Which statement identifies the primary signaling event caused by repeated muscle contractions?

• A. Release of calcium
• B. Increase in lactate
• C. Decrease in NADH
• D. Activation of insulin receptor

Answer: (A)

The key signal involved in initiating muscle contraction is the rise of calcium in the cytosol. When an action potential travels into the muscle fiber, calcium channels in the sarcoplasmic reticulum open and calcium floods the cytoplasm. This calcium binds to troponin C on the thin filaments, causing a shift in the regulatory proteins that exposes the myosin-binding sites on actin. With calcium present, cross-bridge cycling can proceed, producing contraction. During repeated contractions, this calcium release and reuptake cycle keeps driving each contraction in sequence.

Lactate increase happens as a metabolic consequence of glycolysis under limited oxygen, helping regenerate NAD+, but it isn’t the primary signal that starts contraction. Changes in NADH levels reflect metabolic redox state rather than the trigger for contraction. Activation of the insulin receptor is a distinct signaling pathway; contraction can promote glucose uptake through other mechanisms, but it is not the immediate signal that initiates muscle contraction itself.