Does exercise lead to a bigger heart?

Chris - Well actually, it does cause a bigger heart and it can happen for both good reasons and bad reasons. If you have a very high workload on your heart because you, for instance, have high blood pressure all the time, then you can develop cardiac hypertrophy and this is where the muscle of the heart becomes thickened, bigger, and beefier in order to do more work and overcome the extra load applied to the heart by the high blood pressure. Now, that's a bad thing, obviously; sometimes the muscle can become so big that, in fact, it has an oxygen demand which is bigger than the coronary arteries are capable of delivering via the blood. So, people can get angina - pain in their chest - caused by the heart not getting enough oxygen - because the heart has become pathologically large.

More normally though, people who do a lot of exercise along the lines of what you've been describing, Olympic athletes for example, footballers for example, you'll often hear them say that their resting pulse rate is really low. They might say, for example, that, compared with a normal person who has a resting pulse rate of say, 70 beats per minute, their heart beats only 40 or maybe in some cases, 30 times per minute. Now the reason that can get away with beating more slowly is because their heart has become larger in response to their regular training and the demands they place upon it. And that means that, because it's bigger, it's working more efficiently. With every beat of the heart, it actually pumps out more blood than a smaller heart in an untrained person. So, as a result, they don't have to make their heart beat as often because every time it does beat, it pumps out two beats worth of blood, so their pulse rate can come down. And, if you measure the maximum output from their heart, you'll find that their maximum output can be say, 30 litres in a minute when they're working really hard, compared with say, 25 litres in someone who's unfit and untrained. However, the maximum number of beats their heart can make in a minute will be lower than an unfit person because a bigger heart takes longer to fill up with blood, so they have to slow down their pulse rate and they just make their heart eject more. This form of cardiac enlargement is not thought to be bad for you. It's a physiological response to increased workload and, in the short term at least, we think that it isn't harmful.