Which phase of a muscle twitch follows the latent period and involves active cross‑bridges and peak tension?

• A. Contractile period
• B. Latent period
• C. Relaxation period
• D. Tetanus phase

Answer: (A)

Phases of a muscle twitch and when peak tension occurs. After a stimulus, there’s a latent period with no measurable tension as calcium is released and the muscle sets up for contraction. Once calcium is ready for action, the contractile period begins. During this phase, active cross-bridges form and myosin heads pull on actin, producing force that rises to a peak as cross-bridge cycling is at its strongest. After the peak, the relaxation period follows as calcium is pumped back and cross-bridges detach, causing tension to fall. The term tetanus refers to a sustained contraction from rapid, repeated stimulation, not a single twitch phase.