Effects of exercise duration on the interrelationship between lactate and carbohydrate utilisation

A recently proposed model has determined a half maximal constant (kel) to describe the relative rate of carbohydrate utilisation (relCHO) as a function
of blood lactate concentration (BLC) (Beneke et al., 2009: European Journal of Applied Physiology, 105, 257–263). The time constant of BLC onset kinetics
increases from 2 to 4 min between 2 and 7 mmol.l-1 (Beneke, 2003: European Journal of Applied Physiology, 89, 95–99), while the corresponding value in VO2 and VCO2 is 0.3–0.8 min. Whether the latter differences in dynamics affect kel determined at incremental tests with different stage durations is unknown. This study analyses kel at the 2nd, 4th or 6th min of an incremental test stages. The study was approved by the University of Essex Ethics committee. Eleven healthy males (age mean 29.7, s = 5.6 years, height 180, s = 5.9 cm, body mass 74.5, s = 6.2 kg) completed an incremental cycling test at 50 rpm. The workload was initiated with and increased by 0.5 W kg-1 body mass every 6 min stage until exhaustion. Capillary blood samples were drawn from the hyperaemic earlobe at rest and at the end of every 2, 4 and 6 min. VO2 and VCO2 were measured and analysed for estimating CHO and fat oxidation using indirect calorimetry for the last 30 s of 2nd, 4th and 6th min of each incremental stage. relCHO was calculated as a percentage of maximal CHO combustion using 100% of the given VO2 for CHO; and plotted as a sigmoid function of BLC: relCHO = 100/(1+kel/BLC2) to approximate kel for the analyses of 2nd, 4th and 6th min of each stage. relCHO was higher in 4th and 6th than 2nd min at submaximal intensities between 40 and 80% (peak power) (P <0.01), and also higher in 6th than 4th and 2nd min at 60–80% (P< 0.01). BLC was higher in 6th and 4th than 2nd min at intensities above 45% (P <0.01), and also higher in 6th than 4th and 2nd min at intensities above
60% (P< 0.01). Estimates of kel showed no significant difference (1.558, s=1.214; 1.217, s= 0.779 and 1.164, s= 0.788 (mmol.l-1)2) for 2nd, 4th and 6th min, respectively. kel was moderately correlated between 2nd and 4th, and2nd and 6th min (r = 0.65; P = 0.05; r = 0.67, P< 0.05, respectively). kel showed stronger correlation between 4th and 6th min (r= 0.90, P< 0.01). Stage duration does not seem to affect kel significantly. However, higher correlations between values determined after 4 and 6 min seem to promote incremental load tests with longer stage durations.