Metabolic Factors Limiting Performance in Marathon Runners

Benjamin I. Rapoport,Philip E. Bourne

doi:10.1371/journal.pcbi.1000960

Benjamin I. Rapoport, Philip E. Bourne

Open Access

https://doi.org/10.1371/journal.pcbi.1000960

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Abstract

Each year in the past three decades has seen hundreds of thousands of runners register to run a major marathon. Of those who attempt to race over the marathon distance of 26 miles and 385 yards (42.195 kilometers), more than two-fifths experience severe and performance-limiting depletion of physiologic carbohydrate reserves (a phenomenon known as ‘hitting the wall’), and thousands drop out before reaching the finish lines (approximately 1–2% of those who start). Analyses of endurance physiology have often either used coarse approximations to suggest that human glycogen reserves are insufficient to fuel a marathon (making ‘hitting the wall’ seem inevitable), or implied that maximal glycogen loading is required in order to complete a marathon without ‘hitting the wall.’ The present computational study demonstrates that the energetic constraints on endurance runners are more subtle, and depend on several physiologic variables including the muscle mass distribution, liver and muscle glycogen densities, and running speed (exercise intensity as a fraction of aerobic capacity) of individual runners, in personalized but nevertheless quantifiable and predictable ways. The analytic approach presented here is used to estimate the distance at which runners will exhaust their glycogen stores as a function of running intensity. In so doing it also provides a basis for guidelines ensuring the safety and optimizing the performance of endurance runners, both by setting personally appropriate paces and by prescribing midrace fueling requirements for avoiding ‘the wall.’ The present analysis also sheds physiologically principled light on important standards in marathon running that until now have remained empirically defined: The qualifying times for the Boston Marathon.

Highlights

Energy Management in Endurance Runners as a Public Health ConcernRecent years have witnessed dramatic increases in the number of amateurs participating in major endurance running events, world-class marathons such as those in Boston, New York, Chicago, London, and Berlin, for which enrollment has increased by more than an order of magnitude in four decades, from hundreds of runners in the 1970s to the tens of thousands who will compete in each of the largest marathons in the 2010 season [1]
There is a hierarchy of metabolic processes, defined by the rate at which ATP can be produced to power muscle contractions: The anaerobic processes, hydrolysis of phosphocreatine and conversion of glycogen to lactate, produce at most 73:3 and 39:1 mmol ATP s{1, respectively; by contrast, the aerobic processes, which involve the complete oxidation of muscle glycogen, liver glycogen, or adipose-tissue-derived fatty acids, produce at most 16:7, 6:2 or 6:7 mmol ATP s{1, respectively [25]
The present study demonstrates that glycogen storage capacity is only a performance-limiting factor in runners of low and moderate aerobic capacities, or with relatively small leg muscles

Summary

Introduction

Energy Management in Endurance Runners as a Public Health ConcernRecent years have witnessed dramatic increases in the number of amateurs participating in major endurance running events, world-class marathons such as those in Boston, New York, Chicago, London, and Berlin, for which enrollment has increased by more than an order of magnitude in four decades, from hundreds of runners in the 1970s to the tens of thousands who will compete in each of the largest marathons in the 2010 season [1]. Energy Management in Endurance Runners as a Public Health Concern. Among endurance athletes, including distance runners, cyclists, and others, exhausting physiologic carbohydrate reserves is referred to as ‘hitting the wall’ or ‘bonking,’ and athletes engage in a variety of practices, collectively known as ‘carbohydrate loading,’ designed to avoid such catastrophic failure. Energy management has traditionally been perhaps the greatest area of physiologic uncertainty in marathon running: How much carbohydrate does a given runner require to complete the race, and how can a particular runner avoid exhausting his or her carbohydrate reserves, knowing that such depletion will result in a drastic, abrupt, and painful decrease in performance?

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