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The oxygen cost of breathing and blood flow requirements of the respiratory muscles during exercise are discussed along with the implications for limitation of locomotor muscle and exercise performance. Findings show that the oxygen cost of the hyperpnea achieved during very heavy exercise may approach 15% or more of [spacing dot above]VO2max under conditions that require extraordinary levels of ventilatory work. These conditions include those in the highly trained endurance athlete (at [spacing dot above]VE > 150 l[middle dot]min-1), the older athlete (at [spacing dot above]VE of 110-120 l[middle dot]min-1), and athletic cursorial mammals at[spacing dot above]VO2max-all of whom experience significant expiratory flow limitation and sometimes even complete ventilatory limitation during heavy or maximum exercise. Rates of blood flow to the respiratory muscles under these peak exercise conditions may equal or exceed those to the limb locomotor muscles. The hypothesis is advanced that excessive requirements of ventilatory work (and therefore [spacing dot above]VO2 and blood flow) during heavy exercise may cause reflex vasoconstriction of locomotor muscles resulting in curtailment of endurance exercise performance.

(C)1996The American College of Sports Medicine