What is most directly responsible for the coupling of excitation to contraction of skeletal muscle fibers?

The calcium ions.

Explanation:

The most directly responsible action for the excitation or contraction of skeletal muscle fibers is the flow of calcium ions. The calcium ions that are stored in the sarcoplasmic reticulum are released upon the stimulation of the muscle and allow it to contract.

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Calcium ions.

Explanation:

Calcium ions are present in the sarcoplasm reticulum and are directly responsible for the coupling of excitation to contraction of skeletal muscle fibers. Propagation of action potentials along with sarcolemma causes a voltage sensitive protein to alter the shape and this alteration releases calcium ions by opening the calcium releasing ion channels to flood the sarcoplasm.

So, the calcium ions are directly responsible for the coupling of excitation to contraction of skeletal muscle fibers.