Electromyographic Activity in Lower Limb Muscles is Temporally Associated with the Slow Phase of Oxygen Uptake During Cycling

Although the "slow" phase of pulmonary oxygen uptake (VO2) appears to represent energetic processes in contracting muscle, electromyographic evidence tends not to support this. The present study assessed normalized integrated electromyographic (NIEMG) activity in eight muscles that act about the hip, knee and ankle during 8 min of moderate (<ventilatory threshold) and very heavy (>ventilatory threshold) cycling in six male cyclists. VO2 was measured breath by breath during four repeated trials at each of the two intensities. Moderate and very heavy exercise followed a 4-min period of light exercise (50 W). During moderate exercise the slow VO2 phase was absent and NIEMG in all muscles did not increase after the first minute of exercise. During very heavy exercise, the slow VO2 phase emerged (time delay=58 +/- 16 s) and increased progressively (time constant=120 +/- 35 s) to an amplitude (0.83 +/- 0.16 L/min) that was approximately 21% of the total response. This slow VO2 phase coincided with a significant increase in NIEMG in most muscles, and differences in NIEMG activities between the two intensities revealed "slow" muscle activation profiles that differed between muscles in terms of the onset, amplitude and shape of these profiles. This supports the hypothesis that the slow phase VO2 is a function of these different slow muscle activation profiles.