The correct answer is D. Muscle adaptation principle.
Explanation
The principle of muscular adaptation is the name given to the muscular capacity that allows the muscles to adapt to a training load determined by intensity, volume and rest and to show an improvement, which becomes into a feeling that indicates that the activity is easier for the body after some time. According to the example of the question "After lifting weights for two weeks, Joe finds that the weights have been lifting do not feel very heavy to him anymore" is an example of the principle of muscular adaptation, because his muscles have developed resistance to the weight that he has been lifting for two weeks and that is why he feels that it is easier to lift the same weight, so it would be necessary to increase the load so that his muscles are tested again. So, the correct answer is D.
