3972
INTRODUCTION
The metabolic and mechanical requirements of human walking
influence a broad array of structural, functional and health
relationships. This global functional importance has stimulated a
body of scientific literature that now spans more than a century and
encompasses a variety of experimental objectives. These range from
basic biological inquiry to applied efforts to predict speed, energy
expenditure and other variables in laboratory and field settings.
However, in spite of the extensive scientific consideration that
human walking has received, some aspects of basic understanding
remain limited.
A primary example of incomplete contemporary understanding
is the body size dependency long observed for the metabolic
requirements of this gait. As would be expected, larger individuals
do expend more energy than smaller ones when the metabolic energy
expended is expressed in absolute terms. However, the differences
observed are not directly proportional to body mass. When expressed
on a per kilogram basis, the energy expended to walk a fixed distance
or at a given speed can be as much as two to three times greater
for smaller versus larger individuals. At present, a quantitative
explanation for the relationship between body size and the energy
cost of human walking has not been established.
The greater mass-specific metabolic rates consistently observed
for smaller versus larger human walkers have been considered from
several perspectives. Ontogenetic approaches have appropriately
considered both maturation (DeJaeger et al., 2001; Morgan et al.,
2002) and body size (McCann and Adams, 2002) but have not
resolved their quantitative importance. Mechanical approaches have
estimated that the mass-specific mechanical work that small children
and adults perform during walking differs only marginally (Cavagna
et al., 1983; Bastien et al., 2003; Schepens et al., 2004) and therefore
does not account (Schepens et al., 2004) for the much larger
differences observed in metabolic cost. The current lack of
quantitative understanding is reflected in the use of different
generalized equations to estimate the energy expended by adult
(ACSM, 2006; Pandolf et al., 1971) and child populations (Morgan
et al., 2002). In both cases, population-specific equations predict
the same mass-specific metabolic rates for individuals who differ
in height and mass.
A potential explanation for the apparent body-size dependency
of the metabolic cost of human walking is a corresponding rate
dependency in executing the mechanics of each walking stride
(Alexander, 1976; Heglund and Taylor, 1988). Clearly, the
shorter statures of smaller versus larger walkers require more,
and more frequent, strides in order to travel any fixed distance
or at any given speed. If the mechanical components of each
walking stride were to require the same expenditure of metabolic
energy per kilogram of body mass, shorter walkers might have
greater mass-specific metabolic rates simply because they take
more frequent strides. This possibility seems most plausible if
shorter and taller individuals were to walk in dynamically similar
ways, i.e. with both stride lengths and times related to the body’s
length (L
b
) by some constant proportion. Although widely
embraced (DeJaeger et al., 2001; McCann and Adams, 2002;
Cavagna et al., 1983), the validity of the dynamic similarity
assumption is not strictly known. Thus, the simple possibility that
the energy cost per stride at equivalent speeds may be the same
for short and tall individuals has not been evaluated.
The Journal of Experimental Biology 213, 3972-3979
© 2010. Published by The Company of Biologists Ltd
doi:10.1242/jeb.048199
The mass-specific energy cost of human walking is set by stature
Peter G. Weyand
1,
*, Bethany R. Smith
2
, Maurice R. Puyau
3
and Nancy F. Butte
3
1
Locomotor Performance Laboratory, Department of Applied Physiology and Wellness, Southern Methodist University, Dallas, TX
75205, USA,
2
Locomotion Laboratory, Department of Kinesiology, Rice University, Houston, TX 77005, USA and
3
USDA/ARS
Children’s Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine, Houston, TX 77030, USA
*Author for correspondence (pweyand@smu.edu)
Accepted 6 September 2010
SUMMARY
The metabolic and mechanical requirements of walking are considered to be of fundamental importance to the health,
physiological function and even the evolution of modern humans. Although walking energy expenditure and gait mechanics are
clearly linked, a direct quantitative relationship has not emerged in more than a century of formal investigation. Here, on the basis
of previous observations that children and smaller adult walkers expend more energy on a per kilogram basis than larger ones
do, and the theory of dynamic similarity, we hypothesized that body length (or stature, L
b
) explains the apparent body-size
dependency of human walking economy. We measured metabolic rates and gait mechanics at six speeds from 0.4 to 1.9 m s
–1
in
48 human subjects who varied by a factor of 1.5 in stature and approximately six in both age and body mass. In accordance with
theoretical expectation, we found the most economical walking speeds measured (J kg
–1
m
–1
) to be dynamically equivalent (i.e.
similar U, where Uvelocity
2
/gravity · leg length) among smaller and larger individuals. At these speeds, stride lengths were
directly proportional to stature whereas the metabolic cost per stride was largely invariant (2.74±0.12 J kg
–1
stride
–1
). The tight
coupling of stature, gait mechanics and metabolic energy expenditure resulted in an inverse relationship between mass-specific
transport costs and stature (E
trans
/M
b
L
b
–0.95
, J kg
–1
m
–1
). We conclude that humans spanning a broad range of ages, statures and
masses incur the same mass-specific metabolic cost to walk a horizontal distance equal to their stature.
Supplementary material available online at http://jeb.biologists.org/cgi/content/full/213/23/3972/DC1
Key words: metabolism, scaling, locomotion, biomechanics.
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