Restrictions in Locomotor Skeletal Muscle Perfusion, Oxygen Supply and VO2 During Supramaximal Exercise in Humans: 831: 1:30 PM - 1:45 PM

Perfusion to dynamically contracting human skeletal muscle is tightly regulated to match O₂ delivery to the metabolic demand, but this precise regulation is impaired at exercise intensities close to VO₂max. Whether O₂ delivery to locomotor muscles is restricted when O₂ demand is even higher during supramaximal exercise remains unexplored.

PURPOSE: This study examined the role of energy demand on locomotor muscle perfusion, O₂ delivery and O₂ uptake (VO₂), and their effects on supramaximal exercise performance.

METHODS: We measured systemic and leg hemodynamics, O₂ transport, VO₂ and O₂ extraction during constant load supramaximal (SUP) (498±16 W; 110% of peak power; 2.1±0.1min; mean±SEM) and incremental (INC) cycling and knee-extensor exercise (KE) in 12 trained male subjects (28±2 years, 84±3 kg, 4.7±0.2 L-min^-1 VO₂max). Data were analyzed by repeated measures one-way ANOVA and Tukey's honestly significant difference (HSD) post hoc procedure. Statistical significance was accepted at p<0.05.

RESULTS: During SUP, cardiac output (Q), O₂ delivery and VO₂ reached peak values after 60-90s and remained stable until exhaustion. In contrast, stroke volume (SV), systemic and leg vascular conductance peaked after 60-90s, but declined thereafter (p<0.05) in parallel to an increase in mean arterial and central venous pressure. Compared to INC cycling, peak systemic VO₂, leg O₂ extraction and leg VO₂ were lower(4.4±0.2. vs. 4.7±0.2L-mm^-1; 85.5±1.6vs. 88.1±1.3 %;3.6±0.3vs. 3.9±0.3 L-min¹ in SUP and INC, respectively; all p<0.05), while Q and systemic O₂ delivery reached similar peak values (27. 1±1.0 vs. 27.6±1.0 L-min¹; 5.5±0.1 vs. 5.8±0.1 L-min¹), but were lower upon exhaustion (p<0.05). No differences were observed in peak or exhaustion systemic O₂ extraction, leg blood flow, vascular conductance or leg O₂ delivery, despite the 48 W difference in work rate. With the lower O₂ delivery per unit of work and lower peripheral O₂ extraction, leg VO₂ per unit work was lower during SUP compared to INC cycling and KE (7.0±0.5, 8.4±0.4 and 12.2±0.7 ml O₂ min^-1 W^-1 respectively; p<0.05).

CONCLUSIONS: Restrictions in locomotor muscle blood flow during supramaximal cycling is associated with a lower vascular conductance, indicative of enhanced sympathetic vasoconstrictor activity. The restricted O₂ delivery as well as a reduced peripheral O₂ extraction limit supramaximal muscle aerobic energy turnover, resulting in a reduced VO₂ per unit of work despite the higher energy demand. Supported by the Gatorade Sports Science Institute and the Novo Nordisk Foundation