Hormonal and metabolic responses to exercise in subject of high and low work capacities

Hormonal and metabolic responses to submaximal were studied in eight normal subjects. Four subjects had high maximal oxygen uptakes (455 +/- 0.49/min) and were classified as "fit" and four had low maximal oxygen uptakes (2.13 +/- 0.32 1/min) and were classified as "unfit". All subjects exercised for 20 minutes at 750 kpm/min on a cycle ergometer, which represented 35% and 85% of the maximal oxygen uptake for the fit and the unfit subjects respectively. The unfit subjects had a higher heart rate and CO2 output at the same oxygen uptake when compared with the fit group. There was a high blood lactate in the unfit subjects and also a greater rise in blood glucose, plasma cortisol and serum growth hormone during exercise in these subjects. Free fatty acids increased in the fit subjects, but not in the unfit and serum insulin was depressed in both groups. Thus, the cardiorespiratory fitness is an important determinant of the hormonal and metabolic response to submaximal exercise, which differed considerably when fit and unfit subjects exercised at identical absolute levels of power output. These differences have implications for the control of energy substrates used during exercise.