What is the most important intensity?

Performance in an endurance event comes from the highest average speed. Training determines what the highest average speed–“race pace”–will be. The higher the race pace, the faster the time. The faster the time, the better the performance.

So the more race pace training an athlete can tolerate, the better prepared the athlete will be. They’ll be able to make race pace faster or they’ll be able to extend the time they can maintain it.

So the key to improving performance in an endurance event is to train as much as possible at race pace. But how it’s trained is not what most people think.

How is race pace trained?

The key to race pace training is to do “as much as possible”. But that begs the question, as much as possible when? When a novice is starting to train? (No.) Before an important event? (Sometimes.) Or five years from now when the athlete is more developed? (Hell yeah.)

And what determines how much is possible? What maximizes the potential volume of training at race pace?

First, the bad news: Your gut instinct is wrong.

The first instinct of novice athletes is to go as hard as possible as often as possible. The thinking goes like this: “If I can push hard for five minutes today, tomorrow I’ll be able to do six. The next day, seven. If I keep beating my head against the wall, I can’t help but get stronger.”

Even worse, this approach is often applauded by the uninformed HIIT-trend crowd.

Even more confusing to a novice is that the head-beating approach will work. But only for a while. Then the athlete will stagnate while the rating of perceived exertion stays the same. The athlete continues working “hard”, but race pace stops increasing and often slows.

Why does chronic high-intensity training decrease performance?

The confusing thing is that high-intensity exercise has both positive and negative effects. At the start, the positive effects outweigh the negative. The athlete gets faster. But if high-intensity training is chronic, the negative effects will overwhelm the positive. The athlete then gets slower.

So a net advantage becomes a net disadvantage. Like investing with leverage, what creates a short-term benefit is a long-term hazard.

Here’s why:

Each time it’s used, high-intensity exercise creates negative byproducts. With these metabolites, the aerobic system sustains some damage. Aerobic capacity shrinks. At the outset, the increase in performance outweighs the decrease in aerobic capacity. The novice sees that it’s working and does even more.

For a recreationalist, the reduction in aerobic capacity doesn’t matter. Performance isn’t a priority; benchmarks and time trials aren’t used. So the decrease in aerobic capacity–and ultimately performance–will go unnoticed. Like setting a skin track that’s too steep, the recreationalist feels like they’re working hard even though they’re gaining less ground. But it’s the feeling that counts, so they’re happy.

But for an athlete working toward a goal, performance is key. How a workout feels is less important than what the workout does. A lot of high-intensity training will create short-term gains. But six weeks later, the gains will stop as the decrease in aerobic capacity outweighs the gains in speed. And then the athlete starts to slow.

As gains slow, stop, and reverse, the novice gets frustrated. He thinks the obvious thing to do is even more high-intensity, so he doubles down on a losing hand. The aerobic system sustains more damage. Aerobic capacity is further reduced, and the decline continues.

The thinking goes like this:

“If high-intensity works, do more! If it doesn’t work, do more!”

Why does low-intensity training make an athlete faster?

In contrast, complementing high-intensity training with low-intensity fixes the problem. Training at an intensity below the aerobic threshold (not the anaerobic) helps in several ways:

• It repairs the damage to the aerobic system from high-intensity training. Low-intensity training regenerates mitochondria (the aerobic engines in our cells). And it does so without eliminating the increase in speed from higher-intensity training;
• Once restored, our mitochondrial mass continues to grow;
• As mitochondrial mass grows, the ability to absorb greater amounts of metabolites grows;
• As absorption capacity grows, the athlete can tolerate more high-intensity training;
• The more high-intensity that’s tolerated, the more race pace training is possible;
• The more race pace training, the better the athlete will perform in a goal event. (Race pace will increase or–better yet–extend.)

More low-intensity training means more race pace training

Chronic, short-term, high-intensity training is a steep staircase that ends in a cliff. You can bound upward, but then you’re faced with a choice: walk back down or jump. A performance decline is the only outcome.

In contrast, a more gradual staircase is more sustainable. A proper, patient mix of high- and low-intensity will maximize your genetic potential. And if you plod up that gradual staircase long enough, what you find hard today will be easy. Your five- and ten-year gains will shock you.

But the gains happen at a much slower rate. Patience is key.