If a muscle biopsy reveals normal ATP concentration but persistent muscle stiffness and slow relaxation following voluntary contraction, which mechanism explains this phenomenon?

💡 Explanation

Defective myosin ATPase prevents the detachment of myosin heads from actin filaments, causing prolonged cross-bridge cycling and persistent stiffness, because ATP is needed for detachment. Therefore, stiffness results from impaired detachment, rather than impaired calcium handling or ATP production, which would primarily affect contraction strength.

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