To determine the predictability of blood lactate accumulation from excess CO
2 output derived from bicarbonate buffering of lactic acid during constant exercise, eight normal active volunteers were studied during three stages of constant exercise on a cycle ergometer. Three work rates consisted of 100% (stage I), 120% (stage II) and 150% (stage III) of each subject's anaerobic threshold (AT), each of which was lasted for 4 min. Excess CO
2 output (Ex CO
2, ml) at each stage of constant exercise was estimated from the integral of difference between total VCO
2.and aerobic VCO
2 (from regression line for VCO
2 and VO
2 at exercise intensities below the AT obtained in incremental exercise test). Ex CO
2 Per body mass (Ex CO
2·mass
-1) was increased progressively with blood lactate (La) accumulation from rest to each stage of constant exercise. Mean values (± SD) in the measured La accumulation (ΔLa,
measured) and predicted La accumulation (ΔLa,
predicted) at three stages of constant exercise were 1.82 ± 0.83 vs 3.19 ± 1.70 for stage I, 5.58 ± 3.47 vs 7.09 ± 3.28 for stage II and 12.19 ± 2.36 vs 12.74 ± 1.83 mmol·l
-1 for stage III, respectively. There was a significant difference between ΔLa,
measured and ΔLa,
predicted at Stage I (p<0.05), but no significant differences between these two variables at stage II and III. The averaged difference from ΔLa,
predicted to ΔLa,
measured at stage III (0.55 rrunol·l
-1) showed a tendency to be smaller than stage I (1.38 mmol·l
-1) and II (1.50 mmol·l
-1). On the other hand, ΔLa,
predicted was found to correlate very closely with ΔLa,
measured (r=0.954, P<0.001, n=20). The results of this study suggest that the changes of La accumulation could be predicted from excess CO
2 0utput generated in constant exercises above the AT.
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