Overload

The overload principle in training states that the body will adapt to the stresses placed upon it. The more you do, the more you are capable of doing. This is how all the training adaptations occur in exercise and training. When you stress the body through lifting a weight that the body is unaccustomed to lifting, the body will implement physiological changes that will allow it to complete the work. This concept is applies to cardiovascular training. If you stimulate the cardiovascular system to do work not previously done, it will make changes to the body to be able to handle that task better the next time. This is how people get stronger, bigger, faster and increase their physical fitness level.