The crossover point (COP) refers to the increase in CHO oxidation with increasing exercise intensity (1) which posits that, at any exercise intensity above 85%, zero energy can come from fat oxidation. This is the basis for the belief that CHO is the obligatory fuel for high intensity exercise (2) and that muscle glycogen depletion in the active muscles is the favored explanation for the fatigue that develops during prolonged exercise (3-5). We define the reverse crossover point (RCOP) as the point during prolonged, steady-state endurance exercise at which energy from fat oxidation exceeds that from carbohydrate (CHO) oxidation despite maintained exercise intensity. Unlike the conventional COP which reflects a substrate shift due to rising exercise intensity and catecholamine stimulation, the RCOP reflects a time-dependent shift in substrate use, potentially driven by declining glycogen availability, falling insulin levels, and progressive upregulation of lipolysis and fat oxidation pathways (6).
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