Which term is NOT a basic type of muscle contraction?

• A. Isotonic
• B. Concentric
• C. Eccentric
• D. Isometric

Answer: (A)

The key idea is how muscles change length during contraction. The three basic ways are isometric, where the muscle activates but doesn’t shorten; concentric, where the muscle shortens as it contracts; and eccentric, where the muscle lengthens while still producing force. Isotonic describes a dynamic situation—movement against a constant external resistance—where the muscle changes length, but it’s not a single, standalone mode of contraction. In practice, an isotonic action includes both a shortening phase (concentric) and a lengthening phase (eccentric) during the movement. Because isotonic isn’t one of the fundamental contraction modes on its own, it’s the term that isn’t a basic type of muscle contraction.