When does glycogen storage capacity constrain maximal marathon speed?

<p>Computed approximations of total energy consumed as carbohydrate over the course of a marathon, as a function of running speed, in runners with various aerobic capacities. Each colored line corresponds to a particular value of aerobic capacity, , in terms of milliliters of oxygen per kilogram body mass per minute, as labeled (Dark Orange, 35; Orange, 40; Yellow, 45; Light Yellow, 50; Light Green, 55; Green, 60; Aqua, 65; Light Blue, 70; Blue,75; Purple, 80; Violet, 85; Magenta, 90; Red, ). The vertical scale is expressed in terms of kilocalories of energy consumed per kilogram of body mass over 42.195 km (26 miles and 385 yards), the length of a marathon; the corresponding total energy consumption for runners of various masses is shown along the horizontal trend lines, beneath the values of body mass labeled along the top edge of the plot. Running speed is expressed in kilometers per hour along the lower horizontal axis, and as total time to complete a marathon at the corresponding speed along the upper horizontal axis. The dashed, horizontal red lines show the estimated maximum energy storage capacity in runners with maximally glycogen-loaded livers and glycogen-loaded muscles in a state of maximal glycogen supercompensation (144 kcal glycogen per kilogram leg muscle), for whom the leg muscles powering running constitute the indicated percentages of total body mass (right-sided vertical axis). The shaded region indicates the range of supranormal energy storage capacities available to a typical male runner, whose leg muscles constitute approximately 21.4% of his total body mass; the boundaries of the shaded region correspond to typical and maximal values of muscle glycogen density for trained endurance athletes (80 and 144 kcal glycogen per kilogram leg muscle, respectively). See the text for a detailed explanation.