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The Phsyiology of Running: Understanding Why You're Tired and How Your Body Adapts

Updated on March 3, 2013
A group of runners halfway through a 1500m run during an invitational meet at TCNJ.
A group of runners halfway through a 1500m run during an invitational meet at TCNJ. | Source

The girls in the picture to the right are all college athletes. They're halfway through running a 1500m race; that's just under a mile. Oh, and they're going to complete that race in about 5 minutes.

Think you can do that? If you're an average, ordinary person, then probably not. There's no shame in that. You're simply not trained for it.

If you do want to get in shape and be able to run longer, farther, and faster, then it would help to understand a bit of the physiology behind running. You see, your body gets tired, but it gets tired in different ways and for different reasons. A proper training regimen - whether you're a college athlete or a weekend warrior - needs to address each of those issues if you're going to be able to run comfortably at your target pace.

When you're running, different parts of your body are working together. Your heart is pumping blood throughout the body, your lungs are taking in oxygen, and your muscles are trying to convert that oxygen into energy so that your legs can move. We're going to look at each of these components of physiology separately.

The Role of the Cardiovascular System in Running

The purpose of your heart is to move blood throughout the blood. This blood carries oxygen to your muscles, and it then allows waste products to move out of your muscles.

When you're sitting at rest, your heart will beat at a leisurely pace of 60 to 70 beats per minute. There just isn't much demand for the oxygen that blood brings to your muscles.

Once you start exercising, though, you'll notice your heart rate increases. Everyone has experienced this at some point or another. Go for a quick job, and within a couple minutes your heart might feel like it's about to jump out of your chest.

Why? Well, once your legs start working overtime they need more oxygen to convert into energy. There's a bottleneck in the system - the delivery system. The blood needs to flow more rapidly, so your heart needs to beat faster.

The good news is that your heart can be trained like any other muscle, and it will actually improve in performance very quickly. With only a couple of weeks of sustained exercise, your heart will beat stronger.

This allows you to exercise for longer periods of time without stopping because you think your heart is going to explode. You'll also notice that your resting heart rate will drop.

This is one of the first and easiest adaptations that your body makes. The first couple weeks of an exercise routine can be very difficult, but if you keep with it long enough for your cadiovascular system to strengthen itself you'll feel much more comfortable. The key is to run at a low intensity so that you can stress your heart without stressing the other parts of your body.

The Role of the Respiratory System in Running

Once you get your heartbeat under control, the next thing you'll realize is that you start breathing very hard. This is especially a problem when you exercise at a high intensity, like sprinting a short distance or trying to run a long distance at too quickly of a pace.

In fact one of the biggest mistakes beginning runners make is trying to run quickly. Your run will be much more comfortable if you maintain a slow, "conversational" pace than one that pushes your breathing to be more rapid.

What's going on here is that your muscles don't have enough oxygen. In this case, however, it's not your heart's fault. It's pumping perfectly fine and delivering blood. There's just not enough oxygen in the blood to keep your muscles supplied, and your muscles aren't taking that oxygen up rapidly enough.

So your lungs kick into overdrive and force you to take deeper, more rapid breaths. Eventually, you reach a stopping point. Your muscles are so depleted of oxygen that your brain more or less forces you to stop so that you can catch your breath and replenish your stores.

Running experts often talk about "oxygen debt," and this occurs when you run quicker than a certain pace, which we'll cal your VO2 max. If you run at or below your VO2 max, you'll stay in a kind of equilibrium, and breathing shouldn't be too much of a problem. But as your pace quickens, your breathing will also quicken until you have to stop.

Unlike your cardiovascular system, this takes much longer to adapt. If you push yourself and run at moderate to high intensities, you can slowly force your body to adapt and take up oxygen more efficiently. The result is that you can run at quicker paces over time, but it takes a lot of work and effort to get there.

Glycolysis Explained

Finally, Good Old Fashioned Fatigue

Besides an increased heart rate and an increased breath rate, the other symptom of "being tired" is good old fashioned muscle fatigue. If you're in good enough shape to run a decent distance, then eventually you're legs will feel tired and dead even if your heart feels fine and your lungs feel fine.

This happens as you use up the energy available in your muscles and draw down on their stores of sugar. During extended periods of moderate to intense exercise, your muscles will eventually fall back on a chemical reaction that reproduces glycogen (sugar). Your muscles use oxygen to transform this sugar into the energy required to move your legs. No sugar, no movement.

This chemical reaction has a side effect - the creation of lactic acid. At low intensities, this lactic acid is recycled in the chemical reaction. There's an equilibrium point, called your lactate threshold. Once you start working at an intensity higher than that, you begin to accumulate lactic acid in your muscles.

The most direct symptom of this is that your legs feel... dead. That's because they've pretty much exhausted their back-up system for energy production, and you've got nothing left in the tank.

Your muscles can be trained to handle this cycle more efficiently more effectively. Just like improving your respiration, you need to run at moderate intensities to help your body adapt to extended periods of exercise. Essentially, you push your body past it's lactate threshold, and slowly it improves... allowing you to run farther and faster.

Beginner's Running Regimen

Putting It All Together: A Training Regimen

You might think that to get better at running, you need to just get out there and run. To some extent, that's true. For a true beginner, it helps to just get out and move around.

But, if you understand a bit about the physiology of running you can more effectively build a training regimen for yourself. Track coaches and exercise scientists have made this pretty sophisticated, and the average person doesn't need to know all the details.

But you do need to recognize why you're tired - because your heart isn't pumping fast enough, because you're not getting enough oxygen, or because your muscles are overworked. You can then determine what you need to train, and prevent one problem from getting in the way of fixing a different one.

For example, the first thing you should do is jog at a low intensity until your heart adapts. If you start running at a quick pace, then your lungs will give out within a couple minutes. You won't be able to stress the heart long enough to force it to adapt.

Likewise, if you're trying to improve your respiration, you don't want to run at a slow pace. If you don't push your body to take up oxygen more quickly and more efficiently, it won't. Your body is, essentially, lazy. It'll only adapt to the demands that you place on it.

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