Blood flow and muscle oxygenation during low , moderate and 1 maximal sustained isometric contractions

34 A reduction of blood flow to active muscle will precipitate fatigue and sustained isometric 35 contractions produce intramuscular and compartmental pressures which can limit flow. The 36 present study explored how blood flow and muscle oxygenation respond to isometric 37 contractions at low, moderate and maximal intensities. Over two visits, 10 males (26±2 years; 38 mean±SD) performed 1min dorsiflexion contractions at 30, 60 and 100% of maximal voluntary 39 contraction (MVC) torque. Doppler ultrasound of the anterior tibial artery was used to record 40 arterial diameter and mean blood velocity and calculate absolute blood flow. The tissue 41 oxygenation index (TOI) of tibialis anterior was acquired with near-infrared spectroscopy 42 (NIRS). There was a progressive increase in blood flow at 30% MVC (peak of 289±139% 43 resting value); no change from rest until an increase in the final 10s of exercise at 60% MVC 44 (peak of 197±102% rest); and an initial decrease (59±30% resting value) followed by a 45 progressive increase at 100% MVC (peak of 355±133% rest). Blood flow was greater at 30 and 46 100 than 60% MVC during the last 30s of exercise. TOI was ~63% at rest and, within 30s of 47 exercise, reached steady-state values of ~42%, ~22% and ~22% for 30, 60 and 100% MVC, 48 respectively. Even maximal contraction of the dorsiflexors is unable to cause more than a 49 transient decrease of flow in the anterior tibial artery. Unlike dynamic or intermittent isometric 50 exercise, our results indicate blood flow is not linearly graded with intensity or directly coupled 51 with oxygenation during sustained isometric contractions. 52