The principal difference between isometric and isotonic exercises is the
a. use of free weights and weight machines
b. rate of calories burned during exercise
c. change in muscle length during exercise
d. type of muscles exercised

The answer is C. They may sound similar but they are totally different. The prefix iso- signifies being uniform or equal. It is in the meaning of the root words these exercises differ. Isotonic exercise means doing equal resistance while changing the length of the muscle. On the other hand, Isometric exercise signifies doing equal resistance but the muscle should stay at a fixed position.