During an isometric twitch contraction of a single muscle fibre, the 'series elastic component' (SEC) must be stretched before external force is recorded. Which structure primarily constitutes the SEC in skeletal muscle?

• A Actin and myosin filaments themselves, which deform elastically before force transmission
• B The Z-disc proteins (alpha-actinin, Cap-Z) which store elastic energy during contraction
• C Titin (connectin) filaments within the sarcomere and tendinous/aponeurotic connective tissue of the muscle ✓
• D The T-tubule membrane system which must be depolarised before mechanical force is generated

Explanation

The series elastic component (SEC) in the Hill muscle model refers to the elastic structures in series with the contractile element. The primary contributors in skeletal muscle are: (1) titin (connectin) — the giant elastic protein spanning from M-line to Z-disc within each sarcomere, which stretches during force development; and (2) the extracellular connective tissue, tendons, and aponeuroses (largely type I collagen). During isometric contraction, cross-bridges generate force that first elongates the SEC; only when SEC is stretched sufficiently does force appear at the external insertion. This explains the electromechanical delay between EMG onset and force production. Actin and myosin filaments are relatively inextensible.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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