The body mass index, or BMI, is a key index that relates to a person's height and weight.

However, if you are an athlete or generally active person, your body weight may be high not because of fat but because of muscle. Muscle is denser than fat and an abundance of muscle means more weight and therefore a higher BMI. But in such a situation, it is not unhealthy because it does not have the associated health risks that come with a high BMI reading.