In determining an optimal level of stress, it is important to consider the constructs of the progressive overload principle. As a result of stress, fitness levels will improve. As fitness levels improve, an increase in the intensity of the stress should follow. Coaches and athletes have tremendous control in determining the effectiveness of training programs. Intensity, duration, and frequency are variables that, when intelligently administered, can substantially influence adaptation.
Intensity is arguable the most important of the three factors of the progressive overload principle.
Determining the appropriate level of intensity is always a challenge. If the difficulty of a training session is extreme while recovery time is limited then the athlete will experience overtraining which can result in a fatigue oriented injury. Conversely, if intensity is low and recovery time too great then there will be limited gains in adaptation. Intensity must be directed toward the athletes needs and specific to the requirements of the sport. If the intensity level is high the duration will be short.