Isotonic contractions are characterized by which of the following?

• A. Limb movement due to muscle shortening or lengthening
• B. Tension without length change
• C. Heartbeat
• D. Muscle contraction without any force

Answer: (A)

Isotonic contractions involve the muscle changing length to produce movement while generating a relatively constant force. When you lift or lower a limb, the muscle shortens (concentric action) or lengthens under control (eccentric action) as the joint moves, with the tension staying about the same. This is different from an isometric contraction, where the muscle tenses but the joint doesn’t move because the muscle length stays the same. The heartbeat is a rhythmic action of the heart muscle, not the general pattern described by isotonic contractions. A contraction that involves no force isn’t a true contraction. So, moving a limb by the muscle changing length best defines isotonic contractions.