Influence of exercise intensity on respiratory muscle fatigue and brachial artery blood flow during cycling exercise

Purpose: During high intensity exercise, both respiratory muscle fatigue and cardiovascular reflexes occur; however, it is not known how inactive limb blood flow is influenced. The purpose of this study was to determine the influence of moderate and high exercise intensity on respiratory muscle fatigue and inactive limb muscle and cutaneous blood flow during exercise. Methods: Twelve men cycled at 70 and 85 % V ̇ O 2 max for 20 min. Subjects also performed a second 85 % V ̇ O 2 max test after ingesting 1,800 mg of N-acetylcysteine (NAC), which has been shown to reduce respiratory muscle fatigue (RMF). Maximum inspiratory pressures (P _Imax), brachial artery blood flow (BABF), cutaneous vascular conductance (CVC), and mean arterial pressure were measured at rest and during exercise. Results: Significant RMF occurred with 85 % V ̇ O 2 max (P _Imax, -12.8 ± 9.8 %), but not with 70 % V ̇ O 2 max (P _Imax, -5.0 ± 5.9 %). BABF and BA vascular conductance were significantly lower at end exercise of the 85 % V ̇ O 2 max test compared to the 70 % V ̇ O 2 max test. CVC during exercise was not different (p > 0.05) between trials. With NAC, RMF was reduced (p < 0.05) and BABF was significantly higher (~30 %) compared to 85 % V ̇ O 2 max (p < 0.05). Conclusions: These data suggest that heavy whole-body exercise at 85 % V ̇ O 2 max leads to RMF, decreases in inactive arm blood flow, and vascular conductance, but not cutaneous blood flow.