During an isometric contraction, a muscle develops maximum tension at its resting length (Lo). This length-tension relationship is explained at the sarcomere level by:

• A At Lo, titin (connectin) elastic element is at maximum stretch, contributing maximum passive force
• B At Lo, actin and myosin filament overlap is optimal, maximizing the number of cross-bridges that can form between actin's active sites and myosin heads ✓
• C At Lo, calcium release from sarcoplasmic reticulum is maximal because T-tubule geometry is optimal
• D At Lo, troponin-C has maximal calcium affinity, allowing more cross-bridge cycling

Explanation

The length-tension relationship in skeletal muscle is directly explained by the degree of actin-myosin filament overlap. At resting (optimal) length Lo, the sarcomere length is approximately 2.0-2.2 µm, maximizing overlap between thin (actin) filaments and the cross-bridge bearing portions of thick (myosin) filaments. This maximizes the number of simultaneously cycling cross-bridges and thus active tension. At lengths shorter than Lo, actin filaments from opposite Z-discs overlap and interfere with cross-bridge formation. At lengths longer than Lo, reduced filament overlap leaves fewer actin active sites accessible to myosin heads. Titin contributes passive tension at very long sarcomere lengths but not at Lo. The T-tubule geometry and calcium release kinetics do not primarily determine the length-tension curve.

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

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