The principle of resistance training that suggests that muscles should be gradually required to do more than they are used to doing is
a. duration.
b. tension.
c. specificity.
d. overload.

The principle of resistance training that suggests that muscles should be gradually required to do more than they are used to doing is D. OVERLOAD.

The principle of overload states that a greater than normal stress or load on the body is required for training adaptation to take place.

Overload refers to the amount of load or resistance, providing a greater stress, or load, on the body than it is normally accustomed to in order to increase fitness.