Influence of Respiratory Muscle Work on VO2 and Leg Blood Flow During Submax Exercise (original) (raw)

1999, Medicine and Science in Sports and Exercise

Wetter, Thomas J., Craig A. Harms, William B. Nelson, David F. Pegelow, and Jerome A. Dempsey. Influence of respiratory muscle work on V O 2 and leg blood flow during submaximal exercise. J. Appl. Physiol. 87(2): 643-651, 1999.-The work of breathing (W b) normally incurred during maximal exercise not only requires substantial cardiac output and O 2 consumption (V O 2) but also causes vasoconstriction in locomotor muscles and compromises leg blood flow (Q leg). We wondered whether the W b normally incurred during submaximal exercise would also reduce Q leg. Therefore, we investigated the effects of changing the W b on Q leg via thermodilution in 10 healthy trained male cyclists [maximal V O 2 (V O 2 max) ϭ 59 Ϯ 9 ml•kg Ϫ1 •min Ϫ1 ] during repeated bouts of cycle exercise at work rates corresponding to 50 and 75% of V O 2 max. Inspiratory muscle work was 1) reduced 40 Ϯ 6% via a proportional-assist ventilator, 2) not manipulated (control), or 3) increased 61 Ϯ 8% by addition of inspiratory resistive loads. Increasing the W b during submaximal exercise caused V O 2 to increase; decreasing the W b was associated with lower V O 2 (⌬V O 2 ϭ 0.12 and 0.21 l/min at 50 and 75% of V O 2 max , respectively, for ϳ100% change in W b). There were no significant changes in leg vascular resistance (LVR), norepinephrine spillover, arterial pressure, or Q leg when W b was reduced or increased. Why are LVR, norepinephrine spillover, and Q leg influenced by the W b at maximal but not submaximal exercise? We postulate that at submaximal work rates and ventilation rates the normal W b required makes insufficient demands for V O 2 and cardiac output to require any cardiovascular adjustment and is too small to activate sympathetic vasoconstrictor efferent output. Furthermore, even a 50-70% increase in W b during submaximal exercise, as might be encountered in conditions where ventilation rates and/or inspiratory flow resistive forces are higher than normal, also does not elicit changes in LVR or Q leg .