Recent studies have demonstrated that single-leg knee extensor (KE) exercise elicits high mass-specific blood flow (Q) which, if incremented toward maximum, in the presence of additional muscle recruitment would soon outstrip the heart's pumping capacity and blood pressure would fall. Thus incremental KE exercise provides the opportunity to determine the intensity at which, if at all, quadriceps muscle hemodynamics are altered during incremental exercise that involves a substantially greater muscle mass. Leg Q was measured during incremental KE exercise and again with superimposed incremental two-legged knee extensor exercise with incremental arm cranking (A+L) in trained subjects (n = 5). Leg Q and vascular conductance (VC) increased with work rate (WR) to reach high levels [Q = 385.7 +/- 26 and 342.3 +/- 15 ml.min-1.100 g-1 for KE and A+L exercise, respectively; VC at 90% of maximum WR (WRmax) = 79 +/- 5 and 75 +/- 6 ml.min-1. mmHg-1 for KE and A+L exercise, respectively], but the Q/WR and VC/WR relationships in KE and A+L exercise were not different. Maximum O2 consumption (VO2max) and the VO2max/WR relationship of the quadriceps were also unaffected by the additional muscle mass recruited. Despite a significantly greater net femoral venous norepinephrine (NE) outflow at both 90 and 100% of WRmax in A+L exercise (WRmax = 4,216 +/- 1,601 and 901 +/- 99 ng/ml for A+L and KE exercise, respectively; P < 0.05), leg Q continued to rise linearly with WR.(ABSTRACT TRUNCATED AT 250 WORDS)