Timed synchronization of muscle contraction to heart beat enhances muscle hyperemia

Blood flow (BF) to exercising muscles is susceptible to variations of intensity, and duration of skeletal muscle contractions, cardiac cycle, blood velocity, and vessel dilation. During cyclic muscle activity, these elements may change proportionally with or without direct optimal temporal alignment, likely influencing BF to active muscle. Ideally, the pulsed delivery of blood to active muscle timed with the inactive phase of muscle duty-cycle would enhance the peak and average BF. To investigate the phenomenon of muscle contraction and pulse synchronicity, electrically-evoked muscle contractions (trains of 20Hz, 200ms duration) were synchronized with each systolic phase of the blood velocity spectrum (aBVS). Specifically, unilateral quadriceps contractions matched in-phase (IP) with the aBVS were compared to contractions matched out of phase (OP) with the aBVS in 10 healthy participants (26±3 years). During each trial, femoral BF of the contracting limb and central hemodynamics were recorded for 5 minutes with an ultrasound doppler, a plethysmograph, and a cardioimpedence device. At steady-state (5th min) IP BF (454±30 mL/min), MAP (113±3 mmHg) and vascular conductance (4.3±0.2 mL/min/mmHg) were significantly lower (all, P<0.001) in comparison to OP BF (784±25 mL/min), MAP (108±2 mmHg) and vascular conductance (6.7±0.2 mL/min/mmHg). On the contrary, no significant difference (all, P>0.05) was observed between IP and OP central hemodynamics (HR:79±10 vs. 76±11 bpm, CO:8.0±1.6 vs. 7.3±1.6 L/min) and ventilatory patterns (VE:14±2 vs. 14±1 L/min, VO2:421±70 vs. 397±34 mL/min). The results suggest that muscle contractions occurring during OP and that do not interfere with aBVS elicit a maximization of muscle functional hyperemia.