Intensity

Intensity refers broadly to the amount of effort you put into an activity—whether it is fairly easy or fairly difficult for you to complete the task. The greater the intensity, the greater the training response (body adaptations such as getting stronger), at least to a point. The intensity of cardiovascular exercise determines how hard the heart has to work. The higher your heart rate, the more intense the activity, and we can therefore use our heart rate as a way of monitoring intensity of a cardiovascular activity.

The intensity of a strength activity, on the other hand, is related to how much force is required to move some type of resistance. As the amount of resistance increases, so do the amount of force one must exert and the resultant physiological response. Intensity, like overload, works on a continuum. At the lowest intensity an exercise does not elicit a training response, while at the highest intensity the chance of injury increases, as does the stress on bodily systems.

For example, your heart rate increases when you stand, but the increase is usually so minor that it does not stimulate a training response. If you suddenly jump up out of your chair and sprint to the door, however, your heart rate will probably rise to near maximal levels. Not only could you not keep going at a sprint pace, but you could also injure a muscle or sustain a more serious injury. Optimal training intensities tend to range from moderate to high intensity, and you must balance your workouts with appropriate recovery periods during which your body can make the necessary physiological adaptations.