As your understanding of athletic training becomes more sophisticated, one of the first concepts you come across is that of training stimulus. In simple English, training stimulus refers to what the body gets out of a particular workout.

Discussion of training stimulus abounds in circles that use MAF (Maximum Aerobic Function)—also known as the Maffetone Method—as their main framework for training.

The overarching mandate of the MAF Method is to protect the body. That is the best way for it to tolerate stresses, grow from training, and produce a great race performance. The party responsible for these functions is the body’s aerobic system, which oxidizes fats (burning them in the presence of oxygen) to provide a stable and long-lasting energy supply.

In endurance events, “protecting the body” means that the aerobic system must provide almost all the energy utilized during exercise. In power events, the aerobic system should be buttressing the function of the anaerobic system—which provides vast amounts of quick energy by burning sugars without oxygen—and still be strong enough to take charge for the duration of the recovery period.

For those who have already committed to developing their aerobic systems (by training at a low relative intensity), an issue inevitably arises: in long workouts that should occur theoretically at a low intensity, people accidentally (and often) end up rising above the desired intensity for a few seconds.

This brings up a crucial question: does this change the training stimulus?

There are several ways of answering this question. We can observe whether our speed at the aerobic threshold decreases after a month of training. We can go out and get a heart rate variability app that tracks our body’s autonomic readiness. We can even go get lab tested to see if our VO2 Max has decreased.

(If these terms mean nothing to you, don’t worry. Unless you’re an elite athlete who redlines for a living, they don’t need to. That’s the point.)

The body isn’t a black box. Action and circumstance affect it in ways that we can readily experience (when we know what to look for). A critical caveat: In this post, I’m only discussing the interpretation of experience before and after a workout. Using our subjective experience to measure and manage training stimulus in real time brings a whole other level of complexity.

Let’s abstract away from training for a second, and leave all that exercise terminology behind. Suppose you are on a long, leisurely birdwatching hike. You stop every few minutes to take notes, and you loiter every now and then with your binoculars as you try to make out the species of a bird in the distance. But 4 times over the course of this hike, you saw a novel bird just around the bend. Excited, you raced to take a picture.

How do you return from that hike? You are energized, renewed, invigorated. In spite of those few short sprints, the hike was a “low-intensity” experience.

Here’s another example: you’re back in your hometown after 5 years on a family visit. There’s been parties and get-togethers every day, and you’ve had ample time to catch up with all your friends.

But two things happened: the second day, you had the great misfortune of being mugged. And then the day after that, a former business partner caught up to you at a stop sign. He’s had a spell of bad luck—and in that short encounter, saw fit to threaten you and your family (over what you had thought was water under the bridge).

99% of the time, everything was pleasant and relaxing. But, for 10 minutes, the ground shook. That was enough for you to leave town with a new and unexpected wariness. Even the language—“two things happened”—tells you what the primary experience was.

This is also the case in athletic training. Put another way, the same body that has to glean meaning from that unexpectedly stressful visit (in order to be able to adapt to the next threat) is the same one that you take to the gym or out on the trails. That same body has to figure out whether it makes more sense to treat a particular training event as an “endurance workout” or a “strength workout.”

When a run feels “rejuvenating”—it’s very likely that’s exactly what it’s doing for your body. (The opposite holds true as well.)

You can break down the experience of being mugged in ever finer detail, and identify sensory and psychological stressors, and observe their physiological and neurological effects . . . but you don’t really have to.

Don’t get me wrong—you’ll get far more data about the effects of a divorce or a family vacation if you go get an fMRI every time something happens. That is a fact. (You can probably make better lifestyle choices when you know for sure whether your amygdala lights up when you see pictures of your former spouse.) But you don’t need an fMRI to be spot on—in a general sense—when asked what either experience did for your mental and physical health.

You can say the same about phenomena such as autonomic readiness (of the nervous system), which contributes to produce our subjective feelings of readiness for a wide variety of tasks.

Our experience of readiness doesn’t just happen to co-occur with our physiological readiness. Look at it from an evolutionary point of view: we didn’t have heart rate variability apps or monitors “waaay back when.” Our experience of readiness has to emerge from the fact that our nervous system, metabolism, hormonal system, and motor capabilities are actually ready for whatever it is we feel ready for. This is essentially the same line of argument that Tim Noakes (in his immortal book Waterlogged) uses to argue that the best measure of physiologically relevant dehydration is the subjective experience of thirst.

(In the same book, Noakes also argues that the fact that this even needs to be argued shows just how disconnected from the obvious we’ve become.)

If the subjective and the physiological weren’t part and parcel of the same system (to say that they’re “linked” is a gross misrepresentation), we’d all be dead. In other words, our heart rate variability monitor isn’t really going to change until we feel ready—and if it does change but we still don’t feel ready, we can be quite sure that there’s some other measurable physiological parameter out there that explains why.

The biggest mistake we can make is to listen to our pet parameter while disregarding the conclusion of a built-in measuring device capable enough to have outcompeted every other life form on the savannah—a device without which Neil Armstrong would have made it to the orbit but not the surface of the moon.