What is the period of a muscle twitch during which the muscle relaxes as Ca2+ re-enters the sarcoplasmic reticulum?

• A. Period of relaxation
• B. Contraction
• C. Wave/Temporal summation
• D. Treppe

Answer: (A)

During a muscle twitch, the relaxation period is the phase when the muscle length returns to its resting state as Ca2+ is pumped back into the sarcoplasmic reticulum. When cytosolic Ca2+ levels fall because the Ca2+-ATPase pumps are actively transporting calcium into the SR, the troponin-tropomyosin complex re-covers the actin binding sites, cross-bridges detach, and the muscle relaxes. How quickly relaxation happens depends on how fast Ca2+ is removed from the cytosol, which ties to ATP availability for the pumps. This phase is distinct from contraction, which is the active shortening driven by cross-bridge cycling; patterns like wave/temporal summation and Treppe refer to how stimuli are applied over time rather than the single twitch’s relaxation phase.