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1 The Copenhagen Muscle Research Centre
2 Department of Anaesthesia, Rigshospitalet, University of Copenhagen, Denmark
Reductions in systemic and locomotor limb muscle blood flow and O2 delivery limit aerobic capacity in humans. To examine whether O2 delivery limits both aerobic power and capacity, we first measured systemic haemodynamics, O2 transport and O2 uptake 
during incremental and constant (372 ± 11 W; 85% of peak power; mean ±
S.E.M.) cycling exercise to exhaustion (n
= 8) and then measured systemic and leg haemodynamics and 
during incremental cycling and knee-extensor exercise in male subjects (n
= 10). During incremental cycling, cardiac output 
and systemic O2 delivery increased linearly to 80% of peak power (r2
= 0.998, P < 0.001) and then plateaued in parallel to a decline in stroke volume (SV) and an increase in central venous and mean arterial pressures (P < 0.05). In contrast, heart rate and 
increased linearly until exhaustion (r2
= 0.993; P < 0.001) accompanying a rise in systemic O2 extraction to 84 ± 2%. In the exercising legs, blood flow and O2 delivery levelled off at 73–88% of peak power, blunting leg 
per unit of work despite increasing O2 extraction. When blood flow increased linearly during one-legged knee-extensor exercise, 
per unit of work was unaltered on fatigue. During constant cycling, 
, SV, systemic O2 delivery and 
reached maximal values within 
5 min, but dropped before exhaustion (P < 0.05) despite increasing or stable central venous and mean arterial pressures. In both types of maximal cycling, the impaired systemic O2 delivery was due to the decline or plateau in 
because arterial O2 content continued to increase. These results indicate that an inability of the circulatory system to sustain a linear increase in O2 delivery to the locomotor muscles restrains aerobic power. The similar impairment in SV and O2 delivery during incremental and constant load cycling provides evidence for a central limitation to aerobic power and capacity in humans.
(Received 9 March 2005;
accepted after revision 27 April 2005;
first published online 28 April 2005)
Corresponding author J. González-Alonso: The Copenhagen Muscle Research Centre, Rigshospitalet, Section 7652, Blegdamsvej 9, DK-2100 Copenhagen Ø, Denmark. Email: jga{at}cmrc.dk
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