Abstract

The relationships between plasma epinephrine threshold (TE), norepinephrine threshold (TNE), and blood lactate threshold (Tlact) were examined during incremental cycling in 10 untrained males. When oxygen uptake (VO2) measured at each threshold was expressed as VO2max, the threshold occurred at 40.8+/-2.4, 45.5+/-3.0, and 46.6+/-3.0 for Tlact, TE, and TNE, respectively. The average VO2 and power output values obtained at Tlact and TE were not found to be significantly different. However, Tlact and TE occurred simultaneously in 6 subjects, whereas TE occurred at a higher work stage than Tlact in the other 4 subjects. The mean VO2 and power output values determined at TNE were found to be significantly greater than the values obtained at Tlact. These two thresholds occurred together in 4 subjects, while TNE was observed to occur at a higher work stage than Tlact in the other 6 subjects. None of the differences between TE and TNE were significantly different. Although plasma norepinephrine concentrations were much greater than those for epinephrine at a given power output, the two catecholamine thresholds occurred simultaneously in 7 subjects. The results are not consistent with the hypothesis that the increase in plasma catecholamine levels during incremental exercise is the sole determinant of the lactate threshold. It is also possible that a decrease in muscle pH, due to increased lactic acid, stimulated a reflex increase in sympathetic outflow and a subsequent rise in catecholamine levels. Verf.-Referat