During sustained isometric contraction of the quadriceps, blood flow to the muscle paradoxically decreases. Which physiological mechanism explains this?

• A Metabolic hyperemia is overwhelmed by hypercapnia-induced systemic vasoconstriction
• B High intramuscular pressure during forceful contraction mechanically compresses intramuscular vessels, reducing perfusion ✓
• C Increased sympathetic vasoconstrictor tone during isometric exercise overrides local vasodilatory metabolites
• D Arteriovenous shunting bypasses the muscle capillary bed during contraction

Explanation

During sustained isometric contraction at high force (>15-20% of maximum voluntary contraction), intramuscular pressure can exceed perfusion pressure, mechanically compressing capillaries and arterioles and impeding blood flow. This is the basis for 'muscle pump failure' during sustained (not rhythmic) contractions — blood flow falls, ischemic metabolites accumulate causing pain (Jendrassik's phenomenon), and this limits isometric endurance. Rhythmic isotonic contractions actually enhance muscle blood flow via the muscle pump effect. Options A and C are not primary mechanisms; option D is not a physiological mechanism in skeletal muscle.

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

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