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Updated: Feb 23, 2022

by Jonathon Sullivan MD, PhD, SSC, PBC

You use your aerobic system to recharge your anaerobic system. That takes time.

Rest between sets
Adequate rest between sets is essential.

In a previous post, I waxed all philosophical and existential, using the analogy of the candle-clock to noodle out some musings on being, temporality, and our human potential as Athletes of Aging. I am prone to these attacks of philosophical pretension from time to time.

This week, I’m going to stick with the candle analogy, but I’m going to bring it down to earth, all the way down, to the level of muscle biology and how we train. I am prone to these attacks of full-frontal nerdity from time to time.

I need therapy. Lots of it.

The candle-clock isn’t just a philosophical analogy. We really are candles, engaged in combustion. Combustion is a fancy word that simply means burning. In combustion, a fuel is oxidized by an oxidant, and the oxidant is reduced by the fuel (thus, combustion is a redox reaction).

So, for example, you can use gaseous fluorine to oxidize…well, almost anything. You can make steel burn with fluorine. In this case, the steel is the fuel and fluorine the oxidant. Don’t try this at home, kids.

In common experience, however, fuels are usually hydrocarbon-containing substances like propane, gasoline, wood, or sugar, and the oxidant is usually oxygen. The reaction will yield carbon dioxide and water as chemical products. It will also release energy in the form of heat, light, and, in certain cases, work. This is what happens in your car’s engine: gasoline and oxygen are combined, the reaction is initiated by a spark, and the fuel and oxidant react explosively to produce work (the piston is driven through the cylinder). Carbon dioxide, carbon monoxide, and water are released as exhaust.

Notwithstanding the nitty-gritty chemical and stoichiometric details, the exact same thing is happening in your cells. Glucose and fat (hydrocarbon fuels) are combined with an oxidant (oxygen) to produce energy, water, and carbon dioxide “exhaust,” along with some heat. This process is literally a combustion reaction, although we usually use a different term to distinguish it from the fiery, explosive, inefficient, and “messy” form of combustion observed in campfires, candles, and Cadillacs.

That term is cellular respiration, and it refers to the highly controlled, stepwise, enzyme-mediated process of oxidation in living systems.

As you are probably aware, the energy released by respiration is stored in the form of ATP, and it is ATP that directly fuels all the biochemical processes of our cells, including the movement of muscle fibers. Most but not all of this ATP is produced in the mitochondria, where oxygen is combined with carbs and fat to release their energy and capture it in the form of ATP. A small amount of ATP can also be produced by a simple anaerobic “cracking” of glucose called glycolysis (in which the oxidant is literally a derivative of niacin). This process is inefficient, and releases only a fraction of the energy available in the sugar, but it is very fast, meaning it is useful for high power-output activities.

This raises an important point. Oxidation of biological fuels right down to the carbon dioxide “ash” will produce lots of energy—but not very quickly. This means that “aerobic” oxidation of glucose and fat are not particularly powerful, because power is work done quickly. Glycolysis is less efficient, but more powerful, because it makes ATP fast. But for the most demanding, high-power output activities, we can’t wait to make ATP. It has to just be there.

And it is there. Our muscles have a supply of ATP at hand for instantaneous, high-power contractions. But that supply is very limited, only a few seconds at most. Fortunately, this extremely high-powered phosphagen system can continue for up to 60-90s because of phosphocreatine, a depot of high-energy phosphate that can “recharge” ATP on the spot, as long as the phosphocreatine lasts.

In other words, between the small amount of ATP already sitting in your muscle, your limited pool of phosphocreatine, and some help from glycolysis, you can generate the high power output you need to get through a heavy set of deadlifts or presses. If you don’t dawdle.

But after a minute or two, you’re going to be cooked. This is why you can run all afternoon, but nobody in their right mind does deadlift marathons. Low-power repetitive movements like running or pedaling can rely on combustion, but the “anaerobic” demands of heavy lifting are a very different thing, far beyond the capacity of aerobic metabolism (combustion) to support.

So you have a powerful, instantly accessible system that stores energy to function like a supercharged battery, paired to a high-capacity, low-power system that burns fuel like an engine. And just like in your car, these two systems are integrated. Your battery has the juice to start your car and turn the engine over…but not for very long. Your engine can't start itself, but it has the capacity to run all day—and to recharge the battery in the process.

This is exactly what is happening in a workout. Allow me to use nerdity to illustrate. If you engage in a high-intensity effort, like a set of squats, you will significantly deplete the amount of phosphocreatine in your muscles. The rate at which that “anaerobic fuel” is restored is called the kPCr, the phosphocreatine rate constant. But the kPCr is a measure of the tissue’s aerobic efficiency, not of its anaerobic capacity.

You use your aerobic system to recharge your anaerobic system.

By now, maybe you’re wishing I’d go back to the existentialist babblings. But there’s actually a point here, and the point is exactly this: If you find that your car battery is almost dead, and you somehow have just enough juice in it to start the motor, what do you do? Unless you’re an idiot, you leave the motor running. You go back inside for a coffee or you drive around for 30 or 40 minutes. You let the combustion in the engine recharge the battery…and that takes time.

You have to do the same thing in the gym. If you do a maximally heavy set of five, wait just long enough to emit a self-congratulatory grunt, scratch yourself, briefly ogle another lifter, and then try to go again, you are setting yourself up for failure, ridicule, ostracism, and shame. All because it has somehow escaped your notice that your muscle kPCr is not infinitely fast, meaning you do not instantly restore phosphocreatine and ATP to your tired myocytes.

You have to wait. How long? Long enough. That usually means at least 4-5 minutes, and almost always longer than that. The heavier you lift, and for more reps, the longer you have to rest. This cycle is a sort of reflection of the stress-recovery-adaption regime. Just as we must recover after a heavy workout so that adaptation and an increase in strength and tissue growth can occur, so also must we rest between sets, to allow our aerobic mitochondrial engines to re-charge our cytosolic anaerobic batteries.

And so it is that one of the most critically important pieces of training equipment in the gym or in your workout bag is the humble kitchen timer. Use it.

Jonathon Sullivan MD, PhD, SSC, PBC is a retired emergency physician and research physiologist, and the owner and head coach of the Greysteel Strength and Conditioning Clinic in Farmington Hills, Michigan, which specializes in training adults over 50. He is the author of The Barbell Prescription: Strength Training for Life After Forty, with Coach Andy Baker.

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