High intensity interval training is recommended to athletes twice per week and involves short bouts of exercise which elevates the heart rate over a 10-20 minute period. It can be used in any form of exercise such as sprinting, cycling hills or swimming but also involves the use of kettlebells and other equipment. Periods of high intensity usually lasting 30s-60s are followed by periods of recovery usually lasting 30s-90s. These are usually repeated 5-8 times until the session is over. This form of exercise is excellent for increasing overall fitness and building muscle mass.