3 (moderate) to 4 (somewhat heavy).

For running, 3 is a pace where you are “able to maintain a conversation without getting out of breath while running”. 4 is “slight ‘push’ but still at a pace where you could speak a few sentences while running”. Somewhere around here is the inflection point called the Aerobic Threshold. At a pace below this threshold your muscles can get all the oxygen they need from your lungs, so they don’t produce lactate. At a pace above this threshold, your muscles can’t get enough oxygen from your lungs and thus start using anaerobic processes to produce energy, which results in lactate building up in your muscles.

It’s widely recommended when running to either keep your workouts easy and aerobic or intense and anaerobic (see here). In other words, if you’re going to go anaerobic, get the most of it by skipping Zone 2 and going straight for Zone 3. (Zone 2 is anaerobic, but it’s the least efficient kind of anaerobic exercise.)

The same general principle applies to pretty much any other form of training you might do, whether that’s studying, working, or lifting weights. Avoid Zone 2 as much as possible. Move most of your work from Zone 2 into Zone 1, and move the rest from Zone 2 into Zone 3. In other words, make 80% of your work easier, and the remaining 20 % harder.

If you take away nothing else, just remember to avoid Zone 2. All else held equal, this will help you get the same amount of work done with significantly less stress. Lowering the overall volume of perceived exertion is the key to preventing burnout and exhaustion.

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