My favorite kind of questions are those that most people just never think to ask, questions about things we take for granted and so never ask why it has to be so.
When I ask those kinds of questions it often turns out that what I took for granted is actually wrong. I love that! And even if my understanding was correct I usually learn a lot by asking the question.
So, here is a question I asked recently, “Why do I breathe harder and my heart beats faster when I do something strenuous?”
At first glance the answer is obvious. I’m working hard I need more oxygen and more blood flow.
So far, so good. When our muscles do work they need energy. The “fuel” our muscles burn is a chemical called ATP. As you may have read in my last post, we produce ATP (mostly) through an aerobic process, that is to say, a process that requires oxygen. So when we first start a strenuous activity of any kind, our muscles send a signal demanding more oxygen. The increase in heart rate and breathing harder is our body’s response to that increased demand for oxygen.
So, it stands to reason, that if my heart and lungs get fitter, I’ll send more oxygen to my muscles. My muscles will have more energy and I’ll be able to do more of whatever.
Not so fast! Turns out, that’s not quite true for a couple of reasons. We actually almost always have a lot more oxygen than we need.
1) Oxygen is transported to our muscles via hemoglobin. Hemoglobin naturally attracts oxygen so it’s great for transporting oxygen to energy hungry muscles. What it’s not so great at is releasing the oxygen once it’s reached the muscle. We transport a lot more oxygen than we use because greedy hemoglobin is reluctant to give up the goods.
There is a metabolic adaptation called the Bohr effect that changes the structure of hemoglobin so it gives up oxygen more readily.
2) The aerobic part of energy production takes place in the mitochondria of your muscle cells. In order for it to happen, the oxygen from your lungs must meet up with the energy you consume from food and they must meet inside the mitochondria. Oxygen has no problem entering the mitochondria but the energy from food must be transported into the mitochondria by an enzyme called pyruvate dehydrogenase . This enzyme is rate limted, i.e. no matter what you do it will only do so at a fixed rate.
This means that no matter how much oxygen you are sending to your muscle cells, the speed at which the cell will be able to create energy is always at the same . You can’t train this through exercise. Chemistry doesn’t care how hard you work.
3) Your muscles fibers activate and then recover. Some do so pretty quickly, a few seconds. Some require days of recovery. No matter how much oxygen you consume or how much blood you pump, your muscle fibers won’t fire again until they have recovered.
The only way to get a given muscle fiber to do “more” is to make the muscle fiber stronger so it accomplishes more work per activation. You cannot increase the frequency of activation.
My racing heart and panted breath is all wasted if they don’t result in my being able to do more. Aerobics or “cardio” are exercise designed to get the heart to beat faster and lungs to work harder but that is only part of the puzzle. You must also address the processes I’ve explained above,
- • make the oxygen more readily available to the muscle cells,
- • you must deliver the energy from food to the cells at a speed that maximizes the oxygen consumed (or find an alternate way to produce the energy, more about that in a future post) and
- • make the muscle fibers stronger so more work is done per activation.
And here is the kicker, if you can do those things, the signal your body sends to accelerate your breathing and heart rate, will never get sent and you can skip that unpleasantness all together.
I promise all this science is going somewhere. Stay with me and I’ll explain very soon how to exercise less and get fitter.