IMPORTANCE OF THE CROSSOVER CONCEPT IN EXERCISE METABOLISM

1. The 'crossover' concept is a model of substrate supply during exerc ise which makes the following predictions. 2. Lipid is the major fuel (approximately 60%) for noncontracting skeletal muscle and the body at rest. 3. Energy flux, as determined by exercise intensity, is the maj or factor in determining the balance of substrate utilization during e xercise, Thus, moderate and greater exercise intensities increase cont raction-induced muscle glycogenolysis and glycolysis, increase recruit ment of fast-twitch muscle fibres, increase sympathetic nervous system activity and down-regulate mitochondrial fatty acid uptake. 4. Glycog en and glucose utilization scales exponentially to relative exercise p ower output with a greater gain in glycogen than in glucose use at hig h power The relationship between free fatty acid flux and power output is an inverted hyperbola, Consequently, at high power outputs, the ro le of lipid oxidation is diminished. 5. Factors such as endurance trai ning, energy supply, as influenced by dietary manipulation, and prior exercise play secondary roles in determining the balance of substrate utilization during exercise. 6. Comparisons of the metabolic responses in subjects engaged in activities requiring vastly different metaboli c rates or comparisons of subjects of different gender, age or trainin g status require normalization of data to total energy flux.