A weightlifter lowers a heavy barbell slowly while the biceps actively resists the descent. Compared with lifting the same load concentrically, this lengthening contraction generates greater force per active motor unit and preferentially damages which structure?

• A The sarcolemma and associated Z-line elements of the sarcomere ✓
• B The neuromuscular junction presynaptic terminal
• C The sarcoplasmic reticulum calcium pump exclusively
• D The myelin sheath of the innervating motor nerve

Explanation

Eccentric contractions develop high tension because fewer motor units carry the full load while cross bridges are forcibly stretched and mechanically disrupted. The damage falls on the sarcolemma and Z lines, producing delayed-onset muscle soreness with elevated creatine kinase one to two days later. Neural structures are intact, and SERCA dysfunction is not the characteristic injury pattern, which rules out the remaining options.

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

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