Figure 1.
Relative use of fat and carbohydrate as metabolic fuels depends on exercise intensity.
Fractional usage of carbohydrate (plasma glucose plus muscle glycogen, blue filled curve, ) and fat (plasma free fatty acids plus muscle triglycerides, red filled curve,
) are shown as functions of relative exercise intensity,
. (Based on the work of Romijn and colleagues: Points plotted correspond to data points from the 1993 study [24], and corresponding error bars are computed as described in the Methods section.)
Figure 2.
When does glycogen storage capacity constrain maximal marathon speed?
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.
Figure 3.
Distance to ‘The Wall’ for endurance runners.
Computed distance athletes can run before completely depleting glycogen reserves (‘hitting the wall’), as a function of running intensity (expressed as a percent of ), relative leg muscle mass (leg muscle mass as a fraction of total body mass), and muscle glycogen density. The distance an athlete can run before ‘hitting the wall’ decreases with increasing levels of exertion, and as the shaded rectangular region labeled ‘Will “Hit the Wall”’ indicates, if this distance is less than 42.195 kilometers (26 miles and 385 yards) then the runner will not be able to complete a marathon without experiencing complete glycogen depletion (at least not without refueling midrace). Each colored curve corresponds to a particular density of muscle glycogen, as labeled (Red, 40; Orange, 60; Yellow, 80; Green, 100; Blue, 120; Purple,
). The colored curves each correspond to athletes whose leg muscles constitute 21.4% of total body mass and are loaded with glycogen at a particular density (relative liver mass has been assumed constant at 2.5% of total body mass, and liver glycogen density has been assumed maximized at
); a shaded region around each colored curve fills the region corresponding to relative leg muscle masses of 15% to 25% total body mass. Darker shading indicates overlapping regions and identical failure distances for different sets of physical parameters: The area of densest shading straddles the 21-mile line for athletes running at intensities of 80% to 95%
, indicating that many different athletic builds and levels of glycogen loading are subject to failure at these intensities around mile 21, which has been empirically identified as the distance at which marathon runners most commonly ‘hit the wall.’ By contrast, few runners will ‘hit the wall’ before mile 11, or when running a marathon at less than 55%
.
Figure 4.
Estimating the aerobic capacity of a typical runner.
Computed approximations of (in terms of milliliters of oxygen per minute per kilogram body mass), as a function of estimated fraction of maximum heart rate while running at a given speed, are shown as a set of colored curves. Each line corresponds to a particular running speed (Orange, 4 mph (
); Light Green, 5 mph (
); Green, 6 mph (
); Dark Green, 7 mph (
); Light Blue, 8 mph (
); Blue, 9 mph (
); Dark Blue, 10 mph (
); Purple, 11 mph (
); Magenta, 12 mph (
); Red, 13 mph (
)). See the text for a detailed explanation.