The 40% Rule: US Navy Seals Reveal Their Secret for Fighting Fatigue

The other day, I came across this quote about the 40% rule in a book I’m reading:

“The US Navy Seals have a theory that no matter how bad we feel or how tired we get, we’re actually nowhere near the limit of what we can achieve; the brain is deceiving us to save the body from pain. In fact, they think we’re only 60 per cent there.

“It’s a way of saying that by drawing on our mental strength we can push through any situation. When the chips are down, tell yourself you have at least 40 per cent left in the tank.”

Is There Any Science Behind The 40% Rule?

Does the 40% rule have any science behind it, or is it just some macho BS?

The 40% rule isn’t really a rule. Rather, it’s a way of saying that you have much more left in your reserve tank than you think, even when it feels like you’ve reached the limit of what you’re capable of.

Whether it’s a 40% rule or 20% or 5% doesn’t matter too much. The point is that your mind, and not just your muscles, has a big say in deciding when it’s time to throw in the towel.

In fact, there’s plenty of research out there to show that your brain makes a contribution to endurance performance that is independent of what’s going on below your neck.

Here’s a closer look at the research and what it all means for you.

Why Do Your Muscles Get Tired?

The traditional view is that your muscles tire because they reach some kind of physical limit.

Either they run out of fuel, or the accumulation of metabolites – substances produced as a by-product of muscular contractions – interferes with their ability to keep going.

In the 1990’s, Professor Tim Noakes came up with the Central Governor Theory, which suggests that your brain “paces” your muscles to keep them back from the brink of exhaustion.

When the brain decides it’s time to quit, it creates the distressing sensations you interpret as muscle fatigue.

The idea is that your brain stops your body running out of fuel, so there’s always something left in case of an emergency.

In one trial, Noakes and his team recruited seven experienced cyclists and asked them to complete two 100-kilometer time trials on exercise bikes.

On several occasions during the trial, the cyclists were asked to sprint for 1000 or 4000 meters. Electrical sensors taped to their legs were used to measure nerve impulses traveling to their muscles.

During exercise, your body never uses all the available muscle fibers in a single contraction. Instead, it spreads the load by recruiting fresh fibers as needed.

If fatigue was due to muscle fibers hitting some kind of limit, the number of fibers used during each pedal stroke should increase as the fibers tire and the body attempts to compensate by recruiting a larger fraction of the total.

But Noakes and his team found the opposite.

As fatigue set in, electrical activity in the cyclists’ legs dropped — even during the sprints, when they were trying to cycle as fast as they could.

The cyclists may have felt as though they’d reached their physical limit. But there were still considerable reserves they could theoretically tap into.

In another study study, a team of Spanish researchers found that subjects reached physical exhaustion even with enough fuel in the tank to power another 7–8 minutes of exercise at the same intensity at which exhaustion occurred.

The Central Governor Theory is just that — a theory. One that has been criticized by other researchers, with some calling for it to be abandoned altogether.

Samuele Marcora, a professor in exercise physiology at the University of Kent, dismisses the Central Governor Theory as a “flawed model that is increasingly out of favor.”

This doesn’t mean that Marcora discounts the importance of the mind in fatigue.

Rather, he thinks that endurance performance is ultimately limited by perception of effort.

“Endurance performance is directly determined exclusively by psychological factors,” claims Marcora. “The two most important ones are perception of effort and motivation.”

You decide to “give up” either because the effort required to complete a task exceeds the greatest effort you’re willing to exert, or because the effort to complete the task is so high that continuing for any longer is beyond your perceived ability.

Put differently, when the effort is perceived as maximal, or when the effort required eclipses the amount of effort you’re willing to exert, you stop.

In short, there’s an upper limit to how much you’re willing to suffer before you decide to quit. And that limit is always reached before you’re physically unable to continue.

Tired Mind, Tired Body

Back in 2009, Marcora published a study titled Mental Fatigue Impairs Physical Performance in Humans, where he compared the performance of two groups of cyclists.

The first group spent 90 minutes watching a couple of documentaries. Group two spent those same 90 minutes performing a mentally demanding task.

Then, subjects in both groups got on a bike and pedaled until they couldn’t pedal any more.

The riders who watched the documentaries averaged nearly 13 minutes of cycling before they hit exhaustion. In contrast, the mentally fatigued group lasted less than 11 minutes.

In other words, mental fatigue led to a 15% dip in performance.

There were no differences in oxygen consumption or heart rate between the two groups. However, the mentally fatigued group had a higher rating of perceived exertion. That is, from the start of the cycling test, they felt like they were working harder.

Marcora talks more about his research in the video below, if you want to watch.

Brain Training Improves Physical Performance

Marcora has also done some research with the British Ministry of Defence, to see if “brain training” can improve physical performance in soldiers.

In the 12-week study, soldiers were assigned to one of two groups. Both groups did one hour of cardio on an exercise bike, three times a week. However, one of the groups performed a mentally fatiguing task while they pedaled.

At the end of the study, both groups showed similar increases in V02 max, a common indicator of physical fitness.

However, when the soldiers were asked to do a “time to exhaustion test,” where they rode at a specific percentage of their VO2max until they couldn’t continue, there was a marked difference in performance.

The control group saw their time to exhaustion improve by 42 percent. However, soldiers who combined physical training with mental exercise improved by 126 percent, three times as much as the control group.

Combining the physical and mental training led to a big jump in performance. Something about increasing the fitness of the brain led to a massive increase in the riders’ physical abilities as well.

Marcora thinks that soldiers doing the mentally challenging activity were strengthening their anterior cingulate cortex, a part of the brain that helps you resist the urge to quit.

The stronger it gets, the more slowly your sense of effort will increase, and the longer you’ll be able to keep going.

“Applying a cognitive strain can produce an enhanced training effect without adding any additional physiological stress,” Marcora says.

“You strengthen that part of your brain and increase your tolerance for effort, so that when the cognitively straining task is eliminated, you are able to endure a much greater physical load.”

Final Thoughts

The two main takeaways are that most of us can keep going even after our brains tell us to stop, and that training your brain may well turn out to be an effective way to improve your physical performance.

The 40% rule isn’t really a rule. Rather, it’s a way of acknowledging that your brain makes a contribution to endurance performance that appears to be completely independent of what goes on below your neck.

The mind always poops out before the body,” wrote Arnold Schwarzenegger. “Remember that. If you have doubts, if you don’t really believe, then you will fail. You create your own limits.”

It turns out he might have been right.