MUSCLE BLOOD-FLOW AT ONSET OF DYNAMIC EXERCISE IN HUMANS

To evaluate the temporal relationship between blood flow, blood pressu re, and muscle contractions, we continuously measured femoral arterial inflow with ultrasound Doppler at onset of passive exercise and volun tary, one-legged, dynamic knee-extensor exercise in humans. Blood velo city and inflow increased (P < 0.006) with the first relaxation of pas sive and voluntary exercise, whereas the arterial-venous pressure diff erence was unaltered [P = not significant (NS)]. During steady-state e xercise, and with arterial pressure as a superimposed influence, blood velocity was affected by the muscle pump, peaking (P < 0.001) at simi lar to 2.5 +/- 0.3 m/s as the relaxation coincided with peak systolic arterial blood pressure; blood velocity decreased (P < 0.001) to 44.2 +/- 8.6 and 28.5 +/- 5.5% of peak velocity at the second dicrotic and diastolic blood pressure notches, respectively. Mechanical hindrance o ccurred (P < 0.001) during the contraction phase at blood pressures le ss than or equal to that at the second dicrotic notch. The increase in blood flow ((Q)) over dot was characterized by a one-component (simil ar to 15% of peak power output), two-component (similar to 40-70% of p eak power output), or three-component exponential model (greater than or equal to 75% of peak power output), where (Q) over dot (t) = (Q) ov er dot(passive) + Delta(Q) over dot(1).[1 - e(-(t - TD1/tau 1))] + Del ta(Q) over dot(2). [1 - e(-(t - TD2/T2))] + Delta(Q) over dot(3). [1 - e(-(t-TD3/tau 3))]; (Q) over dot(passive,) the blood flow during passi ve leg movement, equals 1.17 +/- 0.11 1/min; TD is the onset latency; tau is the time constant; Delta(Q) over dot is the magnitude of blood flow rise; and subscripts 1-3 refer to the first, second, and third co mponents of the exponential model, respectively. The time to reach 50% of the difference between passive and voluntary asymptotic blood flow was similar to 2.2-8.9 s. The blood flow leveled off after similar to 10-150 s, related to the power outputs. It is concluded that the elev ation in blood flow with the first duty cycle(s) is due to muscle mech anical factors, but vasodilators initiate a more potent amplification within the second to fourth contraction.