What Would Happen If You Never Stopped Doing Push-Ups?

​​This is What Would Happen If, a close examination of mundane hypothetical situations. Each week, we look at something that you could do but probably never would, and take it to its logical endpoint. This week: What would happen if you tried to do infinite push-ups?

Fatigue is a funny thing. We set out to do some kind of physical activity, and sooner than we'd like we're doubled over, muscles aching and gasping for breath. The pain, we think, lies in our stupid muscles, but really it's all in our head.

Anyone who is moderately familiar with exercise, at some point, will think the same thing: What if we could just push past the pain? What might happen if we ignored the lizard part of our brains screaming at us to stop? Would we destroy our bodies or make them stronger?

To answer this we spoke with Russell Tupling, a kinesiology professor at the University of Waterloo, and one of the foremost experts on muscle fatigue. As you might guess, you can try and do push-ups forever, but eventually your body will fail you. But, Tupling explains, this isn't because your muscles are small. It's all in the chemistry.

Before we can dive into why you can't just bang out push-ups until the sun implodes, we need to take a crash course in how muscles work. Biomechanics are funny in that they kind of escape any sort of traditional "engine" metaphor — so bear with me.

To do anything, your muscles require a chemical known as adenosine triphosphate, or ATP. You eat food, the energy in that food is stored in your body, and then your muscle cells use ATP to transfer that energy from your body to your muscles, causing them to contract and thus you move. ATP isn't so much a fuel source as it is a medium for energy transfer, I know this sounds unimportant now, but just keep it in mind for later, when it will be very important. With that in mind, your body has two main ways of converting the energy in your body into ATP and thus into motion: aerobic and anaerobic respiration

I know I said biomechanics escape easy metaphor, but think of aerobic respiration as a car engine, and anaerobic as a "Fast And Furious" nitrous injection. One can go for hours on end at a reasonable speed, and the other can give you a boost for a short time.

So: Aerobic respiration. For the sake of explanation, this is the body's low-speed mode. You breathe air into your lungs, your blood carries oxygen to your muscles, which the mitochondria combine with glucose to produce ATP. Generally, this covers "low-intensity" activity from typing on a keyboard to running a marathon. It's a bit of an oversimplification, but anything up to and including endurance-minded exercise relies on aerobic respiration.¹ 

Once you start to dabble into high-intensity activity², like lifting weights or sprinting and so on, your body will rely on anaerobic respiration. This is the body's "high-speed" mode, and doesn't require any oxygen. 

Here's where that nuanced understanding of ATP comes in handy. When your muscle consumes ATP, it doesn't just go away like a car burning fuel. One of ATP's chemical bonds is severed — this provides the energy for the muscle contraction — leaving you with adenosine diphosphate and a stray phosphate. In aerobic respiration, the mitochondria, with the help of oxygen, provide the energy to reconstruct the ADP into ATP. In anaerobic respiration, the lack of oxygen forces your body to use other methods to convert ADP into ATP and keep your muscles going. The thing is, your muscles only have a limited amount of resources to convert ADP without oxygen, so eventually, this anaerobic respiration just kinda stops.

This brings us back to the push-up. Why can't you do them forever? Well, because they rely entirely on anaerobic respiration. "Not only are you not breathing, but your muscles are already contracted strongly enough that they're squeezing the arteries, which are supplying your muscle cells with oxygen," says Tupling. "So it's almost like you're not getting blood flow to the muscles to get them working. And as a result, the energy supply is almost entirely anaerobic during that type of contraction."

Let's walk through this. The first few push-ups probably seem easy. This is because your muscles are drawing directly from its stores of phosphocreatine to generate ATP. It does this all the time. Normally, when you're doing anything it takes a little time for your circular system to deliver the oxygen your mitochondria crave so your muscles use the phosphocreatine as a buffer energy source until the aerobic respiration kicks in. "You have phosphocreatine in the muscle to last, typically 8 to 10 seconds of really all-out, intense exercise," says Tupling. "Before you run out of the phosphocreatine."

After the first few push-ups, you'll start to feel a burning sensation, and you'll start to breathe harder and harder. This is your body trying to get you to ease off to allow for the aerobic respiration to kick in, but you're not going to ease off. You're going to keep doing push-ups. "It feels more difficult to do, you're producing the same amount of force every time," says Tupling. "But your maximum ability to produce force is getting closer, closer and closer to the amount of force you need to do to just push yourself up off the floor."

The continued demand being placed on your muscles starts to deplete the amount of available ATP, resulting in a buildup of ADP and stray phosphates. Metabolic enzymes outside of the mitochondria are now desperately trying to rejoin the ADP to the stray phosphates, but they can only do so much. Eventually, your muscles run out of the chemical resources to create ATP, and will lose the ability to simply contract. "The force that the muscle has to produce to push yourself up, that stays constant. But when the maximum force the muscle can produce gets closer and closer to the force that's required — that's when you hit exhaustion," says Tupling. "You try to push and you just can't do it."

But even though you just did push-ups to the point of not being able to do push-ups anymore, that doesn't mean your muscles are completely destroyed. "If you stop and wait even 15 seconds, you can go back and probably do another five. As soon as you stop, blood flow is going to the muscle, breathing hard, your aerobic metabolism is increased and you're regenerating that ATP, the phosphocreatine is being resupplied. So the metabolism recovers very quickly. And you can go again," says Tupling. "But the reason why you can't do another 30 is because you've also done some damage to the muscles."

Here's where we get into the good part of doing push-ups. Trying to do infinity push-ups makes you feel like you're going to die, but since you're limited by your muscles metabolic rate — that is the amount of available energy in your muscles to do the thing — it's hard to destroy your body in one go. But if you tried to do infinity push-ups every single day, well, that would end up destroying your body. "So there's some part of fatigue that actually persists for a prolonged period of time that's not metabolic anymore — that's actually structural inside the muscle cell," says Tupling. "When you're lowering your body to the floor, the muscles are actually stretching as they're contracting and you're actually tearing them apart. That takes, sometimes, days to recover."

This tearing sounds bad, but in reality, it's good. If you rest and you let your body repair the tears, it'll tack on a little extra, thus resulting increased muscle mass and, you guessed it, bigger muscles. But that's not all. In response to you trying to do as many push-ups as possible, your body will also make a whole host of adaptations: Increased stores of metabolic enzymes, mitochondrial counts, capillary density, red cell density, blood volume and cardiac output to name a few. This is all in the quest to deliver more energy to your muscles for longer periods of time. Do this enough and your body will be able to do more push-ups in a single go, replenish ATP faster so you can recover and do more push-ups, and increase muscle mass and thus the maximum amount of force your muscles can output.

While you'll never be able to do infinite push-ups — the closest humanity has come is Carlton Williams doing 2,682 in an hour — certainly trying to do so won't hurt.​