How and why your body changes when you start running
Whether you believe running is an art or a science, having an insight into how it affects the body can make those tough sessions seem a little more palatable…
Imagine you are just setting off for a run. Your heart rate (the number of times it beats per minute) and stroke volume (the amount of blood pumped out by your heart per beat) both increase, in order to feed the working muscles with lots of oxygen. Oxygen is transported in the blood – and the amount of oxygen-rich blood that flows out of the heart every minute is called your cardiac output (CO). An increased CO is one of the most important changes that will take place as a result of your training.
Why? Because being able to pump out more blood per minute means the heart doesn’t have to beat so many times as it once did in order to deliver the same amount of oxygen. That’s also why your resting heart rate drops as you get fitter. To put it in practical terms, if running a nine-minute mile took your heart rate up to 160bpm before, after a few weeks of regular training, that same pace might only elevate it to, say, 140bpm. You’ll only get your heart rate back up to 160bpm if you run faster. But the great thing is, it won’t feel any harder, as your fitness level has gone up a notch.
The oxygen-rich blood travels through a vast network of tiny capillaries, which are delicate enough to allow the exchange of gases, nutrients and waste products. Once it arrives at the muscles, they pick up the oxygen, offload some carbon dioxide and it makes its way back to the heart. But the muscle cells don’t take all the oxygen that the blood is carrying. In fact, your muscle cells’ capacity to extract oxygen from blood is one of the critical factors in your running performance. And the good news is that regular running actually increases your oxygen extraction capabilities, by triggering the growth of more capillaries in the muscle, creating a larger surface area for oxygen to be absorbed through. The average non-runner has 3-4 capillaries per muscle fibre while a well-trained runner might have 5-7 per fibre.
The maximum rate at which oxygen can be extracted from the air and used by the muscle is that mysterious Vo2 max you often hear runners talk about. It is partly determined by your sex (women have a lower Vo2 max than men at all levels) genetics and age, but it will almost certainly increase as you get into the swing of regular running. As a general example, a sedentary woman may have a Vo2 max of 35 ml/kg/min while a highly trained man may be closer to 60 ml/kg/min.
But what’s the big deal about all this oxygen, anyway? Well, when enough oxygen is flowing through the bloodstream to meet energy needs, the ‘powerhouses’ of the muscle cells, (the mitochondria), are able to use it to produce energy from the breakdown of a special substance called adenosine triphosphate (ATP). Since the body can only store enough ATP to last for approximately two seconds, it has to be continually broken down in order to sustain any form of activity. But when there isn’t enough oxygen coming through to meet demand, the muscle cells have to make ATP without oxygen, or anaerobically.
This is far less efficient when it comes to an endurance activity such as running, as it results in the accumulation of lactic acid in the muscles. The lactic acid is removed, but if it is produced at a faster rate than it can be taken away, you cross what is known as the ‘lactate threshold’. Physiologically, the lactate threshold is the last point at which lactate is being removed as fast as it is being produced. It’s that feeling when your legs are like concrete, your stomach is churning and your inner voice is screaming ‘STOP!’
Although this point is reached all too soon when you’re a novice runner, your aerobic capacity will soon improve, pushing up the threshold point (closer to your Vo2 max) and allowing you to work at higher intensities without it feeling so tough, and without the negative effects of anaerobic metabolism interfering with your performance.
A number of factors play a part in this change. Firstly, as you become fitter, the number and size of mitochondria increases to cope with the higher demand for energy production.
Like any muscle, the heart – when faced with all that extra work – gets stronger (the left side of the heart, which pumps blood out, can actually get bigger) – and believe it or not, the amount of blood in your body also increases, particularly the volume of red blood cells, which act as the oxygen ‘courier’.
Another bonus of regular aerobic training is that it teaches the body to use fat as its energy source, instead of carbohydrate. Sounds good? It is, and not just because utilising fat means you’ll have less of it clinging to your thighs and tummy (not to mention your heart) but also because it allows precious glycogen, the body’s stored form of carbohydrate, to be ‘spared’ or saved. Since we can only store a limited amount of glycogen in the body, it’s a good thing to hang on to it where possible, and use fat, which is usually available in unlimited supply! Incidentally, as you improve as a runner, the amount of glycogen you can store will also increase substantially. To read about a sports drink that will help convert your body fat into a source of energy click here
So there you have it. I think you’ll agree that you don’t need to be a scientist to appreciate the host of positive changes that take place when you become a real runner…