Does more fatigue mean more fitness?

Training causes fatigue and fitness to appear in sequence. Then athletes confuse the degree of fatigue with future fitness. Sometimes, it’s the opposite.

I’ve had a sinus infection for three weeks. On day 17, I started taking antibiotics. The next day, I felt much better. Did the antibiotics help?

The order of events makes it tempting to think that I’m better because of the medication:

  • Felt bad -> took antibiotics -> feel better.

But what if I hadn’t taken the antibiotics? Would I have felt better on Day #18 anyway? No one can answer that. The sample size is too small.

In my case, the presence of antibiotics and the improvement in my symptoms co-exist. But that doesn’t mandate a relationship between the two. They correlate, but they don’t have a causal relationship. My experience is only anecdotal. It proves nothing.

Does fatigue cause fitness?

A sketch showing the non-relationship between fatigue and fitness

Fatigue and fitness are like my sinus infection. Two things appear close together in time.

After high-volume or high-intensity training, we get tired. Then, after some recovery, fatigue decreases and fitness increases.

But that doesn’t mean that fitness leads to fitness. Fatigue and fitness co-exist, so we assume that fitness is dependent on fatigue.

But assuming causation leads to training mistakes. More training means more fatigue, but that doesn’t always mean more fitness. More might be better, or it could be worse.

  • Is it possible to have a training stimulus and response, but no fatigue? Yes, the desired response may not require noticeable fatigue. (When an athlete is aerobically deficient, acute fatigue will never be present).
  • Is it possible to have a stimulus and fatigue, but no response? Yes, the stimulus may be non-specific. Cycling for a marathon will not create event-specific fitness.
  • Is it possible to have a stimulus and fatigue, but a negative response? Yes, if the stimulus is anti-specific or too intense. Weight-training to increase leg mass will worsen rock climbing ability. Ultramarathons do enough damage that racers are less fit afterward than before.

Proper training is applying the right load at the right time, not the largest load as often as possible. If we apply the wrong load at the wrong time, we’ll get the wrong effect.

Proper training is planned with purpose. Random activity will lead to random changes in fitness.

Going too long or too hard can overwhelm our ability to adapt. The change in fitness could be slower than required, not happen at all, or reverse. The right load depends on several factors:

  • Individual response;
  • Training history;
  • Current fitness;
  • Current level of recovery; and
  • The requirements of the goal event.

Fatigue is a training byproduct, not a stimulus

So does being tired always mean we’ll be fitter? And more tired means even more fitness? Too often we conclude that it does. Then we use the strong-like-bull-smart-like-tractor approach and grind ourselves into the ground.

If fatigue were a predictor of fitness, then it should always work. But it doesn’t. For example:

  1. A long trans-Atlantic flight in a cramped seat will make me tired.
  2. A lot of long, easy runs below aerobic threshold won’t.
  3. Which will make me fitter?

So if fatigue is not a driver of fitness, what good is it?

Fatigue is a traffic light for effort

The purpose of fatigue is to gauge stress. As the severity climbs, our remaining bandwidth for training narrows.

When the light is red

When the light is red, the choice is obvious but counter-intuitive. Rest and recovery are the best choices for long-term fitness. Training should be easy, short, and low stress.

But the classic mistake when over-fatigued is to drive harder.

The light is red… I have an idea! If we make it more red, then I’ll be able to go faster!

Um… what? 🤔 Destroying ourselves becomes a badge of honor, even though it’s counter-productive. It’s a gratification problem: Badges feel good, even when they’re the wrong badges.

Pushing through when over-fatigued will destroy fitness, not magically defy gravity. If you’re tired, go easy or go short so you can recover.

When the light is yellow

The hardest level of fatigue to interpret is when there’s some fatigue but not a lot.

When training aerobic capacity, some mild, chronic fatigue is common. It’s usually a good sign. When training strength or power, fatigue is intense and acute.

But when is enough enough? When does the chronic fatigue from base building become overtraining? How long should it persist?

If the fatigue is intense will it also be short-lived? Or was it too much? Will it take too much recovery and interrupt the training progression?

The yellow caution light represents a fine line. Coaches and experienced athletes walk it every day. Beginners usually blow right by it, thinking the fatigue can’t possibly be enough.

When the light is green

When the light is green, the choice to train is obvious. But selecting the right type, intensity, and duration is important.

  • Where are we in the macrocycle?
  • What’s the next desirable response in the training plan?
  • In theory, what’s the most likely stimulus for that response?
  • In practice, how is the athlete likely to respond?
  • How much of the stimulus will be beneficial? And how much would be too much?
  • How recovered does the athlete need to be for the next key stimulus?
  • How fast does this athlete recover?

If you’re feeling fresh, go hard or go long in a way that makes you fitter for your goal event. Choose a training load that will improve your preparation or maximize your performance.

Don’t choose something that only tells you you worked hard.

A sketch of fatigue as a traffic light
Fatigue is a traffic light for effort. More is not always better.

Don’t be fooled by fatigue

My sinus infection is much improved. The two-week chills are gone and my nose is clear. But I’m still taking antibiotics.

If I went by feel, I’d stop taking them. But that would be an emotional response, not an intelligent one. The sample size of antibiotic use is in the millions. The statistics show that a full course of antibiotics is the best course. What I feel or believe is irrelevant.

In training, erring on the conservative side of fatigue is a time-tested principle. We can’t fill a leaking bucket by making the hole bigger. Find a happy medium, ideally under supervision, for how much fatigue you are likely to be able to handle.

How much fatigue we think we can handle is irrelevant (and probably wrong). Being tired only tells you the choice was challenging. It doesn’t tell you if it was the right choice.