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 ± 2 yr) 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-min 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; 56.0 ± 21.6 ml/min] to 80%WR_max (679 ± 190 ml/min; 70.6 ± 24.8 ml/min), but no change was seen from 80%WR_max to 100%WR_max Blood velocity (49.5 ± 11.5 to 58.1 ± 11.6 cm/s) and brachial diameter (0.49 ± 0.05 to 0.50 ± 0.06 cm) showed concomitant increases (P < 0.05) with blood flow from 60% to 80%WR_max, whereas 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 ± 22 s; 80%WR_max 51 ± 20 s; 100%WR_max 51 ± 23 s) than a-vO_2diff (60%WR_max 29 ± 9 s; 80%WR_max 29 ± 5 s; 100%WR_max 20 ± 5 s), but not different from oxygen uptake (60%WR_max 44 ± 25 s; 80%WR_max 43 ± 14 s; 100%WR_max 41 ± 32 s). 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.NEW & NOTEWORTHY This study evaluated blood flow regulation and oxygen uptake during small muscle mass forearm exercise with high to maximal intensity. Despite utilizing only a fraction of cardiac output, blood flow reached a plateau at 80% of maximal work rate and regulated peak oxygen uptake. Furthermore, the results revealed that muscle contractions dictated bulk oxygen delivery and yielded three times higher peak blood flow in the relaxation phase compared with mean values.