It is common for an athlete's resting heart rate to be lower than the average rate for healthy persons. Details on the same, as well as what it means, are provided below. The primary reason athletes engage in heart rate training is to raise their anaerobic threshold. The anaerobic threshold is the point at which the body begins using an abundance of oxygen; thus, it is a measure of an individual's ability to tolerate exercise of a sustained nature. The heart rate of an athlete will usually be lower than that of a non-athlete during rest. This is because the muscles used by athletes require more energy than those not used regularly, and so the body adapts by reducing certain other activities that use up energy. These include movements such as breathing and some blood flow to the reproductive organs. The result is that the amount of energy available to the muscles when they are idle is greater than that for a person who does not use these muscles. Athletes' hearts also tend to be smaller than those of non-athletes. This is because much of the work of an athlete's muscles is done by the heart, and so there is less need for large chambers. Also, since most sports involve some type of activity that requires fast thinking and quick action, it helps if the brain has enough oxygen to function properly. Smaller hearts and brains mean that athletes can perform better over a long period of time.