Refers to placing demands on the body systematically and progressively over time to cause physiologic adaptation.

Overload.

Explanation:

Overload may be defined as the second most important principle that main aim is to improve the physical fitness of the individual. Overload improves the strength of the individual.

Different muscles are involved in the process of overload. Overload can be achieved by lifting the heavier objects for few time. The stress on the muscle is applied during the overload. The continuous overload changes the physiologic adaptation of the body.

Thus, the answer is overload.  

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