Fixed time versus fixed distance protocols for the blood lactate profile in athletes

Laboratory studies of blood lactate accumulation often use a fixed time protocol to define the onset (4 mM) of blood lactate accumulation (OBLA) or other indices of blood lactate concentration. For practical reasons, field studies with athletes often use a fixed distance protocol to accomplish the same goal. Whether these variations of protocol are comparable has not been established. We studied 10 subjects in the laboratory during fixed time (4 minute) and fixed distance (2 km) exercise protocols on a racing bicycle attached to a wind load simulator. The fixed distance studies required 3-6 minutes to complete. We also studied the subjects during fixed distance (2 km) rides in the field. In the laboratory there were no systematic differences in the velocity (34.3 4.6 vs 34.2 +/- 4.6 km.hr-1), VO2 (2.78 +/- 0.60 vs 2.84 +/- 0.62 liters.min-1), or heart rate (159 +/- 16 vs 155 +/- 14 beats.min-1) at OBLA in the fixed time vs fixed distance protocols. The correlation coefficients for velocity (r = 0.97), VO2 (r = 0.97) and heart rate (r = 0.94) further indicate the similarity of results. In the field study there was a significant difference in velocity (29.9 +/- 4.8 vs 34.2 +/- 4.6 km.hr-1) but not heart rate (155 +/- 18 vs 155 +/- 15 beats.min-1) at OBLA versus the fixed distance laboratory study. The correlations for velocity (r = 0.47) and heart rate (r = 0.93) support these data. The results suggest that the practical modification of the lactate profile technique of using fixed distance versus fixed time exercise stages does not systematically influence the outcome, at least for exercise stage durations approximating 4 minutes.