This study compared the relationship of maximum oxygen uptake and skeletal muscle enzyme activities to the submaximal exercise intensity eliciting 4 mM blood lactate (OBLA). Twelve subjects performed both cycle (Cy) and treadmill (Tr) submaximal exercise with step-wise increments each fourth minute. Blood lactate concentration and oxygen uptake (VO2) were determined during the final minute of each step. Peak VO2 during exhaustive exercise was also measured on each ergometer. Biopsies were taken from the gastrocnemius (gast) and vastus lateralis (vl) muscles as representatives of muscles recruited during Tr and Cy exercise, respectively. Citrate synthase (CS), phosphofructokinase (PFK), and lactate dehydrogenase (LDH) activities were assayed. Peak VO2 was 10% greater and the VO2 at OBLA was 16% greater during Tr compared to Cy exercise. The percent of peak VO2 at OBLA was 85% and 79% for Tr and Cy exercise, respectively. The absolute enzyme activities were not different in the two muscles, however the ratio LDH/CS was greater in the vl than in the gast. The results indicate that the absolute differences between Cy and Tr exercise in peak VO2 are not commensurate with the differences in the relative exercise intensity at which OBLA occurs.
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