Exercise Physiology/Fundamentals of Exercise Physiology

Overload Principle
If you habitually overload a system, it will respond accordingly by adapting to the demands of the stressor. Continuing to subject a system to progressively higher demands not only triggers adaptive responses but also fosters enhanced resilience, as the system undergoes continuous refinement to meet and even surpass the challenges imposed by the stressors. This iterative process of overloading and adaptation contributes to the system's ongoing development and capacity for handling increased loads.

Individuality Principle
While the physiological response to a particular stimulus are predictable, the precise responses to varying stimuli will vary among individuals and subpopulations.

Specificity Principle
Only the system or body part stressed will adapt. For example, someone will not build muscle in the arm from walking because that body part is not being stressed in that activity.

Reversibility Principle
There reversibility principle, also aptly referred to as the "use it or lose it" principle states: whereas overloading will result in training adaptations, inactivity (also referred to as "detraining") will result in a return to baseline (pre-training levels) of adaptation. Training adaptations are gradually lost with inactivity over time.