A Test of the Metabolic Cost of Cushioning Hypothesis during Unshod and Shod Running

Abstract

This study aimed to investigate the effects of surface and shoe cushioning on the metabolic cost of running. In running, the leg muscles generate force to cushion the impact with the ground. External cushioning (surfaces or shoes) may reduce the muscular effort needed for cushioning and thus reduce metabolic cost. Our primary hypothesis was that the metabolic cost of unshod running would decrease with a more cushioned running surface. We also hypothesized that because of the counteracting effects of shoe cushioning and mass, unshod running on a hard surface would have approximately the same metabolic cost as running in lightweight, cushioned shoes.
To test these hypotheses, we attached 10- and 20-mm-thick slats of the same foam cushioning used in running shoe midsoles to the belt of a treadmill that had a rigid deck. Twelve subjects who preferred a midfoot strike pattern and had substantial barefoot/minimalist running experience ran without shoes on the normal treadmill belt and on each thickness of foam. They also ran with lightweight, cushioned shoes on the normal belt. We collected V˙O2 and V˙CO2 to calculate the metabolic power demand and used a repeated-measures ANOVA to compare between conditions.
Compared to running unshod on the normal belt, running unshod on the 10-mm-thick foam required 1.63% ± 0.67% (mean ± SD) less metabolic power (P = 0.034) but running on the 20-mm-thick foam had no significant metabolic effect. Running with and without shoes on the normal belt had similar metabolic power demands, likely because the beneficial energetic effects of cushioning counterbalanced the detrimental effects of shoe mass.
On average, surface and shoe cushioning reduce the metabolic power required for submaximal running.

Full text

A Test of the Metabolic Cost of Cushioning
Hypothesis during Unshod and Shod Running
KRYZTOPHER DAVID TUNG, JASON R. FRANZ, and RODGER KRAM
Department of Integrative Physiology, University of Colorado, Boulder, CO
ABSTRACT
TUNG, K. D., J. R. FRANZ, and R. KRAM. A Test of the Metabolic Cost of Cushioning Hypothesis during Unshod and Shod Running.
Med. Sci. Sports Exerc., Vol. 46, No. 2, pp. 324–329, 2014. Purpose: This study aimed to investigate the effects of surface and shoe
cushioning on the metabolic cost of running. In running, the leg muscles generate force to cushion the impact with the ground. External
cushioning (surfaces or shoes) may reduce the muscular effort needed for cushioning and thus reduce metabolic cost. Our primary
hypothesis was that the metabolic cost of unshod running would decrease with a more cushioned running surface. We also hypothesized
that because of the counteracting effects of shoe cushioning and mass, unshod running on a hard surface would have approximately the
same metabolic cost as running in lightweight, cushioned shoes. Methods: To test these hypotheses, we attached 10- and 20-mm-thick
slats of the same foam cushioning used in running shoe midsoles to the belt of a treadmill that had a rigid deck. Twelve subjects who
preferred a midfoot strike pattern and had substantial barefoot/minimalist running experience ran without shoes on the normal treadmill
belt and on each thickness of foam. They also ran with lightweight, cushioned shoes on the normal belt. We collected V
˙O
2
and V
˙CO
2
to
calculate the metabolic power demand and used a repeated-measures ANOVA to compare between conditions. Results: Compared to
running unshod on the normal belt, running unshod on the 10-mm-thick foam required 1.63% T0.67% (mean TSD) less metabolic power
(P= 0.034) but running on the 20-mm-thick foam had no significant metabolic effect. Running with and without shoes on the normal belt
had similar metabolic power demands, likely because the beneficial energetic effects of cushioning counterbalanced the detrimental
effects of shoe mass. Conclusions: On average, surface and shoe cushioning reduce the metabolic power required for submaximal
running. Key Words: ECONOMY, ENERGETICS, ENERGY COST, SHOES, BAREFOOT
Although our ancestors ran on natural surfaces with-
out shoes, most modern recreational and competitive
runners do so on artificial surfaces with cushioned
shoes. In running, the leg muscles generate force to cushion
the impact with the ground. External cushioning (surfaces or
shoes) may reduce the muscular effort needed for cushion-
ing and thus reduce the metabolic cost of running (13). To
test this idea, we specifically investigated how the cushion-
ing properties of surfaces and shoes affect the metabolic cost
of running.
The elastic and viscoelastic properties of surfaces
(17,18,20) and treadmills (15,16) can negatively, neutrally, or
positively affect the metabolic cost of running. For example,
Lejeune et al. (17) found that running on sand was 1.6 times
more expensive than on a firm floor surface. In contrast, Pugh
(20) found no metabolic difference between running on an
artificial rubberized track and a traditional cinder track. But,
when McMahon and Greene (18) designed and built a
‘‘tuned’’ indoor running track with substantial elastic recoil,
they found that competitive times for the distance running
events were faster on the new track, suggesting reduced met-
abolic cost. Classic and modern research-grade treadmills
have rigid decks, comparable in vertical stiffness to asphalt or
concrete surfaces. However, many modern treadmills, espe-
cially those used for fitness, have decks with fixed or adjust-
able stiffness and damping qualities that appear to increase
the metabolic cost of running (15). In contrast, Kerdok et al.
(16) built a unique research treadmill with adjustable vertical
stiffness and minimal damping which reduced the metabolic
cost of running by as much as 12% with a surface deflection
of È2 cm. Kerdok et al. also found that the subjects ran with
less flexed knees on the low-stiffness treadmill, which pre-
sumably reduced the knee extensor torque required and thus
reduced the metabolic cost of generating force with the
quadriceps muscles.
Like surfaces and treadmills, running shoes have been shown
to have negative (8,9,19), neutral (8,13,20,22), and positive
(13) effects on the metabolic cost of running. For example, Perl
et al. (19) found that running in heavy shoes (with substan-
tial damping properties) required more metabolic energy than
lightweight, ‘‘minimal’’ shoes. Frederick et al. (14) established
that shoe mass incurs a predictable metabolic penalty (1% per
100 g per shoe). However, in a different study, Frederick et al.
Address for correspondence: Kryztopher David Tung, M.S., Locomotion
Lab, Department of Integrative Physiology, University of Colorado,
Boulder, CO 80309-0354; E-mail: kryztophert@gmail.com.
Submitted for publication February 2013.
Accepted for publication July 2013.
0195-9131/14/4602-0324/0
MEDICINE & SCIENCE IN SPORTS & EXERCISE
Ò
Copyright Ó2013 by the American College of Sports Medicine
DOI: 10.1249/MSS.0b013e3182a63b81
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(13) observed that, despite the mass of the shoes, the sub-
maximal rate of oxygen consumption (V
˙O
2
) did not differ
between running in well-cushioned shoes and barefoot. Note
that the treadmill used in that study had a rigid deck. To try
and explain their results, Frederick et al. (13) hypothesized
that submaximal V
˙O
2
during barefoot running includes a ‘‘cost
of cushioning’’ the body and thus lightweight, well-cushioned
shoes might reduce the metabolic cost of running. Indeed,
Franz et al. (11) showed that when shoe mass is factored out,
cushioned shoes can require 3%–4% less metabolic power
than running without shoes. However, Franz et al. could not
definitively attribute the energy savings specifically to the
shoe cushioning.
Differences in footstrike (e.g., rearfoot vs midfoot) and/or
other shoe-related factors (e.g., heel height, flexibility, mo-
tion control elements) could conceivably enhance or blunt
any metabolic cost savings due to cushioning. Thus, we
designed an experiment to isolate and measure the metabolic
effects of shoe cushioning in a novel way—by attaching the
same foam used in running shoe midsoles to the belt of
a rigid-decked treadmill. This approach isolated the inde-
pendent variable (cushioning) and eliminated possible con-
founding shoe construction factors. This allowed us to
measure the effects of shoe cushioning without having our
subjects wear shoes. We first hypothesized that the metabolic
cost of unshod running would decrease on a cushioned sur-
face. We also hypothesized that, on the normal rigid tread-
mill surface, unshod running would have approximately the
same metabolic cost as running with lightweight, cushioned
running shoes due to counteracting effects of shoe mass and
shoe cushioning.
METHODS
We present data for 12 healthy runners (10 M/2 F; mean T
SD, age = 30.2 T9.1 yr, mass = 68.5 T6.5 kg, and height =
174.5 T5.9 cm). These subjects reported running an average
of 79.4 T60.5 kmIwk
j1
, of which 59.5 T50.0 kmIwk
j1
(range = 11–177 kmIwk
j1
) were barefoot or in minimal
shoes. Subjects reported that their typical training speed av-
eraged 3.5 T0.6 mIs
j1
(range = 2.9–4.8 mIs
j1
). Our sample
size was based on the recommendations of Frederick (12),
who reported that, with an expected coefficient of variation of
1.5%–2% for repeated within-day measurements of oxygen
uptake, a 1%–2% mean difference could be resolved with a
sample size of 10–15 subjects. Indeed, with careful attention
to detail, Roy and Stefanyshyn (21) were able to discern
economy differences of 1% between shoe conditions using a
sample size of 13. Thus, we collected data for 14 subjects.
However, we had to exclude the data from two subjects be-
cause they exhibited RER 91.0, indicating that they were not
in metabolic steady state (3). Subject inclusion criteria were
as follows: 918 yr of age; midfoot strike preference both
shod and unshod; run at least 25 kmIwk
j1
, of which at least
8kmIwk
j1
were barefoot or in minimal running footwear
(e.g., Vibram Five Fingers
Ò
) for at least 3 months; injury free;
self-reported ability to sustain 3.3 mIs
j1
running pace for at
least 60 min; and meeting the medical criteria of the Ameri-
can College of Sports Medicine for minimal risk for exercise
(1). On the basis of subject reports and our inclusion criteria,
completing the experimental protocol was of low to moder-
ate intensity and duration for all subjects. We included only
runners with a midfoot strike pattern because asking runners
to rearfoot strike without shoes on a hard treadmill surface
might have increased the risk of injury. The University of
Colorado Institutional Review Board approved the study pro-
tocol, and all subjects gave their written consent after being
informed of the nature of the study.
To verify that the subjects preferred to run with a midfoot
strike pattern (4), we asked them to run at their typical train-
ing pace across a 30-m runway equipped with a force plat-
form (Advanced Mechanical Technology Inc., Watertown,
MA) to which a sheet of paper was affixed. We attached
small pieces of felt marker to each subjects’ right foot at 90%,
70%, and 33% of their foot length (measured between the
heel and the distal end of the second toe). We collected the
force plate data at 1000 Hz and tracked the center of pressure
relative to the data points provided by the ink dots left on
the paper as per Cavanagh and Lafortune (4). We classified
subjects as midfoot strikers if the center of pressure at ini-
tial contact was between 33% and 70% of foot length and
rearfoot strikers if the center of pressure originated posterior
to the 33% mark (4).
During a single experimental session, subjects completed
a 5-min standing trial, a 10-min unshod running acclima-
tion trial (with no surface cushioning), and then four 5-min
running trials. A 3-min rest period separated each of the
running trials. In all running trials, subjects ran at a speed of
3.35 mIs
j1
on a Quinton 18-60 motorized treadmill (Quinton
Instrument Company, Bothell, WA) that we modified to have
a calibrated digital readout for speed. Note that this treadmill
has a rigid steel deck and a thin belt with no significant
cushioning or damping properties. For the duration of the
experiment, subjects wore very thin, slip-resistant yoga socks
for traction and hygienic purposes.
In random order, subjects completed one shod (Nike Free
3.0 V2; È211 g per shoe) running condition on the normal
treadmill belt surface and three unshod running trials: on the
normal treadmill belt surface (Unshod 0 mm), with 10-mm-
thick slats of foam attached to the belt (Unshod 10 mm), and
with 20-mm-thick slats of foam attached to the belt (Unshod
20 mm) (Fig. 1). The foam slats (length width thick-
ness; 18.8 cm 33.7 cm 10 mm and 21.4 cm 37.3 cm 
20 mm) consisted of the same material used in the midsole
of the Nike Free running shoes (Phylite
Ò
; 60% Phylon
Ò
and 40% rubber with an Asker Type C durometer reading
of 52–58). We drilled two 2.5-cm-diameter holes along the
left and right lateral edges of each foam slat through which
we sewed short loops of 2.5-cm-wide nylon webbing. We
sewed continuous strips of hook Velcro
Ò
to two 2.5-cm-
wide straps of nylon webbing (one left and one right) and
routed the strap through the small loops. We glued strips of
METABOLIC COST OF CUSHIONING DURING RUNNING Medicine & Science in Sports & Exercise
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the hooked part of the VelcroÒto the left and right edges of
the treadmill belt. Thus, the slats of foam could be easily put
in place and removed. Overall, we created a ‘‘tank-tread’’ of
foam slats that covered the entire length of the treadmill belt.
During the running trials, we offered verbal instructions to
each subject to maintain a midfoot strike pattern whether
shod or unshod. Further, we confirmed foot strike through-
out each trial visually as well as with high-speed video re-
cordings (Casio EX-FH20; 210 frames per second). We did
not control the stride frequency or stride length so as to
compare normal unshod and shod running. We determined
each subject’s contact time and stride frequency from the video
recordings using Windows Movie Maker (Microsoft Corpora-
tion, Redmond, WA) averaged over five consecutive strides.
During the standing and running trials, we used an open-
circuit respirometry system (TrueOne 2400; Parvo Medics,
Sandy, UT) to analyze the subject’s expired gases and cal-
culate the STPD rates of oxygen consumption (V
˙O
2
) and
carbon dioxide production (V
˙CO
2
). Before each experiment,
we calibrated the system using reference gases and a 3-L
syringe. We averaged V
˙O
2
,V
˙CO
2
, and RERs for the last
2 min of each 5-min trial. As noted, two subjects had to be
excluded because their RER values exceeded 1.0. The RER
values for each of the 12 remaining subjects were below 1.0.
We report not only gross V
˙O
2
values in milliliters per kilogram
per minute (mLIkg
j1
Imin
j1
) but also the average standing
value (mean TSD: 4.84 T0.39 mLIkg
j1
Imin
j1
) to allow cal-
culation of net V
˙O
2
. We normalized V
˙O
2
and V
˙CO
2
using
the subject’s body mass while unshod. From V
˙O
2
and V
˙CO
2
,
we calculated gross metabolic power in watts per kilogram
(WIkg
j1
) using Brockway’s equation (2). We agree with
Fletcher et al. (10) who suggested that metabolic power is
more representative of running economy than V
˙O
2
alone, but
we report both metabolic power and V
˙O
2
for the convenience
of the reader.
A Shapiro–Wilk test and Mauchly test of sphericity respec-
tively confirmed that metabolic cost, contact time, and stride
frequency were normally distributed (P90.24, P90.08, and
P90.07, respectively) and each had equal variance across
conditions (P= 0.63, P= 0.15, and P= 0.62, respectively). A
repeated-measures ANOVA then tested for significant main
effects of cushioning (0, 10, and 20 mm) on V
˙O
2
,grossmet-
abolic power, contact time, and stride frequency. When a sig-
nificant main effect was detected, we performed post hoc
pairwise comparisons. We also compared shod and un-
shod conditions using paired-samples t-tests. We used a
criterion of PG0.05 for statistical significance.
RESULTS
Treadmill surface cushioning significantly decreased V
˙O
2
and metabolic power. On average, V
˙O
2
and metabolic
power for unshod running were 1.47% (P= 0.015) and 1.63%
(P=0.034)lesson10mmoffoamcushioningcomparedto
the rigid surface, respectively (Table 1 and Fig. 2). However,
those measures for running on 20 mm of foam cushioning
were not significantly different from those for running on the
rigid surface (P= 0.602 and P=0.605,respectively).Wedid
find considerable individual variation with respect to the
effect of surface cushioning on metabolic demand (Table 2
and Fig. 3). However, 10 of the 12 subjects had lower V
˙O
2
values and 8 of the 12 subjects required less metabolic power
for the 10-mm-thick foam surface compared to the rigid sur-
face. On the rigid treadmill surface, V
˙O
2
and metabolic
power for running unshod and shod were not significantly
different (P= 0.533 and P=0.182,respectively).
Average stride frequencies for unshod running on the
cushioned surfaces were not significantly different from un-
shod running on the normal rigid surface, but stride frequency
was a modest 2.5% slower during shod running (PG0.01)
(Table 1). Hence, stride length was 2.5% longer (average
of È5 cm). Likewise, ground contact times were not different
FIGURE 1—Treadmill with a ‘‘tank tread’’ of foam slats. Each slat is
separated by a 4-mm gap, which was considered to be negligible and
imperceptible by our subjects.
TABLE 1. Metabolic and kinematic variables.
Unshod 0 mm Unshod 10 mm Unshod 20 mm Shod
V
˙O
2
(mLIkg
j1
Im
j1
)39.17 T3.68 38.58 T3.27* 38.99 T2.88 39.36 T3.09
Metabolic power (WIkg
j1
)13.40 T1.28 13.18 T1.09* 13.34 T1.08 13.59 T1.15
Stride frequency (Hz) 1.625 T0.11 1.666 T0.129 1.651 T0.136 1.585 T0.101**
Contact time (s) 0.238 T0.013 0.237 T0.016 0.240 T0.017 0.252 T0.015**
Values are mean TSD.
*PG0.05.
**PG0.01.
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between the unshod conditions, but contact time was 5.9%
longer during the shod condition (PG0.01) (Table 1).
DISCUSSION
In this study, we tested the metabolic cost of cushion-
ing hypothesis, which states that running involves a ‘‘cost of
cushioning’’ the body against impact (13). Specifically,
we quantified the isolated effects of shoe cushioning on the
metabolic cost of running, while controlling for footstrike
pattern, barefoot/minimalist running experience, and footwear.
Supporting our first hypothesis, we found that, on average, the
metabolic cost of unshod running was significantly reduced
when subjects ran on a 10-mm-thick foam-cushioned surface
compared to a normal rigid treadmill surface. Supporting our
second hypothesis, on the normal, rigid treadmill surface, the
metabolic cost of unshod running was not significantly differ-
ent from running with lightweight, cushioned running shoes.
To further clarify, our first hypothesis stated that the meta-
bolic cost of unshod running would decreasewith a cushioned
surface. While 10 mm of surface cushioning did elicit a lower
metabolic costthan the rigid treadmill surface alone, 20 mm of
surface cushioning did not, on average, further reduce meta-
bolic cost. We suspect that there may be an optimal cushion-
ing thickness for each individual, which minimizes his/her
metabolic power demand. This optimum likely depends on
many factors including cushioning hardness (durometer),
body mass, and footstrike preference.
The elastic and viscoelastic (damping) properties of run-
ning shoes and surfaces can combine to influence the met-
abolic cost of running. Kerdok et al. (16) found that the
metabolic cost of running on an elastic, adjustable-stiffness
treadmill steadily decreased with decreased stiffness. In con-
trast, we did not find that the metabolic cost of running
steadily decreased with thicker foam cushioning. It is likely
that the treadmill surface of Kerdok et al. had much less
damping than our foam surfaces and thus, that of running
shoes. Indeed, excessive damping may have negative meta-
bolic effects. In another treadmill running study, Hardin et al.
(15) reported that metabolic cost increased with a lower-
stiffness treadmill surface that also had greater damping.
Treadmill surface properties should be considered when
interpreting studies of footwear energetics and biomechanics
and when designing future studies.
Our data also support our second hypothesis, that is, un-
shod running would have approximately the same metabolic
cost as running with lightweight shoes due to counteract-
ing effects of cushioning and mass. In a previous study from
our laboratory, Franz et al. (11) investigated the effects of
adding mass to the feet on the metabolic cost of shod and
unshod running. In accordance with the classic findings
from Frederick et al. (14), Franz et al. (11) found that every
100 g of mass added to each foot increased the rate of ox-
ygen consumption by È1%, both with and without shoes.
Based on this ‘‘1% rule’’ alone, we would expect that run-
ning in the 210-g shoes used in the present study would be
FIGURE 2—Percent difference in metabolic power for unshod cush-
ioned conditions compared to the unshod 0-mm condition. Data are
mean TSE. *Significant difference (P= 0.034) between the unshod
10-mm and unshod 0-mm conditions.
TABLE 2. Metabolic power (WIkg
j1
) for each individual subject.
Subject No.
Unshod
0mm
Unshod
10 mm
Unshod
20 mm Shod
116.40 15.77 16.05 16.31
213.94 13.77 13.49 13.83
311.80 11.98 12.08 12.01
413.75 12.96 13.15 13.72
512.54 12.62 13.06 13.07
614.68 14.22 14.43 14.36
712.94 12.70 12.58 13.02
814.23 13.83 13.81 13.95
912.58 12.31 12.26 12.88
10 13.18 13.36 13.38 14.27
11 12.16 11.99 12.57 12.12
12 12.62 12.70 13.19 13.50
Mean 13.40 13.18 13.34 13.59
SD 1.28 1.09 1.08 1.15
FIGURE 3—Individual percent differences in metabolic power for the
unshod cushioned surface conditions compared to the unshod on the
rigid treadmill surface condition.
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2.10% more expensive than running unshod on the normal
rigid treadmill surface. However, we found that those con-
ditions elicited similar metabolic costs. Running unshod on
the 10 mm of foam cushioning (approximately the thickness
of the shoe midsole in the forefoot region) afforded an en-
ergetic savings of 1.63%. Thus, it appears that the positive
effects of shoe cushioning counteracted the negative effects
of added mass, resulting in a metabolic cost for shod running
approximately equal to that of unshod running.
While our stride kinematics results for unshod versus shod
conditions were similar to those of previous studies, our data
for unshod running on cushioned surface conditions provide
new insight. We found that stride frequency was 2.5% slower
for shod running, which is similar but somewhat less than
the 3.3%, 3.4%, 3.9%, 5.1%, and 5.7% values reported by
Franz et al. (11), Divert et al. (8), De Wit et al. (6), Divert
et al. (7), and Squadrone and Gallozzi (22), respectively.
Those studies varied in what factors were controlled for (e.g.,
footstrike type). A slower stride frequency at a fixed speed
equates to longer strides while running shod compared to
unshod. What aspects of shoes are responsible for the longer
strides? Franz et al. (11) found that the longer strides were not
due to shoe mass. Other authors have suggested that shorter
strides are selected during barefoot running to reduce loading
or, in other words, shoe cushioning allows for longer strides
with similar loading. In the present study, we found that stride
frequency did not differ between unshod running on the hard
surface and the cushioned surfaces. Thus, the stride frequency/
length differences between unshod versus shod running
cannot be attributed to the cushioning properties of the shoe.
Similarly, our contact time data for unshod versus shod
running were akin to those of previous studies. We found that
contact time was 5.9% longer for shod running, similar to the
2.4%, 4.1%, 5.0%, 5.7%, and È9% differences reported by
Divert et al. (7), Kerdok et al. (16), De Wit et al. (6) Divert
et al. (8), and Clarke et al. (5), respectively. But again, we
found that running unshod on hard and cushioned surfaces
had similar contact times. Thus, the contact time differences
between unshod and shod cannot be attributed to the cush-
ioning properties of the shoe. These topics may deserve fur-
ther investigation.
Our study had several limitations. First, to maintain con-
sistency, all subjects ran in the same model of running shoes;
therefore, our findings may not translate to other running
shoe models. Simply due to the demographics of our vol-
unteers, our subjects were predominately male. However,
we have no reason to expect different results for female
runners. Finally, we only studied two thicknesses of one
specific foam cushioning material.
Our results have implications for the design of running
shoes and/or track surfaces. Many competition track surfaces
are extremely hard, presumably to enhance sprint perfor-
mance. Despite the prevalent track hardness, spiked shoes
designed for middle- and long-distance track running events
have almost no cushioning under the midfoot and forefoot.
Our results suggest that distance running spikes with midfoot/
forefoot cushioning (or the use of racing flats) could enhance
performance. Alternatively, track surfaces could be made with
soft, yet elastic properties, allowing athletes to run without
shoes. However, given the interindividual variability that we
noted, the benefits of a soft track would not be uniform and
thus possibly considered to be unfair. Our data suggest that
the design of competition shoes for road racing on paved
surfaces should not overemphasize weight minimization at the
expense of cushioning.
Future studies on the energetics, biomechanics, and neu-
romuscular control of running on different surfaces could be
fruitful. For example, to identify the metabolically optimal
thickness of foam cushioning, future studies could compare
more experimental conditions (e.g., 5, 10, 15, 20, 25 mm).
A complementary approach would be to compare foam sur-
faces of the same thickness (e.g., 10 mm) but with different
hardness properties (i.e., durometer values). Because small
reductions in the metabolic cost of running are most mean-
ingful for competitive runners, it would be useful to repeat
our study at faster running speeds on more aerobically fit
subjects. In addition, it may be interesting to study how run-
ners adapt over time to different cushioned surfaces. Further,
we have not yet elucidated the biomechanical or neuromus-
cular basis for why metabolic cost was reduced onthe 10-mm-
thick cushioned surface. EMG measurements may be able
to detect a reduction in muscle activity, but the trial-to-trial
variability may preclude the detection of small differences.
In summary, we found that a moderate thickness of foam
cushioning generally reduced the metabolic cost of run-
ning. In addition, the metabolic cost of running did not differ
between unshod and shod conditions, presumably because
the positive effect of cushioning was counteracted by the
negative effect of shoe mass. Overall, our data provide clear
support for the cost of cushioning hypothesis of Frederick
et al. (13).
Nike, Inc., donated the foam cushioning and shoes used in this
study but was not involved in the conception, planning, design, or
interpretation of the study. Rodger Kram is a paid consultant for
Nike, Inc.
The results of the present study do not constitute endorsement by
the American College of Sports Medicine.
REFERENCES
1. American College of Sports Medicine. ACSM’s Guidelines for
Exercise Testing and Prescription. 7th ed. Baltimore (MD):
Lippincott Williams & Wilkins; 2006. p. 368.
2. Brockway JM. Derivation of formulae used to calculate energy
expenditure in man. Hum Nutr Clin Nutr. 1987;41C:463–71.
3. Brooks GA, Fahey TD, Baldwin KM. Exercise Physiology: Human
Bioenergetics and Its Applications. 4th ed. New York (NY):
McGraw Hill; 2004. p. 928.
4. Cavanagh PR, Lafortune MA. Ground reaction forces during dis-
tance running. J Biomech. 1980;13:397–406.
http://www.acsm-msse.org328 Official Journal of the American College of Sports Medicine
APPLIED SCIENCES
Copyright © 2014 by the American College of Sports Medicine. Unauthorized reproduction of this article is prohibited.

5. Clarke TE, Frederick EC, Cooper LB. Effects of shoe cushioning
upon ground reaction forces in running. Int J Sports Med. 1983;
4(4):247–51.
6. De Wit B, De Clercq D, Aerts P. Biomechanical analysis of the
stance phase during barefoot and shod running. J Biomech. 2000;
33:269–78.
7. Divert C, Mornieux G, Baur H, Mayer F, Belli A. Mechanical
comparison of barefoot and shod running. Int J Sports Med. 2005;
26(7):593–8.
8. Divert C, Mornieux G, Freychat P, Baly L, Mayer F, Belli A.
Barefoot-shod running differences: shoe or mass effect. Int J Sports
Med. 2008;29(6):512–8.
9. Flaherty RF. Running economy and kinematic differences among
running with the foot shod, with the foot bare, and with the bare
foot equated for weight [dissertation]. Springfield (MA): Springfield
College; 1994. p. 106.
10. Fletcher JR, Esau SP, MacIntosh BR. Economy of running: be-
yond the measurement of oxygen uptake. J Appl Physiol. 2009;
107:1918–22.
11. Franz JR, Wierzbinski CM, Kram R. Metabolic cost of running
barefoot versus shod: is lighter better? Med Sci Sports Exerc. 2012;
44(8):1519–25.
12. Frederick EC. Measuring the effects of shoes and surfaces on the
economy of locomotion. In: Nigg BM, Kerr BA, editors. Biome-
chanical Aspects of Sport Shoes and Playing Surfaces. Calgary
(Canada): University of Calgary; 1983. pp. 93–106.
13. Frederick EC, Clarke TE, Larsen JL, Cooper LB. The effect of
shoe cushioning on the oxygen demands on running. In: Nigg BM,
Kerr BA, editors. Biomechanical Aspects of Sports Shoes and
Playing Surfaces. Calgary (Canada): University of Calgary; 1983.
pp. 107–14.
14. Frederick EC, Daniels JT, Hayes JW. The effect of shoe weight on
the aerobic demands of running. In: Bachl N, Prokop L, Suckert R,
editors. Current Topics in Sports Medicine. Vienna (Austria): Ur-
ban & Schwarzenberg; 1984. pp. 616–25.
15. Hardin EC, van den Bogert AJ, Hamill J. Kinematic adaptations
during running: effects of footwear, surface, and duration. Med Sci
Sports Exerc. 2004;36(5):838–44.
16. Kerdok AE, Biewener AA, McMahon TA, Weyand PG, Herr HM.
Energetics and mechanics of human running on surfaces of dif-
ferent stiffnesses. J Appl Physiol. 2002;92:469–78.
17. Lejeune TM, Willems PA, Heglund NC. Mechanics and ener-
getics of human locomotion on sand. JExpBiol. 1998;201(Pt 13):
2071–80.
18. McMahon TA, Greene PR. Fast running tracks. Sci Am. 1978;239(6):
148–63.
19. Perl DP, Daoud AI, Lieberman DE. Effects of footwear and strike
type on running economy. Med Sci Sports Exerc. 2012;44(7):
1335–43.
20. Pugh LG. Oxygen intake in track and treadmill running with
observations on the effect of air resistance. J Physiol. 1970;207:
823–35.
21. Roy JP, Stefanyshyn DJ. Shoe midsole longitudinal bending
stiffness and running economy, joint energy, and EMG. Med Sci
Sports Exerc. 2006;38(3):562–9.
22. Squadrone R, Gallozzi C. Biomechanical and physiological com-
parison of barefoot and two shod conditions in experienced bare-
foot runners. J Sports Med Phys Fitness. 2009;49(1):6–13.
METABOLIC COST OF CUSHIONING DURING RUNNING Medicine & Science in Sports & Exercise
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