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PORTSIDE  October 2010, Week 5

PORTSIDE October 2010, Week 5

Subject:

Metabolic Factors Limiting Performance in Marathon Runners

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Sun, 31 Oct 2010 23:16:25 -0400

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Metabolic Factors Limiting Performance in Marathon Runners
Benjamin I. Rapoport
PLoS Computational Biology
October 21, 2010
http://www.ploscompbiol.org/article/info%3Adoi%2F10.1371%2Fjournal.pcbi.1000960;jsessionid=A93E721C7C34B977040E260070E88D2D.ambra02

M.D.- Ph.D. Program, Harvard Medical School, Boston,
Massachusetts, United States of America, Department of
Electrical Engineering and Computer Science and Division
of Health Sciences and Technology, Massachusetts
Institute of Technology, Cambridge, Massachusetts,
United States of America

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.

Author Summary

Marathon running, historically perceived as testing the
physiologic limits of human endurance, has become
increasingly popular even among recreational runners. Of
those runners who test their endurance by racing the
marathon distance, however, more than two in five report
`hitting the wall,' the rapid onset of severe fatigue
and inability to maintain a high-intensity pace,
resulting from the near-complete depletion of
carbohydrate stores in the leg muscles and liver. An
apparent paradox of long-distance running is that even
the leanest athletes store enough fat to power back-to-
back marathons, yet small carbohydrate reservoirs can
nevertheless catastrophically limit performance in
endurance exercise. In this study I develop and validate
a mathematical model that facilitates computation of
personalized estimates of the distances at which runners
will exhaust their carbohydrate stores while running at
selected paces. In addition, I provide a systematic
approach to estimating personalized maximum speeds at
which runners can safely complete a marathon, based on
accessible physiologic parameters such as heart rate and
running speed. This analysis provides a quantitative
basis for improving the safety and optimizing the
performance of endurance runners, evaluating midrace
fueling requirements, and estimating limits of
performance in human endurance running, for elite and
recreational runners alike.

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