Blood flow regulation and oxygen uptake during high intensity forearm exercise

The vascular strain is very high during heavy handgrip exercise, but the intensity and kinetics to reach peak blood flow, and peak oxygen uptake, are uncertain. We included 9 young (25±2yr) healthy males to evaluate blood flow and oxygen uptake responses during continuous dynamic handgrip exercise with increasing intensity. Blood flow was measured using Doppler-ultrasound and venous blood was drawn from a deep forearm vein to determine arteriovenous oxygen difference (a-vO_2diff) during 6-minutes bouts of 60, 80 and 100% of maximal work rate (WR_max), respectively. Blood flow and oxygen uptake increased (p<0.05) from 60%WR_max (557±177(SD) mL•min^-1; 56.0±21.6 mL•min^-1) to 80%WR_max (679±190 mL•min^-1; 70.6±24.8 mL•min^-1), but no change was seen from 80%WR_max to 100%WR_max. Blood velocity (49.5±11.5 cm•sec^-1 to 58.1±11.6 cm•sec^-1) and brachial diameter (0.49±0.05cm to 0.50±0.06 cm) showed concomitant increases (p<0.05) with blood flow from 60% to 80%WRmax, while no differences were observed in a-vO_2diff. Shear rate also increased (p<0.05) from 60% (822±196 s^-1) to 80% (951±234 s^-1) of WR_max. The mean response time (MRT) was slower (p<0.05) for blood flow (60%WR_max:50±22s; 80%WR_max:51±20s; 100%WRmax:51±23s) than a-vO_2diff (60%WR_max:29±9s; 80%WR_max:29±5s; 100%WR_max:20±5s), but not different from oxygen uptake (60%WR_max:44±25s; 80%WR_max:43±14s; 100%WR_max:41±32s). No differences were observed in MRT for blood flow or oxygen uptake with increased exercise intensity. In conclusion, when approaching maximal intensity, oxygen uptake appeared to reach a critical level at ~80% of WR_max and be regulated by blood flow. This implies that high, but not maximal, exercise intensity may be an optimal stimulus for shear stress-induced small muscle mass training adaptations.

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