This is an excerpt from Physiology of Sport and Exercise 8th Edition With HKPropel Access by W. Larry Kenney,Jack H. Wilmore & David L. Costill.
The crossover concept was first outlined by Brooks and Mercier1 to describe the relative balance between carbohydrate (CHO) and fat metabolism during sustained exercise. At rest and during exercise at low to moderate intensities (below 60% of maximal oxygen uptake), lipids serve as the main substrate for generating ATP. During high-intensity exercise (above 75% of maximal oxygen uptake), increases in muscle glycogenolysis and the recruitment of more type II muscle fibers promote a shift to CHO as the predominant substrate for generating ATP. The crossover point is the intensity where fat and carbohydrate utilization intersect (see figure 2.15) as the energy from fat decreases and the energy from carbohydrate increases. Beyond this crossover point, further increases in power are met with further increments in CHO utilization and decrements in fat oxidation.
The crossover point is affected by both the exercise intensity and endurance training status. Endurance training results in biochemical adaptations within the muscle fibers that promote and support oxidation of FFAs, including an increase in the number of mitochondria, increased oxidative enzymes, and changes in β-oxidation and the electron transport chain—all important determinants of fat metabolism. The result of training is to allow the body to spare muscle glycogen since carbohydrate stores within the body are limited. These training-induced adaptations shift the crossover point toward higher exercise intensities. Diet (energy supply and stores) and prior exercise play secondary roles in determining the balance of substrate utilization during submaximal exercise.