Leg blood flow during exercise with blood flow restriction: evidence for and implications of compensatory cardiovascular mechanisms.

Proximal limb cuff inflation to 40% arterial occlusion pressure (AOP) is assumed to reduce exercising leg perfusion, creating "blood flow restriction" (BFR). However, no study has validated this assumption. Eighteen healthy young participants (9 F) performed two-legged knee flexion/extension exercise at 25% WR_peak with bilateral cuffs applied to the proximal thigh at 0% AOP (CTL), 20% AOP, and 40% AOP. Leg blood flow (LBF; Doppler and echo ultrasound) and cardiac output (CO; finger photoplethysmography) were measured during rest and exercise. LBF values were doubled to account for both exercising legs. AOP (20% and 40%) reduced exercising LBF in a dose-response manner (P < 0.01). However, the magnitude of the leg blood flow restriction by 40% AOP was progressively attenuated across the exercise bout (5-15 s: 37%, 50-70 s: 20%, 240-300 s: 16%; P < 0.01) due to compensatory increases in leg vascular conductance (LVC) (P < 0.01). Between 5 and 15 s of exercise, 40% AOP significantly reduced CO compared with CTL and 20% AOP (8.0 ± 1.3 vs. 8.4 ± 1.5 L/min, P < 0.001 and 8.5 ± 1.5, P < 0.001). By 240-300 s, there were no significant differences in CO between cuff pressures (all P > 0.13). Pneumatic cuff inflation at 20% and 40% AOP reduces LBF in a dose-response manner, but this impairment was progressively attenuated across the exercise bout by an increase in LVC. Importantly, this compensatory response differed across participants, which may have implications for the degree of adaptations following BFR training. Furthermore, restoration of normal CO during BFR despite compromised limb perfusion suggests that other tissue perfusion is increased as part of the response.NEW & NOTEWORTHY It remained to be determined whether BFR set below 60% AOP impairs leg blood flow during continuous exercise. We showed that BFR at 20% and 40% AOP impairs exercising leg blood flow in a dose-response manner. However, the leg blood flow impairment was progressively attenuated across the exercise bout. Both initial compromise and partial restoration varied across participants, which may have implications for the degree of muscle adaptations following BFR training.