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Medicine & Science in Sports & Exercise:
BASIC SCIENCES/REGULATORY PHYSIOLOGY: Symposium: Mechanisms Which Control [latin capital V with dot above]O2 Near [latin capital V with dot above]O2max: PDF Only

Blood flow and pressure relationships which determine VO2max.

BRECHUE, WILLIAM F.; AMEREDES, BILL T.; BARCLAY, JACK K.; STAINSBY, WENDELL N.

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Abstract

The role of O2 delivery in regulating [latin capital V with dot above]O2max has been studied in an isolated gastrocnemius-plantaris muscle preparation contracting in situ; recent data addressing this issue are presented. [latin capital V with dot above]O2 increases nonlinearly with stimulation frequency reaching a peak at 5 twitches[middle dot]s-1 or 1 tet[middle dot]s-1 (200 ms trains, 50 imp[middle dot]s-1). Further increases in stimulation frequency result in a lower [latin capital V with dot above]O2. Measured [latin capital V with dot above]O2 maxima are less than predicted [latin capital V with dot above]O2 capacity, and peak [latin capital V with dot above]O2 during tetanic contractions is greater than that during twitches. Above 150 imp[middle dot]min-1, [latin capital V with dot above]O2 is directly related to the level of blood flow attained as [latin capital V with dot above]O2/[Latin capital letter Q with dot above] (arterial-venous O2 difference) is fixed by some unknown mechanism. Increasing blood flow, with a pump, during 1[middle dot]s-1 tetanic contractions increases O2 diffusive conductance and peak [latin capital V with dot above]O2. When O2 delivery is reduced, ischemic hypoxia appears to result in more rapid reductions in muscle performance than hypoxic hypoxia because of decreases in perfusion pressure and [Latin capital letter Q with dot above]. 31P-NMR studies reveal that reductions in creatine phosphate and energy charge are similar between ischemia and hypoxia suggesting a common regulator, O2. We conclude that [latin capital V with dot above]O2max is limited by O2 delivery as a result of a limited and uneven distribution of muscle blood flow. These limitations appear secondary to mechanical restraints imposed by contraction duty cycle and vascular compression.

(C)1995The American College of Sports Medicine

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