Problem: The cross-over concept changes through biochemical adaptations in muscles with the consistent aerobic training. Aerobic training recruits more type 1 slow-twitch muscle fibers, increases the density of mitochondria, reduces the stimulation of the sympathetic nervous system during exercise stress. There is less use of glycogen, less secretion of epinephrine and norepinephrine, while lipid oxidation becomes more enhanced. These adaptations from consistent training improve metabolic efficiency, oxygen uptake, and VO2. With improvements in fat metabolism and VO2 max, fat becomes dominant fuel sources for longer period before carbohydrate are used. As the exercise increases, carbohydrates become the dominant fuel source at the crossover point. Need Assignment Help?