Sensitivity of Crossover Concept to Discriminate Different Levels of Performance: A New Approach

The crossover concept (CO) is the relationship between the percentage of carbohydrate oxidation (%CHO) and percentage of Fat oxidation (%Fat) rates with respect to the relative exercise intensity expressed as %VO2 max. Recently, the CO has been determined through indirect calorimetry and estequiometric equations, which have been applied to assess substrate utilization during exercise. PURPOSE: To evaluate which variables of exercise intensity are more sensitive to discriminate performance level and substrate utilization using the CO. METHODS: 2 groups of male cyclists, 8 Elite (EC) and 9 recreational (RC) (mean ± SD, VO2 max: 74.2 ± 6.0 vs. 58.0 ± 6.8, respectively), performed an incremental test on a cycloergometer: Initial work load of 150 W·min-1; increments of 25 W·4min-1 until exhaustion. VO2 and VCO2 were measured by indirect calorimetry, and CHO and Fat oxidation rates were estimated and averaged for the last 3 minutes of each workload through estequiometric equations. Maximum Fat and CHO oxidation rates (FATmax and CHOmax), crossover point (COP), and minimum Fat oxidation rate (FATmin) were calculated and expressed in %VO2 max, W and W·kg-1 according to the CO. A t-test was used to compare the variables between EC and RC groups. Statistical significance was P<0.05. The coefficient of variation (CV) was also applied. RESULTS: The CV was lower for the variables expressed in %VO2max (8.3 %) with respect to the one's expressed in W (17.7 %) and W·kg-1 (19.2 %). When variables associated to the CO were expressed in %VO2max, statistically significant differences were only found for FATmax (EC: 53.8 ± 2.1 %VO2max; RC: 49.1 ± 5.6 %VO2max, P<0.05). However, when expressed in W and W·kg-1, it was found statistically significant differences in all the parameters studied for EC (FATmax: 164 ± 30 vs. 228 ± 16 W, P<0.05; COP: 273 ± 42 vs. 333 ± 31 W, P<0.01; FATmin: 320 ± 55 vs. 381 ± 43 W, P<0.05; CHOmax: 356 ± 35 vs. 425 ± 30 W, P<0.01); and, especially when expressed in W·kg-1 (FATmax: 2.1 ± 0.3 vs. 3.2 ± 0.2 W·kg-1, P<0.001; COP: 3.6 ± 0.4 vs. 4.7 ± 0.5 W·kg-1, P<0.001; FATmin: 4.1 ± 0.5 vs. 5.3 ± 0.6 W·kg-1, P<0.05; CHOmax: 4.6 ± 0.4 vs. 5.9 ± 0.4 W·kg-1, P<0.001). CONCLUSION: Relative mechanical power (W·kg-1) is the variable that discriminates the best when studying parameters associated to the CO for different levels of performance.