What is the contraction type where tension rises to a maximum and contractions fuse into a smooth, sustained plateau?

• A. Fused (complete) tetanus
• B. Incomplete tetanus
• C. Isotonic contraction
• D. Isometric contraction

Answer: (A)

Rapid, high-frequency stimulation of a motor unit causes the individual muscle twitches to fuse into one continuous contraction, so tension rises to a maximum and remains smooth and constant. This is fused (complete) tetanus. The key idea is that the stimulus is frequent enough that the muscle doesn’t have time to relax between twitches, so calcium stays elevated and cross-bridge cycles keep happening, producing a steady plateau of force. Incomplete tetanus occurs at a lower frequency where some relaxation between contractions happens, so the tension waxes and wanes. Isotonic contractions shorten the muscle while the tension changes as it overcomes a load, and isometric contractions generate force without changing the muscle’s length.