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ENVIRONMENTAL, EXERCISE AND RESPIRATORY PHYSIOLOGY

Thigh muscle activation distribution and pulmonary O₂ kinetics during moderate, heavy, and very heavy intensity cycling exercise in humans

¹Department of Exercise Science and Physiology, School of Health Sciences, Prefectural University of Hiroshima, Hiroshima; ²Center for Tsukuba Advanced Research Alliance, Tsukuba University, Ibaraki; ³Research Center of Sports Sciences, Musashino University, Tokyo; and ⁴Research Center of Health, Physical Fitness and Sports, Nagoya University, Nagoya, Japan; and ⁵Institute of Membrane and Systems Biology, University of Leeds, United Kingdom

Submitted 16 January 2007 ; accepted in final form 20 April 2007

The mechanisms underlying the oxygen uptake (O₂) slow component during supra-lactate threshold (supra-LT) exercise are poorly understood. Evidence suggests that the O₂ slow component may be caused by progressive muscle recruitment during exercise. We therefore examined whether leg muscle activation patterns [from the transverse relaxation time (T2) of magnetic resonance images] were associated with supra-LT O₂ kinetic parameters. Eleven subjects performed 6-min cycle ergometry at moderate (80% LT), heavy (70% between LT and critical power; CP), and very heavy (7% above CP) intensities with breath-by-breath pulmonary O₂ measurement. T2 in 10 leg muscles was evaluated at rest and after 3 and 6 min of exercise. During moderate exercise, nine muscles achieved a steady-state T2 by 3 min; only in the vastus medialis did T2 increase further after 6 min. During heavy exercise, T2 in the entire vastus group increased between minutes 3 and 6, and additional increases in T2 were seen in adductor magnus and gracilis during this period of very heavy exercise. The O₂ slow component increased with increasing exercise intensity (being functionally zero during moderate exercise). The distribution of T2 was more diverse as supra-LT exercise progressed: T2 variance (ms) increased from 3.6 ± 0.2 to 6.5 ± 1.7 between 3 and 6 min of heavy exercise and from 5.5 ± 0.8 to 12.3 ± 5.4 in very heavy exercise (rest = 3.1 ± 0.6). The T2 distribution was significantly correlated with the magnitude of the O₂ slow component (P < 0.05). These data are consistent with the notion that the O₂ slow component is an expression of progressive muscle recruitment during supra-LT exercise.

oxygen uptake; slow component; T2 time; muscle use patterns

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