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SKELETAL MUSCLE AND EXERCISE |
1 Copenhagen Muscle Research Centre, Department of Biomedical Sciences, Panum Institute
2 Department of Human Physiology, Institute of Exercise and Sport Sciences, University of Copenhagen, Denmark
The aim of the study was to examine local muscle metabolism in response to graded exercise when the involved muscle mass is too small to elicit marked hormonal changes and local blood flow restriction. Nine healthy overnight fasted male subjects performed knee extension exercise with both thighs kicking at 25% of maximal power (Wmax) for 45 min (23 ± 1% of pulmonary 
) followed by 35 min of kicking with one thigh at 65% and the other at 85%
Wmax (40 ± 1%
). Primed constant infusion of [U-13C] palmitate and [2H5]glycerol was carried out. Blood was sampled from a femoral artery and both femoral veins, and thigh blood flow was determined by thermodilution. Muscle biopsies were obtained from m. vastus lateralis of both thighs. From rest through exercise at 25, 65 and 85%
Wmax the thigh blood flow (0.3 ± 0.1, 2.5 ± 0.2, 3.5 ± 0.2, 4.1 ± 0.3 l min–1) and oxygen uptake (0.02 ± 0.01, 0.27 ± 0.03, 0.48 ± 0.04, 0.55 ± 0.05 l min–1) increased (P < 0.05). The plasma fatty acids oxidized in the thigh (5 ± 1, 114 ± 15, 162 ± 30, 180 ± 31 µmol min–1) increased (P < 0.05) with exercise intensity, whereas the total thigh fat oxidation (19 ± 6, 312 ± 64, 356 ± 93, 323 ± 120 µmol min–1) increased (P < 0.05) from rest, but remained unchanged through exercise. The thigh glycerol uptake (1 ± 1, 16 ± 4, 24 ± 10, 39 ± 8 µmol min–1) increased significantly from rest through exercise at 25–65 and 85%
Wmax, respectively. Glucose uptake and glycogen breakdown always increased with exercise intensity. In conclusion, in the presence of a high blood flow and oxygen supply and only small hormonal changes, total fat oxidation in muscle increases from rest to light exercise, but then remains constant with exercise intensity up to heavy exercise. However, with increasing exercise intensity, oxidation of plasma free fatty acids increases and accordingly oxidation of other fat sources decreases. These findings are in contrast to whole body measurements performed during graded exercise involving a large muscle mass during which fat oxidation peaks at around 60% of 
.
(Received 16 January 2007;
accepted after revision 19 March 2007;
first published online 23 March 2007)
Corresponding author J. W. Helge: Section of Systems Biology, Dept. of Biomedical Sciences, Panum Institute, Blegdamsvej 3, Dk 2200 n, Copenhagen, Denmark. Email: jhelge{at}mfi.ku.dk
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J. Physiol. 2007 581: 886.
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