Training intensity zones are foundational to a training program because each zone targets a different training effect. This allows you to arrange workouts based on the particular training effects you want to achieve.
Hans Selye, a Hungarian biologist who worked around the middle of the twentieth century, outlined a model that underlies the training process. When you train, you introduce a stimulus, or “stress” to your body. This is followed by a “response” from the body which leads to a physiological “adaptation.” Selye called this stress-response-adaptation process the general adaptation syndrome (GAS).
The work you do during a training session breaks down the body, followed by a recovery phase during which the body rebuilds stronger than before. As a result of this process, you gain fitness, or the ability to perform faster and longer than before.
The overload principle states that any new training gain requires an appropriate training stimulus that is greater than the amount of training stress to which the body is currently adapted. Just as the name of the principle implies, you must “overload” the system to bring about a response and adaptation (after that all-important recovery phase). Keep in mind that the training stimulus must be appropriate. It needs to be an appropriate stress to ratchet up your fitness level incrementally, rather than a stress that completely overwhelms the system and leads to overtraining or injury.
Training stress is associated with your “training load.” Your training load consists of both training volume and training intensity.
Training volume is determined by the total duration (or distance) of your workouts. Volume is easy to measure by simply adding up the duration (or distance) of each training session. One can then talk about weekly training hours (or distances), monthly training hours (or distances), a season’s training hours (or distances), etc. But training requires more than simply logging time (or distance). You also need to take into account the intensity level of the training you do.
Training intensity refers to the effort level put forth during your workouts. Whereas volume refers to the quantity of your training, intensity deals with the qualitative nature of your training. Does that workout interval feel “easy” or “hard”? How fast are you running that interval? What is your heart rate during the interval? How much power are you producing during the interval? As indicated by these questions, intensity can be measured in a number of ways.