Question

excitation of the sarcolemma is coupled or linked to the contraction of a skeletal muscle fiber. what specific event initiates the contraction?
voltage-sensitive proteins change shape.
calcium release from the sarcoplasmic reticulum initiates the contraction.
sodium release from the sarcoplasmic reticulum initiates the contraction.
action potentials propagate into the interior of the skeletal muscle fiber.

Explanation:

When the sarcolemma is excited, the action potential travels along the transverse tubules. This causes voltage - sensitive proteins (dihydropyridine receptors) in the T - tubules to change shape. This shape change then causes the release of calcium ions from the sarcoplasmic reticulum. Calcium ions are the key initiators of muscle contraction as they bind to troponin, causing a conformational change that exposes the myosin - binding sites on actin.
Sodium is not stored in the sarcoplasmic reticulum for muscle contraction initiation. While action potentials do propagate into the interior (via T - tubules), it is the subsequent calcium release that is the direct initiator of contraction.

Answer:

Calcium release from the sarcoplasmic reticulum initiates the contraction.