The _____ principle refers to the need to exercise an organ system beyond its accustomed level to elicit a training adaptation.

• A. Reversibility
• B. Overload
• C. Specificity
• D. Progression

Answer: (B)

Overload is the principle that to elicit a training adaptation you must push an organ system beyond its accustomed level. When you raise the stimulus—whether by increasing intensity, duration, frequency, or resistance—you create a demand the body must meet, leading to adaptations such as greater mitochondrial density, improved capillarization, and enhanced metabolic enzyme activity. If the workload stays the same, the body settles into a steady state and no new adaptations occur.

Progression is related but describes how the overload is increased over time to continue gains; specificity refers to adaptations tailored to the exact type of training, and reversibility concerns the loss of adaptations when training stops. The requirement to exceed the accustomed level to trigger adaptation is the overload principle.