Identify a primary signaling event that occurs due to repeated muscle contractions.

• A. Increase in pyruvate
• B. Activation of AMPK
• C. Decrease in calcium
• D. Release of calcium

Answer: (D)

Repeated muscle contractions initiate a rise in cytosolic calcium, released from the sarcoplasmic reticulum into the muscle fiber. This calcium surge is the primary signal that couples electrical excitation to contraction, allowing calcium to bind troponin and enable actin-myosin cross-bridge cycling. Beyond driving immediate contraction, the calcium signal also acts as a second messenger for training adaptations, activating calcium-dependent pathways (like CaMK and calcineurin) that promote changes in gene expression and mitochondrial biogenesis when contractions occur repeatedly. The other options don’t fit as the initial signaling event: pyruvate increases are a glycolysis byproduct rather than the immediate signaling trigger; AMPK activation arises from energy stress over time rather than the direct contraction signal; a decrease in calcium would hinder contraction, not accompany it. Hence, release of calcium best explains the primary signaling event.