The aerobic threshold is the point where exercise intensity increases enough that the body can no longer supply enough oxygen to cover its total fuel utilization.
(The anaerobic threshold is totally different: it is the point where the rate of anaerobic activity exceeds the body’s ability to keep it in check.)
As work rate increases, the body’s big muscles (let’s call these “exercise muscles” for short) start working harder and harder, increasing the fuel and oxygen demand. More oxygen has to be pulled in, and so the breathing muscles—diaphragm, muscles around the ribcage and various shoulder muscles—also have to work harder.
But these breathing muscles have their own oxygen demand. So the more they (and the exercise muscles) work, the more the overall demand for oxygen rises.
This can’t go on forever: if the overall demand of oxygen rises, the breathing muscles have to work even harder to meet it. What’s happening? The harder the body works to meet its oxygen demands, the greater they become.
At the point at which these demands start to rise in tandem, it’s essentially impossible for the body to cover all of its fueling needs with oxygen. The body then has to start consuming fuel without mixing it with oxygen (also known as anaerobic activity).
This means that there’s a sweet spot just before this tandem increase where the exercise muscles are working somewhat hard—but not hard enough that the breathing muscles have to work significantly harder and dramatically increase their own oxygen demand.
This sweet spot is the aerobic threshold.
Note that this is nowhere near the maximum ability of the body to supply and utilize oxygen (also known as VO2Max). The body can still increase the breathing rate, expanding and contracting the lungs much more and much faster, and it can still increase the heart rate to pump oxygen-laden blood everywhere it needs to go.
Conceivably, it could be pulling in and transporting oxygen at twice the rate (and using it to burn twice the fuel). But doing so would itself have extraordinary oxygen requirements. The body can’t approach its VO2Max and still be spending less oxygen than it is requiring (a.k.a. still aerobic).
The aerobic threshold is critical from a physiological perspective—much more important than the anaerobic threshold, for example—because it is the point where the body has the first reason to worry about its oxygen supply. Anywhere from resting to the aerobic threshold, the body is A-OK. It can essentially continue doing whatever it is doing ad infinitum: it has enough fuel and oxygen going to all of its systems (brain, organs, muscles, etc.) that they are at no risk of shutdown.
Above the aerobic threshold, the game changes: the further you get, the less the body is able to sustain that rate of activity indefinitely. The further above the aerobic threshold, the farther the body is from the conditions that allow it to remain alive over the long-term (e.g. oxygen to cover its entire fueling needs).
Above the aerobic threshold, you’re on Everest. You’re in the Death Zone. You literally don’t have enough oxygen to just keep on doing whatever you’re doing. It’s OK to be up there for a while (and there’s benefits to doing so), but if you don’t come down, you’re gonna die.
(And just like on Everest, if you go up there more often than you can recover from, you’re going to get sick).
If the body is doing anaerobic stuff too often, it just won’t be able to recover—and keep on breaking down.
Training under the aerobic threshold has all kinds of benefits that you just can’t get training above it. To stay under the aerobic threshold, the body has to be able to bring oxygen all the way into the muscles to cover every bit of its energy demands.
While this may seem too obvious to mention, it actually hides a critical point: in order for oxygen to make it all the way into the muscles, it has to get handed down a long ladder of systems, organs, and processes. The lungs have to fill up, and they have to hand the oxygen off to the red blood cells in the bloodstream, which then have to get pumped through the bloodstream and through the capillary networks into the muscles.
(This chain is the aerobic system).
The amount of oxygen that makes its way down to the muscles at any given time is determined by the weakest system in the body (and not the strongest). Let’s say that the lungs have capacity for lots of oxygen, and the red blood cells can carry all that oxygen, and the heart is powerful enough to pump all that blood around the body, but the breathing muscles aren’t very strong. It won’t matter how big the heart or the lungs are, or how much red blood cells are in the body. The body will have to drive those lung muscles extremely hard in order to get the oxygen it needs. (And its oxygen requirements will go up, and boom it’s above the aerobic threshold).
The more you train under the aerobic threshold, the better the body gets at handing oxygen from the nose all the way down to the muscles.
In technical terms, this means that aerobic training strengthens the vertical integration of the body’s aerobic system—“vertical” as in all the way up and down the oxygen ladder.
If there’s a really strong part, it won’t develop much until the weaker parts (that were constraining oxygen flow) catch up. So aerobic training really evens out the body in terms of its ability to transport and utilize oxygen.
Anaerobic training does exactly the opposite. Anaerobic activity literally starts because the body’s rate of exercise exceeded the oxygen transport capabilities of the weakest system.
Training anaerobically means that you’re committing to run the body harder than it has the oxygen for. So the systems that are already strong enough to take the body above the aerobic threshold get stimulated. They get trained, and they get even stronger, while the other systems lag behind. The asymmetry grows, and the athlete grows less resilient, not more.
If you keep doing this too long and too often, and without making sure to train the vertical integration of the aerobic system, you’ll eventually train yourself into a situation where the least capable part of the body gets neglected, and the most capable part of the body gets powerful.
And since that less-capable part was a critical piece of the oxygen puzzle, the body’s ability to use oxygen remains exactly that neglected.
When the whole oxygen chain is strong, aerobic training is awesome. But when there’s one really weak part, aerobic training can be super slow and super boring. But that’s also why it’s so important.
PS: In my next post I’ll discuss how muscle mitochondria—the body’s tiny aerobic motors—relate to the aerobic threshold and to aerobic training in general.