Vasoconstrictor responsiveness through alterations in relaxation time and metabolic rate during rhythmic handgrip...

Increasing the relaxation phase of the contraction-relaxation cycle will increase active skeletal muscle blood flow ( $$ ). However, it remains unknown if this increase in $$ alters the vasoconstriction responses in active skeletal muscle. This investigation determined if decreasing mechanical impedance would impact vasoconstriction of the active skeletal muscle. Eight healthy men performed rhythmic handgrip exercise under three different conditions; "low" duty cycle at 20% maximal voluntary contraction (MVC), "low" duty cycle at 15% MVC, and "high" duty cycle at 20% MVC. Relaxation time between low and high duty cycles were 2.4 sec versus 1.5 sec, respectively. During steady-state exercise lower body negative pressure (LBNP) was used to evoke vasoconstriction. Finger photoplethysmography and Doppler ultrasound derived diameters and velocities were used to measure blood pressure, forearm blood flow (FBF: mL min^-1 ) and forearm vascular conductance (FVC: mL min^-1  mmHg) throughout testing. The low duty cycle increased FBF and FVC versus the high duty cycle under steady-state conditions at 20% MVC (P < 0.01). The high duty cycle had the greatest attenuation in %ΔFVC (-1.9 ± 3.8%). The low duty cycle at 20% (-13.3 ± 1.4%) and 15% MVC (-13.1 ± 2.5%) had significantly greater vasoconstriction than the high duty cycle (both: P < 0.01) but were not different from one another (P = 0.99). When matched for work rate and metabolic rate ( $$ ), the high duty cycle had greater functional sympatholysis than the low duty cycle. However, despite a lower $$ , there was no difference in functional sympatholysis between the low duty cycle conditions. This may suggest that increases in $$ play a role in functional sympatholysis when mechanical compression is minimized.