Which statement best distinguishes isotonic from isometric contractions?

• A. Isotonic contractions cause the muscle to lengthen under constant tension.
• B. Isometric contractions shorten the muscle.
• C. Isotonic contractions change muscle length with constant tension.
• D. Isometric contractions generate movement at a joint.

Answer: (C)

Understanding how muscles contract helps tease apart isotonic from isometric. Isotonic contractions involve the muscle changing length while producing a relatively constant level of tension, allowing movement of a joint. This includes both concentric actions, where the muscle shortens, and eccentric actions, where it lengthens as it controls a load. The statement that best distinguishes isotonic contractions is that they change muscle length with constant tension, which captures both directions of length change during movement. In contrast, isometric contractions generate force without any change in muscle length, so there’s no joint movement, and the other descriptions either limit isotonic to lengthening or claim movement that isn’t characteristic of isometric contractions.