Foot strike patterns and collision forces in habitually barefoot versus shod runners

Abstract

Humans have engaged in endurance running for millions of years, but the modern running shoe was not invented until the 1970s. For most of human evolutionary history, runners were either barefoot or wore minimal footwear such as sandals or moccasins with smaller heels and little cushioning relative to modern running shoes. We wondered how runners coped with the impact caused by the foot colliding with the ground before the invention of the modern shoe. Here we show that habitually barefoot endurance runners often land on the fore-foot (fore-foot strike) before bringing down the heel, but they sometimes land with a flat foot (mid-foot strike) or, less often, on the heel (rear-foot strike). In contrast, habitually shod runners mostly rear-foot strike, facilitated by the elevated and cushioned heel of the modern running shoe. Kinematic and kinetic analyses show that even on hard surfaces, barefoot runners who fore-foot strike generate smaller collision forces than shod rear-foot strikers. This difference results primarily from a more plantarflexed foot at landing and more ankle compliance during impact, decreasing the effective mass of the body that collides with the ground. Fore-foot- and mid-foot-strike gaits were probably more common when humans ran barefoot or in minimal shoes, and may protect the feet and lower limbs from some of the impact-related injuries now experienced by a high percentage of runners.

Full text

LETTERS
Foot strike patterns and collision forces in habitually
barefoot versus shod runners
Daniel E. Lieberman
1
, Madhusudhan Venkadesan
1,2
*, William A. Werbel
3
*, Adam I. Daoud
1
*, Susan D’Andrea
4
,
Irene S. Davis
5
, Robert Ojiambo Mang’Eni
6,7
& Yannis Pitsiladis
6,7
Humans have engaged in endurance running for millions of years
1
,
but the modern running shoe was not invented until the 1970s. For
most of human evolutionary history, runners were either barefoot
or wore minimal footwear such as sandals or moccasins with smal-
ler heels and little cushioning relative to modern running shoes.
We wondered how runners coped with the impact caused by the
foot colliding with the ground before the invention of the modern
shoe. Here we show that habitually barefoot endurance runners
often land on the fore-foot (fore-foot strike) before bringing down
the heel, but they sometimes land with a flat foot (mid-foot strike)
or, less often, on the heel (rear-foot strike). In contrast, habitually
shod runners mostly rear-foot strike, facilitated by the elevated
and cushioned heel of the modern running shoe. Kinematic and
kinetic analyses show that even on hard surfaces, barefoot runners
who fore-foot strike generate smaller collision forces than shod
rear-foot strikers. This difference results primarily from a more
plantarflexed foot at landing and more ankle compliance during
impact, decreasing the effective mass of the body that collides with
the ground. Fore-foot- and mid-foot-strike gaits were probably
more common when humans ran barefoot or in minimal shoes,
and may protect the feet and lower limbs from some of the impact-
related injuries now experienced by a high percentage of runners.
Running can be most injurious at the moment the foot collides
with the ground. This collision can occur in three ways: a rear-foot
strike (RFS), in which the heel lands first; a mid-foot strike (MFS), in
which the heel and ball of the foot land simultaneously; and a fore-
foot strike (FFS), in which the ball of the foot lands before the heel
comes down. Sprinters often FFS, but 75–80% of contemporary shod
endurance runners RFS
2,3
. RFS runners must repeatedly cope with
the impact transient of the vertical ground reaction force, an abrupt
collision force of approximately 1.5–3 times body weight, within the
first 50 ms of stance (Fig. 1a). The time integral of this force, the
impulse, is equal to the change in the body’s momentum during this
period as parts of the body’s mass decelerate suddenly while others
decelerate gradually
4
. This pattern of deceleration is equivalent to
some proportion of the body’s mass (M
eff
, the effective mass) stop-
ping abruptly along with the point of impact on the foot
5
. The rela-
tion between the impulse, the body’s momentum and M
eff
is
expressed as
ð
T
0{
Fz(t)~Mbody(Dvcom zgT)~Mef f ({vfoot zgT)ð1Þ
where F
z
(t) is the time-varying vertical ground reaction force, 0
2
is
the instant of time before impact, Tis the duration of the impact
transient, M
body
is the body mass, v
com
is the vertical speed of the
centre of mass, v
foot
is the vertical speed of the foot just before impact
and gis the acceleration due to gravity at the Earth’s surface.
Impact transients associated with RFS running are sudden forces
with high rates and magnitudes of loading that travel rapidly up the
body and thus may contribute to the high incidence of running-
related injuries, especially tibial stress fractures and plantar
fasciitis
6–8
. Modern running shoes are designed to make RFS running
comfortable and less injurious by using elastic materials in a large
heel to absorb some of the transient force and spread the impulse over
more time
9
(Fig. 1b). The human heel pad also cushions impact
transients, but to a lesser extent
5,10,11
, raising the question of how
runners struck the ground before the invention of modern running
shoes. Previous studies have found that habitually shod runners tend
to adopt a flatter foot placement when barefoot than when shod, thus
reducing stresses on the foot
12–15
, but there have been no detailed
studies of foot kinematics and impact transients in long-term habitu-
ally barefoot runners.
We compared foot strike kinematics on tracks at preferred endurance
running speeds (4–6 m s
21
) among five groups controlled for age and
habitual footwear usage (Methods and Supplementary Data 2). Adults
were sampled from three groups of individuals who run a minimum of
20 km per week: (1) habitually shod athletes from the USA; (2) athletes
from the Rift Valley Province of Kenya (famed for endurance running
16
),
most of whom grew up barefoot but now wear cushioned shoes when
running; and (3) US runners who grew up shod but now habitually run
barefoot or in minimal footwear. We also compared adolescents from
two schools in the Rift Valley Province: one group (4) who have never
worn shoes; and another group (5) who have been habitually shod most
of their lives. Speed, age and distance run per week were not correlated
significantly with strike type or foot and ankle angles within or among
groups. However, because the preferred speed was approximately
1ms
21
slower in indoor trials than in outdoor trials, we made statistical
comparisons of kinematic and kinetic data only between groups 1 and3
(Table 1).
Strike patterns vary within subjects and groups, but these trials
(Table 1 and Supplementary Data6) confirm reports
2,3,9
that habitu-
ally shod runners who grew up wearing shoes (groups 1 and 5) mostly
RFS when shod; these runners also predominantly RFS when barefoot
on the same hard surfaces, but adopt flatter foot placements by dorsi-
flexing approximately 7–10uless (analysis of variance, P,0.05). In
contrast, runners who grew up barefoot or switched to barefoot run-
ning (groups 2 and 4) most often used FFS landings followed by heel
contact (toe–heel–toerunning) in both barefoot and shod conditions.
MFS landings were sometimes used in barefoot conditions (group 4)
*These authors contributed equally to this work.
1
Department of Human Evolutionary Biology, 11 Divinity Avenue,
2
School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA.
3
University
of Michigan Medical School, Ann Arbor, Michigan 48109, USA.
4
Center for Restorative and Regenerative Medicine, Providence Veterans Affairs Medical Center, Providence, Rhode
Island 02906, USA.
5
Department of Physical Therapy, University of Delaware, Newark, Delaware 19716, USA.
6
Department of Medical Physiology, Moi University Medical School, PO
Box 4606, 30100 Eldoret, Kenya.
7
Faculty of Biomedical & Life Sciences, University of Glasgow, Glasgow G12 8QQ, UK.
Vol 463
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©2010

and shod conditions (group2), but RFS landings were infrequent
during barefoot running in both groups. A major factor contributing
to the predominance of RFS landings in shod runners is the cushioned
sole of most modern running shoes, which is thickest below the heel,
orienting the sole of the foot so as to have about 5uless dorsiflexion
than does the sole of the shoe, and allowing a runner to RFS comfort-
ably (Fig. 1). Thus, RFS runners who dorsiflex the ankle atimpact have
shoe soles that are more dorsiflexed relative to the ground, and FFS
runners who plantarflex the ankle at impact have shoe soles that are
flatter (less plantarflexed) relative to the ground, even when knee and
ankle anglesare not different (Table 1).These data indicate that habitu-
ally unshod runners RFS less frequently, and that shoes with elevated,
cushioned heels facilitate RFS running (Supplementary Data 3).
Kinematic differences among foot strikes generate markedly differ-
ent collision forces at the ground, which we compared in habitually
shod and barefoot adult runners from the USA during RFS and FFS
running (Methods and Supplementary Data2). Whereas RFS land-
ings cause large impact transients in shod runners and even larger
transients in unshod runners (Fig. 1a, b), FFS impacts during toe–
heel–toe gaits typically generate ground reaction forces lacking a dis-
tinct transient (Fig. 1c), even on a stiff steel force plate
4,17–19
. At similar
speeds, magnitudes of peak vertical force during the impact period
(6.2 63.7% (all uncertainties are s.d. unless otherwise indicated) of
stance for RFS runners) are approximately three times lower in habi-
tual barefoot runners who FFS than in habitually shod runners who
RFS either barefoot or in shoes (Fig. 2a). Also, over the same percent-
age of stance the average rate of loading in FFS runners when barefoot
is seven times lower than in habitually shod runners who RFS when
barefoot, and is similar to the rate of loading of shod RFS runners
(Fig. 2b). Further, in the majority of barefoot FFS runners, rates of
loading were approximately half those of shod RFS runners.
Modelling the foot and leg as an L-shaped double pendulum that
collides with the ground(Fig. 3a) identifies two biomechanical factors,
0.0
0.1 0.2 0.3 0.4 0.5 0.6 0.7
0.8
1.6
2.4
0.0
0.1 0.2 0.3 0.4 0.5 0.6 0.7
0.8
1.6
2.4
0.0
0.1 0.2 0.3 0.4 0.5 0.6 0.7
0.8
1.6
2.4
Force (body weights)Force (body weights) Force (body weights)
Time (s)
Time (s)
Time (s)
a
b
c
Figure 1
|
Vertical ground reaction forces and foot kinematics for three foot
strikes at 3.5 m s
21
in the same runner. a, RFS during barefoot heel–toe
running; b, RFS during shod heel–toe running; c, FFS during barefoot
toe–heel–toe running. Both RFS gaits generate an impact transient, but
shoes slow the transient’s rate of loading and lower its magnitude. FFS
generates no impact transient even in the barefoot condition.
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Impact force (body weights)
FFS
barefoot
RFS
barefoot
RFS
shod
FFS
barefoot
RFS
barefoot
RFS
shod
0
100
200
300
400
500
600
700
Mean rate of loading
(body weights per second)
a
T
b
T
Figure 2
|
Variation in impact transients. a,b, Magnitude (a) and rate of
loading (b) of impact transient in units of body weight for habitually shod
runners who RFS (group 1; open boxes) and habitually barefoot runners
who FFS when barefoot (group 3; shaded boxes). The rate of loading is
calculated from 200 N to 90% of the impact transient (when present) or to
6.2 63.7% (s.d.) of stance phase (when impact transient absent). The impact
force is 0.58 60.21 bodyweights (s.d.) in barefoot runners who FFS, which is
three times lower than in RFS runners either barefoot (1.89 60.72body
weights (s.d.)) or in shoes (1.74 60.45 body weights (s.d.)). The average rate
of impact loading for barefoot runners who FFS is 64.6 670.1 body weights
per second (s.d.), which is similar to that for shod RFS runners
(69.7 628.7 body weights per second (s.d.)) and seven times lower than that
for shod runners who RFS when barefoot (463.1 6141.0body weights per
second (s.d.)). The nature of the measurement (force versus time) is shown
schematically by the grey and red lines. Boxes, mean 6s.d.;
whiskers, mean 62 s.d.
LETTERS NATURE
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namely the initial point of contact and ankle stiffness, that decrease
M
eff
and, hence, the magnitude of the impact transient (equation (1)
and Supplementary Data4). A RFS impact typically occurs just below
the ankle, under the centre of mass of the foot plus leg, and with
variable plantarflexion (Fig. 3b). Therefore, the ankle converts little
translational energy into rotational energy and most of the trans-
lational kinetic energy is lost in the collision, leading to an increase
in M
eff
(ref. 20). In contrast, a FFS impact occurs towards the front of
the foot (Fig. 3a), and the ankle dorsiflexes as the heel drops under
control of the triceps surae muscles and the Achilles tendon (Fig. 3b).
The ground reaction force in a FFS therefore torques the foot around
the ankle, which reduces M
eff
by converting part of the lower limb’s
translational kinetic energyinto rotational kinetic energy, especially in
FFS landings with low ankle stiffness (Fig. 3a). We note that MFS
landings with intermediate contact points are predicted to generate
intermediate M
eff
values.
The conservation of angular impulse momentum during a rigid
plastic collision can be used to predict M
eff
as a function of the
location of the centre of pressure at impact for ankles with zero
and infinite joint stiffnesses (Supplementary Data 4). Figure 3 shows
model values of M
eff
for an average foot and shank comprising 1.4%
and 4.5% M
body
, respectively, where the shank is 1.53 times longer
than the foot
21
.M
eff
can be calculated, using experimental data from
equation (1), as
Meff ~ÐT
0{Fz(t)dt
{vfoot zgT ð2Þ
Using equation (2) with kinematic and kinetic data from groups 1
and 3 (Methods), we find that M
eff
averages 4.49 62.24 kg for RFS
runners in the barefoot condition and 1.37 60.42 kg for habitual bare-
foot runners who FFS (Fig. 3a). Normalized to M
body
, the average M
eff
is
6.8 63.0% for barefoot RFS runners and 1.7 60.4% for barefoot FFS
runners. For all RFS landings, these values are not significantly different
from the predicted M
eff
values for a rigid ankle (5.5–5.9% M
body
)ora
compliant ankle (3.4–5.9% M
body
), indicating that ankle compliance
has little effect and that there is some contribution from mass above
the knee, which is very extended in these runners (Fig. 3b). For FFS
landings, M
eff
values are smaller than the predicted values for a rigid
ankle (2.7–4.1% M
body
) and are insignificantly greater than those pre-
dicted for a compliant ankle (0.45–1.1% M
body
), suggesting low levels of
ankle stiffness. These results therefore support the prediction that FFS
running generates collisions with a much lower M
eff
than does RFS
running. Furthermore, MFS running is predicted to generate inter-
mediate M
eff
values with a strong dependence on the centre of pressure
at impact and on ankle stiffness.
How runners strike the ground also affects vertical leg compliance,
defined as the drop in the body’s centre of mass relative to the vertical
force during the period of impact. Vertical compliance is greater in
FFS running than in RFS running, leading to a lower rate of loading
(Fig. 3c). More compliance during the impact period in FFS runners
is partly explained by a 74% greater drop in the centre of mass (t-test,
P,0.009), resulting, in part, from ankle dorsiflexion and knee
flexion (Fig. 3b). In addition, like shod runners, barefoot runners
adjust leg stiffness depending on surface hardness
22
. As a result, we
found no significant differences in rates or magnitudes of impact
loading in barefoot runners on hard surfaces relative to cushioned
surfaces (Supplementary Data 5).
Differences between RFS and FFS running make sense from an
evolutionary perspective. If endurance running was an important
behaviour before the invention of modern shoes, then natural selec-
tion is expected to have operated to lower the risk of injury and
discomfort when barefoot or in minimal footwear.Most shod runners
today land on their heels almost exclusively. In contrast, runners who
cannot or prefer not to use cushioned shoes with elevated heels often
avoid RFS landings and thus experience lower impact transients than
do most shod runners today, even on very stiff surfaces (Fig. 2). Early
Ln(le
g
compliance)
Ankle
Barefoot Shod Barefoot Shod Barefoot Shod
Knee Hip
Mean rate of loading (body weights per second) Joint angle change during impact transient (°)
Plantarexion
Strike index
Extension
Flexion
Extension
Flexion
0
100
200
300
400
500
600
700
a
b
c
–6.5 –6.0 –5.5 –5.0 –4.5 –4.0 –3.5 –3.0
y = –1208 – 291x; r = 0.86
y = –370 – 105x; r = 0.95
–15
–10
–5
0
5
10
15
Dorsiexion
Meff relative to body mass (%)
0
2
0.20.0 0.4 0.6 0.8 1.0
4
6
8
10
12
14
Rear-foot Fore-foot Mid-foot
Figure 3
|
Differences during impact between shod RFS runners (group1)
and barefoot FFS runners (group3) at approximately 4 ms
21
.
a, Predicted (lines) and measured(boxes) effective mass, M
eff
, relative to body
mass, versus foot length at impact (strike index) for FFS and RFS runners in
the barefoot condition (Methods). The solid and dotted lines show predicted
M
eff
values for infinitely stiff and infinitely compliant ankles, respectively, at
different centres of pressure. b, During the impact period, FFS runners (filled
boxes) dorsiflex the ankle rather than plantarflexing it, and have more ankle
and knee flexion than do RFS runners (open boxes). Boxes,mean 6s.d.;
whiskers, mean 62 s.d. c, Overall dimensionless leg compliance (natural
logarithm) during the impact-transient period (ratio of vertical hip drop
relative to leg length at 90% of impact transient peak, normalized by body
weight) relative to the rate of impact loading (body weights per second) for
RFS runners (open circles) and FFS runners (filled circles) (shod and unshod
conditions). Compliance is greater and is correlated with lower rates of
loading in FFS impacts than in RFS impacts (plotted lines determined by
least-squares regression; r, Pearson’s correlation coefficient).
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bipedal hominins such as Australopithecus afarensis had enlarged
calcaneal tubers and probably walked with a RFS
23
. However, they
lacked some derived features of the modern human foot, such as a
strong longitudinal arch
1,24
that functionally improves the mass–
spring mechanics of running by storing and releasing elastic energy
25
.
We do not know whether early hominins ran with a RFS, a MFS or a
FFS gait. However, the evolution of a stronglongitudinal arch in genus
Homo would increase performance more for non-RFS landings
because the arch stretches passivelyduring the entire first half of stance
in FFS and MFS gaits. In contrast, the arch can stretch passively only
later in stance during RFS running, when both the fore-foot and the
rear-foot are on the ground. This difference may account for the lower
cost of barefoot running relative to shod running
15,26
.
Evidence that barefoot and minimally shod runners avoid RFS
strikes with high-impact collisions may have public health implica-
tions. The average runner strikes the ground 600 times per kilometre,
making runners prone to repetitive stress injuries
6–8
. The incidence of
such injuries has remained considerable for 30 years despite technolo-
gical advancements thatprovide more cushioning and motion control
in shoes designed for heel–toe running
27–29
. Although cushioned,
high-heeled running shoes are comfortable, they limit proprioception
and make it easier for runners to land on their heels. Furthermore,
many running shoes have arch supports and stiffened soles that may
lead to weaker foot muscles, reducing arch strength. This weakness
contributes to excessive pronation and places greater demands on the
plantar fascia, which may cause plantar fasciitis. Although there are
anecdotal reports of reduced injuries in barefoot populations
30
, con-
trolled prospective studies are needed to test the hypothesis that
individuals who do not predominantly RFS either barefoot or in
minimal footwear, as the foot apparently evolved to do, have reduced
injury rates.
METHODS SUMMARY
We studied five subject groups (Table 1 and Supplementary Data1), both bare-
foot and in running shoes. Habitually shod and barefoot US subjects ran over a
force plate embedded 80% of the way along a 20–25-m-long indoor track. We
quantified joint angles using a three-dimensional infrared kinematic system
(Qualysis) at 240Hz and a 500-Hz video camera (Fastec InLine 500M). African
subjects were recorded on a 20–25-m outdoor track of hard dirt using a 500-Hz
video camera. All subjects ran at preferred speeds with several habituation trials
before each condition,and were recorded for five to seven trials per condition. We
taped kinematic markerson joints and segments in all subjects. Video frames were
analysed using IMAGEJ (http://rsb.info.nih.gov/nih-image/) to measure the
angle of the plantar surface of the foot relative to earth horizontal (plantar foot
angle), as well as ankle, knee and hip angles (Methods). We recorded the vertical
ground reaction force (F
z
) in US subjects at 4,800 Hz using AMTI force plates
(BP400600 Biomechanics Force Platform), and normalized the results to body
weight. The impact-transient magnitude andpercentage of stance were measured
at peak, and the rate of loading was quantified between 200 N and 90% of peak
(following ref.18). When there was no distinct impact transient, the same
parameters were measured at the same percentage of stance plus/minus 1 s.d. as
determined for each condition in trials with an impact transient. The effective
mass (M
eff
) in RFS runners was calculated using the integral of F
z
(equation (2))
between the time when F
z
exceeded 4 s.d. above baseline noise and the time when
the transient peak was reached as measured in RFS runners; the impulse over the
same percentageof stance (6.2 63.7%) was used to calculated M
eff
in FFS runners.
Vertical foot and leg speed were calculated using a central difference method and
the three-dimensional kinematic data.
Full Methods and any associated references are available in the online version of
the paper at www.nature.com/nature.
Received 27 July; accepted 26 November 2009.
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Table 1
|
Foot strike type and joint angles of habitual barefoot and shod runners from Kenya and the USA
Group N
(male/female)
Age (age shod) (yr) Strike-type mode (%)*Joint angle at foot strike Speed (m s
21
)
Condition RFS MFS FFS Plantar foot{Ankle{Knee
(1) Habitually shod adults, USA{8(6/2)19.160.4(,2) Barefoot 83 17 0 216.464.4u0.263.0u12.167.9u4.060.3
Shod 100 0 0 228.366.2u29.366.5u9.166.4u4.260.3
(2) Recently shod adults, Kenya 14 (13/1)23.163.5(12.465.6) Barefoot 9 0 91 3.769.8u18.667.7u21.264.4u5.960.6
Shod 29 18 54 21.867.4u15.066.7u22.264.3u5.760.6
(3) Habitually barefoot adults, USA18(7/1)38.368.9(,2) Barefoot 25 0 75 8.464.4u17.665.8u17.362.5u3.960.4
Shod 50 13 37 22.2614.0u8.1615.9u16.662.4u4.060.3
(4) Barefoot adolescents, Kenya 16 (8/8)13.561.4(never) Barefoot 12 22 66 1.13 66.8u14.668.3u22.865.4u5.560.5
Shod
||
——— — — — —
(5) Shod adolescents, Kenya 17 (10/7)15.060.8(,5) Barefoot 62 19 19 210.169.7u4.1610.9u18.966.5u5.160.5
Shod 97 3 0 219.8610.3u22.769.0u18.466.6u4.960.5
Data shown as mean 6s.d.
*RFS equivalent to heel-toe running; FFS equivalent to toe
–
heel
–
toe running.
{Angle of the sole of the foot or shoe (column 8), or of the ankle (column9), relative to ground. Negative values indicate dorsiflexion relative to standing position; positive values indicate
plantarflexion relative to standing position.
{Joint angles calculated from RFS only.
1Joint angles calculated from FFS only.
||
No shod condition reported because subjects had never worn shoes.
LETTERS NATURE
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©2010

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Supplementary Information is linked to the online version of the paper at
www.nature.com/nature.
Acknowledgements We are grateful to the many volunteer runners who donated
their time and patience. For help in Kenya, we thank M. Sang; E.Anjilla; Moi
University Medical School; E. Maritim; and the students and teachers of Pemja,
Union and AIC Chebisaas schools, in Kenya. For laboratory assistance in
Cambridge, we thank A. Biewener, S. Chester, C. M. Eng, K. Duncan, C. Moreno,
P. Mulvaney, N.T. Roach, C. P. Rolian, I.Ros, K. Whitcome and S.Wright. We are
grateful to A. Biewener, D. Bramble, J. Hamill, H. Herr, L.Mahadevan and
D. Raichlen for discussions and comments. Funding was provided by the US
National Science Foundation, the American School of Prehistoric Research, The
Goelet Fund, Harvard University and Vibram USA.
Author Contributions D.E.L. wrote the paper with substantial contributions from
M.V., A.I.D., W.A.W., I.S.D., R.O.M. and Y.P. Collision modelling was done by M.V.
and D.E.L.; US experimental data were collected by A.I.D., W.A.W. and D.E.L., with
help from S.D’A. Kenyan data were collected by D.E.L., A.I.D., W.A.W., Y.P. and
R.O.M. Analyses were done by A.I.D., D.E.L., M.V. and W.A.W.
Author Information Reprints and permissions information is available at
www.nature.com/reprints. The authors declare competing financial interests:
details accompany the full-text HTML version of the paper at www.nature.com/
nature. Correspondence and requests for materials should be addressed to D.E.L.
(danlieb@fas.harvard.edu).
NATURE
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©2010

METHODS
Subjects. We used five groups of subjects (outlined in Table 1 and Supplemental
Table 1), including the following three groups of adults. Group 1 comprised
amateur and collegiate athletes from the Harvard University community,
recruited by word of mouth, all of whom were habitually shod since early child-
hood. Group 2 comprised Kalenjin athletes from the Rift Valley Province of
Kenya, all training for competition, and recruited by word of mouth in the town
of Kapsabet and at Chepkoilel Stadium, Eldoret. All adult Kenyan subjects were
habitually shod, but 75% did not start wearing shoes and training in running
shoes until late adolescence. Group 3 comprised self-identified habitual barefoot
runners from the USA, recruited through the internet, who run either barefoot
and/or in minimal footwear such as Vibram FiveFingers shoes, defined as lacking
arch support and cushioning. In addition, two groups of adolescent subjects
(aged 11–16 yr) were sampled from two schools in the Kalenjin-speaking region
of Kenya. Group 4 comprised habitually unshod runners (N516; eight male,
eight female) recruited from a rural primary school in the South Nandi District
of Kenya in which none of children have ever worn shoes (verified by observation
and interviews with teachers at the school). Group 5 comprised habitually shod
runners (N516; nine male, seven female) recruited from an urban primary
school in Eldoret in which all of the children have been habitually shod since
early childhood.
For all adults, criteria for inclusion in the study included a minimum of 20 km
per week of distance running and no history of significant injury during the
previous six months. Habitual barefoot runners were included if they had run
either barefoot or in minimal footwear for more than six months and if more
than 66% of their running was either barefoot or in minimal footwear. To
compare habitual barefoot FFS (toe–heel–toe) runners and habitually shod
RFS (heel–toe) runners, we analysed kinematic and kinetic data from subsamples
of six RFS runners from group 1 and six FFS runners from group 3 in greater
depth (Supplementary Data Table 1).
All information on subject running history was self-reported (with the assist-
ance of teachers for the Kenyan adolescents). All subjects participated on a
voluntary basis and gave their informed consent according to the protocols
approved by the Harvard Institutional Review Board and, for Kenyan subjects,
the Moi University Medical School. Subjects were not informed about the hypo-
theses tested before recording began.
Treatments. All subjects were recorded on flat tracks approximately 20–25-m
long. Subjects in groups 1–3 and5 were recorded barefoot and in running shoes.
A neutral running shoe (ASICS GEL-CUMULUS 10) was provided for groups1
and 3, but groups 2 and 5 ran in their own shoes. Subjects in group 4 were
recorded only in the barefoot condition because they had never worn shoes.
For groups 1 and3, two force plates (see below) were embedded at ground level
80% of the way along the track, with a combined force-plate length of 1.2 m.
Force plates were covered with grip tape (3M Safety-Walk Medium Duty
Resilient Tread 7741), and runners were asked to practice running before record-
ing began so that they did not have to modify their stride to strike the plates.
Kenyan runners in groups 2, 4 and 5 were recorded on flat, outdoor dirt tracks
(with no force plates) that were 20–25-m long and cleaned to remove any pebbles
or debris. In all groups, subjects were asked to run at a preferred speed and were
given several habituation trials before each data collection phase, and were
recorded in five to seven trials per condition, with at least one minute’s rest
between trials to avoid fatigue.
Kinematics. To record angles in lateral view of the ankle, knee, hip and plantar
surface of the foot, a high-speed video camera (Fastec InLine 500M, Fastec
Imaging) was placed approximately 0.5 m above ground level between 2.0 and
3.5 m lateral to the recording region and set to record at 500 Hz. Circular markers
were taped on the posterior calcaneus (at the level of the Achilles tendon inser-
tion), the head of metatarsal V, the lateral malleolus, the joint centre between the
lateral femoral epicondyle and the lateral tibial plateau (posterior to Gerdy’s
tubercle), the midpoint of the thigh between the lateral femoral epicondyle
and the greater trochantor of the femur (in groups 2, 4 and 5); the greater tro-
chantor of the femur (only in groups 1 and 3); and the lateral-most point on the
anterior superior iliac spine (only in groups 1 and 3). We could not place hip and
pelvis markers on adolescent Kenyan subjects (groups 4 and 5). IMAGEJ (http://
rsb.info.nih.gov/nih-image/) was used to measure three angles in all subjects: (1)
the plantar foot angle, that is, the angle between the earth horizontal and the
plantar surface of the foot (calculated using the angle between the lines formed by
the posterior calcaneus and metatarsal V head markers and the earth horizontal
at impact, and corrected by the same angle during quiet stance); (2) the ankle
angle, defined by the metatarsal V head, lateral malleolus and knee markers; (3)
the knee angle, defined by the line connecting the lateral malleolus and the knee
and the line connecting the knee and the thigh midpoint (or greater trochantor).
Hip angle was also measured in groups 1 and2 as the angle between the lateral
femoral condyle, the greater trochantor and the anterior superior iliac spine. All
angles were corrected against angles measured during a standing, quiet stance.
Average measurement precision, determined by repeated measurements (more
than five) on the same subjects was 60.26u.
Under ideal conditions, plantar foot angles greater than 0uindicate a FFS,
angles less than 0uindicate a RFS (heel strike) and angles of 0uindicate a MFS.
However, because of inversion of the foot at impact, lighting conditions and
other sources of error, determination of foot strike type was also evaluated by
visual examination of the high-speed video by three of us. We also note that ankle
angles greater than 0uindicate plantarflexion and that angles less than 0uindicate
dorsiflexion.
Additional kinematic data for groups 1 and 3 were recorded with a six-camera
system (ProReflex MCU240, Qualysis) at 240 Hz. The system was calibrated
using a wand with average residuals of ,1 mm for all cameras. Four infrared
reflective markers were mounted on two 2-cm-long balsawood posts, affixed to
the heel with two layers of tape following methods described in ref. 18. The
average of these four markers was used to determine the total and vertical speeds
of the foot before impact.
Kinetics. Ground reaction forces (GRFs) were recorded in groups1 and 3 at
4,800 Hz using force plates (BP400600 Biomechanics Force Platform, AMTI).
All GRFs were normalized to body weight. Traces were not filtered. When a
distinct impact transient was present, transient magnitude and the percentage
of stance was measured at peak; the rate of loadingwas quantified between 200 N
and 90% of the peak (following ref.18); the instantaneous rate of loading was
quantified over time intervals of 1.04 ms. When no distinct impact transient was
present, the same parameters were measured using the average percentage of
stance 61 s.d. as determinedfor each condition in trials with an impact transient.
Estimation of effective mass. For groups 1 and 3, we used equation (2) to estimate
the effective mass that generates the impulseatfootlanding.Thestartoftheimpulse
was identified as the instant at which the vertical GRF exceeded 4 s.d. of baseline
noise above the baseline mean, and its end was chosen to be 90% of the impact
transient peak (a ‘real’ time point among RFS runners, the average of which was used
as the end of the transient in FFS runners who lacked a transient); this resulted in an
impulse experienced, on average, through the first 6.263.7% of s tance. The integral
of vertical GRF over the period of the impulse is the total impulse and was calculated
using trapezoidal numerical integration within theMATLAB 7.7 environment using
the TRAPZ function (Mathworks). Three-dimensional kinematic data of the
foot (see above) were low-pass-filtered using a fourth-order Butterworth filter with
a 25-Hz cut-off frequency. The vertical speed at the moment of impact was found by
differentiating the smoothed vertical coordinate (smoothed with a piecewise-cubic
Hermite interpolating polynomial) of the foot using numerical central difference.
To minimize the effects of measurement noise, especially because we used differ-
entiated data, we used the average of the three samples measured immediately before
impact in calculating the impact speed. M
eff
was then estimated as the ratio of the
vertical GRF impulse (found by numerical integration) and the vertical impact speed
(found by numerical differentiation).
doi:10.1038/nature08723
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