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Abstract

The foot is a complex structure with many articulations and multiple degrees of freedom that play an important role in static posture and dynamic activities. The evolutionary development of the arch of the foot was coincident with the greater demands placed on the foot as humans began to run. The movement and stability of the arch is controlled by intrinsic and extrinsic muscles. However, the intrinsic muscles are largely ignored by clinicians and researchers. As such, these muscles are seldom addressed in rehabilitation programmes. Interventions for foot-related problems are more often directed at externally supporting the foot rather than training these muscles to function as they are designed. In this paper, we propose a novel paradigm for understanding the function of the foot. We begin with an overview of the evolution of the human foot with a focus on the development of the arch. This is followed by a description of the foot intrinsic muscles and their relationship to the extrinsic muscles. We draw the parallels between the small muscles of the trunk region that make up the lumbopelvic core and the intrinsic foot muscles, introducing the concept of the foot core. We then integrate the concept of the foot core into the assessment and treatment of the foot. Finally, we call for an increased awareness of the importance of the foot core stability to normal foot and lower extremity function.
The foot core system: a new paradigm for
understanding intrinsic foot muscle function
Patrick O McKeon,
1
Jay Hertel,
2
Dennis Bramble,
3
Irene Davis
4
To read the full version of
this paper, please visit the
journal online (http://dx.doi.
org/10.1136/bjsports-2013-
092690).
1
Department of Exercise and
Sport Sciences, School of
Health Sciences and Human
Performance, Ithaca College,
Ithaca New York, USA
2
Department of Kinesiology,
Curry School of Education,
University of Virginia,
Charlottesville, Virginia, USA
3
Department of Biology,
University of Utah, Salt Lake
City, Utah, USA
4
Department of Physical
Medicine and Rehabilitation,
Spaulding National Running
Center, Harvard Medical
School, Cambridge,
Massachusetts, USA
Correspondence to
Dr Patrick O McKeon,
Department of Exercise and
Sport Science, School of
Health Sciences and Human
Performance, Ithaca College
Hill Center, Room G66 953
S. Danby Rd, Ithaca, NY
14850, USA;
pmckeon@ithaca.edu
Accepted 27 February 2014
To cite: McKeon PO,
Hertel J, Bramble D, et al.
Br J Sports Med
2015;49:290.
ABSTRACT
The foot is a complex structure with many articulations
and multiple degrees of freedom that play an important
role in static posture and dynamic activities. The
evolutionary development of the arch of the foot was
coincident with the greater demands placed on the foot
as humans began to run. The movement and stability of
the arch is controlled by intrinsic and extrinsic muscles.
However, the intrinsic muscles are largely ignored by
clinicians and researchers. As such, these muscles are
seldom addressed in rehabilitation programmes.
Interventions for foot-related problems are more often
directed at externally supporting the foot rather than
training these muscles to function as they are designed. In
this paper, we propose a novel paradigm for
understanding the function of the foot. We begin with an
overview of the evolution of the human foot with a focus
on the development of the arch. This is followed by a
description of the foot intrinsic muscles and their
relationship to the extrinsic muscles. We draw the
parallels between the small muscles of the trunk region
that make up the lumbopelvic core and the intrinsic foot
muscles, introducing the concept of the foot core. We
then integrate the concept of the foot core into the
assessment and treatment of the foot. Finally, we call for
an increased awareness of the importance of the foot
core stability to normal foot and lower extremity function.
The human foot is a very complex structure, which
allows it to serve many diverse functions. During
standing, it provides our base of support. During
gait, the foot must be stable at foot-strike and
push-off. However, during mid-support, the foot
must become a mobile adaptor and attenuate loads. It
also possesses spring-like characteristics, storing and
releasing elastic energy with each foot-strike. This is
accomplished through the deformation of the arch,
which is controlled by intrinsic and extrinsic foot
muscles. There is evolutionary evidence that the foot
arch architecture and musculature developed in
response to the increased demands of load carriage
and running. The stability of this arch, which we pro-
posed to be the central coreof the foot, is requisite
to normal foot function.
THE RELEVANCE OF CORE STABILITY
TO THE FOOT
Core stability has received much attention in the clin-
ical and athletic arenas. Interest has primarily been
focused on the role of lumbopelvic-hip stability in
normal lower extremity movement patterns.
1
The
muscular system of the lumbopelvic hip complex, or
core, has been described as consisting of local stabili-
sers such as the multidus and transverse abdominis,
and global movers such as latissimus dorsi.
2
The local
stabilisers have small cross-sectional areas and small
moment arms. Therefore, they do not produce large
rotational moments at the respective joints that they
cross. However, they do act to increase intersegmen-
tal stability. Proper function of local stabilisers pro-
vides a stable base on which the primary movers of
the trunk, those with larger cross-sectional areas and
moment arms, can act to cause gross motion. When
core muscles are weak or are not recruited appropri-
ately, the proximal foundation becomes unstable and
malaligned, and abnormal movement patterns of the
trunk and lower extremity ensue.
3
This can lead to a
variety of overuse lower extremity injuries.
47
We propose that the concept of core stability
may also be extended to the arch of the foot. The
arch is controlled with both local stabilisers and
global movers of the foot, similar to the lumbopel-
vic core. The local stabilisers are the four layers of
plantar intrinsic muscles that originate and insert
on the foot. These muscles generally have small
moment arms, small cross-sectional areas and serve
primarily to stabilise the arches. The global movers
are the muscles that originate in the lower leg,
cross the ankle and insert on the foot. These
muscles have larger cross-sectional areas, larger
moment arms, are prime movers of the foot, and
also provide some stability to the arch. With each
footstep, the four layers of intrinsic muscles act to
control the degree and velocity of arch deform-
ation. When they are not functioning properly, the
foundation becomes unstable and malaligned; and
abnormal movement of the foot ensues. This may
manifest in foot-related problems. Plantar fasciitis
is one of the most common overuse injuries of the
foot. It is recognised as a repetitive strain injury
from excessive deformation of the arch.
8
The
importance of the arch musculature in this preva-
lent foot injury is currently underappreciated. This
is underscored by recent articles describing clinical
evidence and guidelines for plantar fasciitis,
9
as
well as posterior tibial tendon dysfunction,
10
medial tibial stress syndrome
11
and chronic lower
leg pain
12
that have no mention of foot strengthen-
ing as a component of the interventions.
Therefore, our purpose was to propose a foot
core system paradigm by (1) describing the evolu-
tion of the human arch for locomotion, (2) delin-
eating the subsystems of the foot core, (3)
reviewing assessment and treatment of the foot
integrating the concepts of foot core stability and
(4) nally discussing future research directions. Our
overall goal was to propose a new paradigm by
which to view foot function, assessment and
treatment.
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THE ORIGIN OF THE HUMAN ARCH
The human foot has evolved from one similar to that of African
apes, where it serves in both arboreal and terrestrial locomotion.
13
The transition from ape-like to human-like foot structure reects a
shift to locomotor habits dominated by terrestrial bipedality.
When walking bipedally, the gait of chimps is compromised by the
absence of structural specialisations that permit the human foot to
operate as a compact, rigid lever system during the latter half of
stance. These include (1) an enlarged and permanently adducted
hallux, (2) shortened lateral digits, (3) compaction and realign-
ment of the tarsal bones to help prevent the mid-tarsal break
observed in the foot of apes
14
and (4) the addition of a well-
dened medial longitudinal arch defended by strong plantar
tensile elements. The condition of the foot arches, absent in apes,
remains controversial. A transverse arch was likely present,
15
but
the crucial medial longitudinal arch was absent or weakly
expressed,
16
implying a poorly dened plantar aponeurosis and
hence greater reliance on muscular effort to resist forces acting on
the toes during late stance. In contrast, even the earliest members
of the genus Homo for which there is adequate evidence (eg, early
Homo erectus) possessed an essentially modern foot structure,
including a well-dened medial longitudinal arch.
17
The modern human body (ie, Homo), especially in musculoskel-
etal design, reects the mechanical demands of endurance
running.
18
That habitual bipedalism was practiced for several million
years by Australopithecus without the hallmark features of the
human foot also suggests that such traits arose in the context of a
new and more demanding locomotor behaviour. A key distinction
between walking and running is the central importance of leg
springsin running but not walking.
19
These springs include a pro-
nounced Achilles tendon and the plantar aponeurosis and spring
ligaments on the inferior aspect of the foot. All are absent in apes
and were either lacking or minimally developed in Australopithecus.
Running also subjects the digits to much larger extension forces
during late stance and toe-off phases than does walking; a strong
plantar aponeurosis offers substantial passive resistance to these
loads. Additionally, mid-stance attening of the longitudinal arch
when running both cushions foot impact and stores recoverable
strain energy in the stretched elastic tissues,
20
but unlike most quad-
rupedal mammals specialised for running, humans retain consider-
able intrinsic foot musculature. These same muscles are reduced and
sometimes completely lost in quadrupedal runners, making internal
stabilisation of the foot mostly passive. Human runners are unique
in needing to control balance during single leg support and for this
reason (unlike quadrupeds) require a foot that is reasonably mobile,
able to accommodate uneven substrates, and actively controlled.
Electromyography (EMG) studies show that plantar intrinsic foot
muscle activity is most consistent among participants during running
and least during walking.
21
While more variable, the intrinsic foot
muscles are routinely active in late stance of walking and may have a
signicant role in controlling load distribution under the foot as well
as augmenting the exor function of the medial longitudinal arch,
especially at higher speeds.
21 22
Although often showing minimal
activity in simple stance, the intrinsic foot muscles are more strongly
recruited when additional loads are added to the participant.
23
Frequent long-distance burden carrying may explain the evolution-
ary transformation from Australopithecus-like to Homo-like body
proportions
24
and might also help account for relatively robust
intrinsic foot muscle development in the human foot.
THEFOOTCORESYSTEM
The theoretical basis of lumbopelvic-hip core stability is rooted
in the functional interdependence of the passive, active and
neural subsystems controlling spinal motion and stability origin-
ally proposed by Panjabi.
25
The passive subsystem consists of the
bony and articular structures, while the active subsystem consists
of the muscles and tendons attaching to and acting on the spine.
The neural subsystem consists of sensory receptors in the joint
capsules, ligaments, muscles and tendons surrounding the spine.
The passive subsystem provides for a balance between mobility
and stability of the vertebral column. The active subsystem con-
sists of two functional muscular components: the local stabilisers
and the global movers.
2
The local stabilisers consist of both the
short, intersegmental muscles that primarily originate and insert
on the spinal column and have short moment arms and act to
increase intersegmental dynamic stability. Proper function of
local stabilisers provides a stable base on which the primary
movers of the trunk can act to cause gross motion. The global
movers cross multiple vertebral segments, have attachments on
the pelvis and thorax, and can exert longer moment arms to
move the trunk and extremities. These include the more super-
cial erector spinae as well as the internal and external oblique
and rectus abdominus muscles. The neural subsystem monitors
spinal motion and forces and sends afferent signals to the central
nervous system. If those afferent signals exceed a given threshold,
efferent signals are sent from the central nervous system to the
appropriate muscles to alter spinal motion and forces.
Hodges
26
further delineated the strategies of lumbopelvic
core stability into controland capacitycomponents. The
control strategy aims to restore coordination of the muscles
acting on the lumbopelvic core while the capacity strategy aims
to provide adequate muscle strength and endurance to prevent
the spine from being mechanically unstable under varying loads.
Ultimately, the control and capacity strategies complement each
other in providing for a stable lumbopelvic core and these same
principles may be applied to the foot core system. The applica-
tion of lumbopelvic core stability concepts to the foot are illu-
strated in gure 1. These concepts as they relate to the ankle
and foot were rst proposed by Jam
27
and we further expand
on their application to the foot core. The description of each of
subsystems follows.
Passive subsystem of the foot core
The passive subsystem of the foot core consists of the bones, liga-
ments and joint capsules that maintain the various arches of the
foot. The functional conguration of the bony anatomy of the
foot results in four distinct arches which include the medial and
lateral longitudinal arches as well as the anterior and posterior
transverse metatarsal arches.
28
While often viewed as separate
structures, McKenzie
29
proposed that these arches coalesce into a
functional half dome responsible for exibly adapting to load
changes during dynamic activities (see gure 2). This half dome
has been thought to be predominantly supported by passive
structures including the plantar aponeurosis (see gure 3A) and
plantar ligaments (see gure 4), however local dynamic support
is also thought to be provided from the intrinsic foot muscles in
the active subsystems and indirectly by the contractions of the
extrinsic foot muscles.
30
Active subsystem of the foot core
The active subsystem consists of the muscles and tendons that
attach on the foot. The local stabilisers of the foot are the
plantar intrinsic muscles that both originate and insert on the
foot, whereas the global movers are the extrinsic muscles that
originate in the lower leg, cross the ankle and insert on the foot
(see gure 5). While there are intrinsic muscles on both the
dorsal and plantar aspects, the plantar intrinsic muscles are most
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commonly described due to their functional link with the longi-
tudinal and transverse arches of the foot half dome.
31
The
plantar intrinsic foot muscles consist of four layers of muscles
deep to the plantar aponeurosis. The rst two layers have
muscle congurations which align with the medial and lateral
longitudinal arches of the foot whereas the deeper layers cong-
ure more so with the anterior and posterior transverse arches
(see gure 6AE). See online supplementary appendix for a full
description of the anatomical and biomechanical contributions
of the intrinsic foot muscles. By examining the synergistic rela-
tionships among these muscles to the relevant bony anatomy
and foot dome conguration, their functional role may be eluci-
dated. Soysa et al
31
summarised the functional qualities of the
intrinsic foot muscles to include being supportive of the foot
arches,
3235
activity and load dependent,
36 37
synergistic
38
and
modulating.
28
See table 1 for the evidence-based descriptions of
these functional qualities.
The extrinsic foot muscles function as the global movers of
the foot core to generate foot motion via their long tendons and
modulate structures within the passive subsystem. For example,
the Achilles tendon from the triceps surae modulates the tension
of the plantar aponeurosis based on their common connection
to the calcaneus. As triceps surae tension increases, so does the
tension on the plantar fascia
39
(see gure 3B). This is critically
important for key events in foot behaviour such as transitioning
from a supple to a rigid body during gait. The orientations of
the extrinsic foot muscle tendons clearly illustrate their ability
to provide dynamic support and control of both the longitu-
dinal and transverse components of the foot dome. These
global movers provide both absorption and propulsion capabil-
ities during dynamic activities.
Neural subsystem of the foot core
The neural subsystem consists of the sensory receptors in the
plantar fascia, ligaments, joint capsules, muscles and tendons
involved in the active and passive subsystems. It is well accepted
that plantar sensation is a critical element to gait and balance
with the contributions of the plantar cutaneous receptors the
most extensively studied.
4044
The sensory contributions of the
intrinsic foot muscles remain less clear. Based on the intrinsic
foot musclesanatomical and biomechanical conguration, these
muscles lack mechanical advantage for producing large joint
motions. Rather, their anatomical positions and alignments
suggest that they are advantageously positioned to provide
Figure 1 The foot core system. The
neural, active and passive subsystems
interact to produce the foot core
system which provides stability and
exibility to cope with changing foot
demands.
Figure 2 Functional half dome
proposed by McKenzie. Note the origin
of the dome is considered to be the
dome of the talus.
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immediate sensory information, via the stretch response, about
changes in the foot dome posture. In contrast to input from
sensory receptors within the passive subsystem (eg, capsuloliga-
mentous and cutaneous receptors), these sensors may be modu-
lated through training to alter their sensitivity to foot dome
deformation.
45
After fatigue of the intrinsic foot muscles via
repetitive isolated contractions of metatarsophalangeal joint
exion, navicular drop during standing increased signicantly in
healthy participants.
33
The authors concluded that the motor
contributions of these muscles led to the change in foot posture,
but this may be more associated with a change in sensory infor-
mation. Muscular fatigue brought about by repetitive contrac-
tions has been shown to decrease joint position sense in other
areas of the lower extremity.
46
This may indicate that not only
do the muscles provide relevant direct support to the passive
subsystem via muscular contraction, but they may also provide
relevant sensory information about foot dome posture in a
similar fashion to the lumbopelvic muscles in relation to trunk
posture.
47
FOOT CORE ASSESSMENT
Little attention has been paid to the clinical assessment of intrin-
sic foot muscles in the musculoskeletal injury literature apart
from few specic conditions such as diabetic neuropathy
48
and
claw toes.
49
Assessments for these conditions have largely been
related to diminished toe exion strength or atrophy of the
intrinsic foot muscles. A recent systematic review concluded that
there is no gold standard for assessing the function of the intrin-
sic foot muscles.
31
Assessment techniques have been categorised
as directand indirectevaluations of intrinsic muscle func-
tion.
31
Direct evaluations have focused on assessment of toe
exion strength while indirect evaluations include imaging tech-
niques and EMG to estimate intrinsic foot muscle function.
Tests focusing on toe exion strength are inherently limited
by the inability to conclusively separate the contributions of the
intrinsic and extrinsic toe exor muscles. Methods of assessment
have included manual muscle testing, toe grip dynamometry,
pedobarography, and a pair of special tests: the paper grip and
intrinsic positive tests.
31
The limitation of all of these measures
is their strict focus on the role of the intrinsic muscles in produ-
cing toe exion, but ignoring their more proximal functions of
supporting the arches of the foot. We assert that the latter is
more important than the former is assessment of foot core
function.
The intrinsic foot muscle test has been proposed as a func-
tional assessment of a patients ability to maintain a neutral
foot posture and medial longitudinal arch height during single
limb stance.
27
To perform this test, the clinician sets the
patients test foot in subtalar neutral with the calcaneus and all
the metatarsal heads on the ground, and asks the patient to
fully extend the toes. The patient then lowers their toes to the
Figure 3 (A) The plantar fascia
alignment. (B) The anatomical and
biomechanical relationship between
the Achilles tendon and the plantar
fascia is depicted. Note the fascial
connection between these two
structures around the calcaneus.
Figure 4 The predominant plantar ligaments of the foot that provide
passive stability to both the longitudinal and transverse aspects of the
foot.
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Figure 5 Insertions of the extrinsic
foot muscle tendons on the plantar
surface of the foot. (A) The insertions
of the exor digitorum longus, exor
hallucis longus and peroneus longus
are depicted. Note the longitudinal
alignment of the exor tendons as it
relates to their functional contributions
to longitudinal foot stability. The
oblique alignment of the peroneus
longus tendon and its midfoot
orientation clearly supports the
transverse arch. (B) The insertion of
the tibialis posterior tendon is depicted
with the tendons of gure 1A cut
away. Note the widespread insertions
of the tibialis posterior tendon across
the tarsals and metatarsals elucidating
its functional contributions to
longitudinal and transverse arch
stability.
Figure 6 The intrinsic foot muscles are presented in their anatomic orientation within the four plantar layers and the dorsal intrinsic muscle.
The numbers correspond to the muscles as follows: (1) abductor hallucis, (2) exor digitorum brevis, (3) abductor digiti minimi, (4) quadratus
plantae (note its insertion into the exor digitorum tendon), (5) lumbricals (note their origin from the exor digitorum longus tendon), (6) exor
digiti minimi, (7) adductor hallucis oblique (a) and transverse (b) heads, (8) exor hallucis brevis, (9) plantar interossei, (10) dorsal interossei and
(11) extensor digitorum brevis.
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ground and is asked to maintain the foot position in single
limb stance for 30 s. The clinician observes for gross changes
in navicular height and overactivity of the extrinsic muscles.
27
Preliminary evidence suggests that the intrinsic foot muscle test
can detect improvements in foot core function after rehabilita-
tion in patients with lower extremity injuries,
34 35
however
further development of the clinimetric properties of this test is
needed.
Both surface and ne wire EMG are methods of testing
intrinsic foot muscle function, although these have primarily
been in laboratory rather than clinical settings. Surface EMG
testing has focused on the abductor hallucis, the most super-
cial intrinsic muscle of the medial longitudinal arch.
32 33
While
EMG crosstalk is typically a concern with muscles in proximity
to each other, this concern is diminished in this case because
the abductor hallucis EMG activity is seen as a surrogate for all
of the medially located intrinsic foot muscles as a whole. Fine
wire EMG testing of the intrinsic foot muscles is ideally per-
formed by using real-time ultrasound imaging to guide and
conrm the location of the indwelling electrode. Kelly et al
36
reported the ability to assess the activation of the abductor hal-
lucis, exor digitorum brevis, dorsal interossei and quadratus
plantae with these methods. At this time, there is a lack of clin-
ical studies that have used either surface or ne wire EMG to
assess plantar intrinsic muscle function in lower extremity
injured patients.
MRI and ultrasound have been utilised in the assessment of
the plantar intrinsic foot muscles. MRI has primarily been used
to assess either the cross-sectional area or the total volume of
specic muscles. For example, Chang et al
50
demonstrated that
patients with unilateral plantar fasciitis had less total volume of
the plantar intrinsic muscles in their forefoot region compared
to their contralateral healthy limbs. Serial MRI examinations
have been used to demonstrate more rapid atrophy of plantar
intrinsic muscles in patients with diabetes with neuropathy com-
pared to patients with diabetes without neuropathy and healthy
controls.
51
Similar muscle volume decits have been identied
with ultrasonography.
48
As with surface EMG, the supercial
location of the abductor hallucis muscle has made this muscle
the primary target for ultrasound measures of cross-sectional
area.
52
Further research is needed to determine if ultrasound of
the plantar intrinsic muscles can be used as a biofeedback tool
during rehabilitation to allow patients to visualise contraction of
specic muscles similar to what has been used with the lateral
abdominal muscles.
53
FOOT CORE TRAINING
Therapeutic exercise of the plantar intrinsic foot muscles has
been traditionally described as occurring during toe exion
exercises such as towel curls and marble pick-ups. While these
exercises certainly do activate some of the plantar intrinsic
muscles, they also involve substantial activation of the exor hal-
lucis longus and exor digitorum longus muscles. Recently, the
short foot exercisehas been described as a means to isolate
contraction of the plantar intrinsic muscles
45 54 55
(gure 7).
The foot is shortenedby using the intrinsic muscles to pull the
rst metatarsophalangeal joint towards the calcaneus as the
medial longitudinal arch is elevated. As the arch raises during
this exercise, it is also referred to as foot doming.
56
We advocate Hodgesconcept
26
of establishing control of
intrinsic foot muscle function before increasing capacity. The
short foot exercise can be viewed as a foundational exercise for
foot and ankle rehabilitation similar to how the abdominal
drawing in manoeuvre (ADIM) is foundational to lumbopelvic
core stability exercise programmes. With the ADIM, emphasis is
placed on the patient learning to sense pelvic neutral and being
able to contract the local stabiliser muscles to draw in the umbil-
icus. Care is taken to not allow activation of any global mover
muscles while executing the ADIM. With the short foot exer-
cise, emphasis should be placed on the patient learning to sense
subtalar neutral with the calcaneus and the metatarsal heads on
the ground and the toes neither exed nor extended (the posi-
tioning described earlier with the intrinsic foot muscle test) and
then being able to shorten the foot by using the plantar intrinsic
muscles. EMG activity of the abductor hallucis, exor digitorum
brevis and quadratus plantae have been shown to increase sub-
stantially with increasing postural demand.
36
Activation of the
abductor hallucis has been shown to be over four times greater
during short foot exercise compared to towel curl exercises in
sitting and unipedal standing.
57
The short foot exercise can be
performed in progression from sitting to bipedal, to unipedal
positions, followed by functional activities such as squats and
single leg hops.
There is increasing evidence to suggest that training the foot
core via short foot exercise progressions can improve foot func-
tion. For example, 4 weeks of short foot exercise training in
healthy individuals reduces arch collapse as assessed by measures
of navicular drop and arch height index, and improve balance
ability.
35
In another study, healthy individuals who completed
4 weeks of short foot exercises demonstrated improved dynamic
balance compared to those who performed 4 weeks of towel
curl exercises.
58
However, postural control gains following a
4-week balance training home exercise programme were equiva-
lent between healthy training groups that did and did not
perform the short foot positioning during their balance exer-
cises.
59
In healthy young adults with pes planus, there were sig-
nicant increases in great toe exion strength and the
cross-sectional area of the abductor hallucis muscle after
4 weeks of short foot exercises and foot orthotic intervention
compared to foot orthotic intervention alone.
57
Preliminary evi-
dence demonstrates improved self-reported function in chronic
ankle instability patients who performed 4 weeks of short foot
Table 1 Functional qualities of the intrinsic foot muscles and their corresponding evidence-based descriptions
Functional quality Description
Supportive of the foot
arches
Diminished function of the intrinsic foot muscles leads to deleterious alterations in foot posture
32 33
whereas training the intrinsic foot muscles
enhances foot posture
34 35
Activity dependent Intrinsic foot muscles are more active in dynamic activities such as walking compared to standing
37
Load dependent As postural demands increase, such as from double to single limb stance, so does the activity of the intrinsic foot muscles
36
Synergistic The intrinsic foot muscles work together as a unit to provide dynamic arch support during the propulsive phase of gait
38
Modulating The intrinsic foot muscles support the foot in its role as a platform for standing and lever for propelling the body during dynamic activities
28
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positioning during balance exercises compared to a group that
did not perform the short foot positioning during their balance
exercises.
60
The training programmes used in these studies form
a framework for functional improvements through foot core
rehabilitation. There are other promising interventions for foot
core training, which may offer strong functional benets.
Role of barefoot/minimal footwear training for the foot core
Barefoot/minimal footwear walking and running may be used as
a training tool to strengthen the foot core system. Robbins and
Hanna
61
reported a signicant reduction in the foot length
(measured radiographically from the anterior aspect of the
calcaneus to the rst metatarsophalangeal joint) following
4 months of barefoot walking and running. The shortened foot
is an indirect measure of foot strengthening as it indicates a
raising of the arch. Muscle size has been directly correlated to
muscle strength.
62
Using this principle, Brüggemann et al
63
measured the cross-sectional area of some of the core muscles
of the foot in runners who trained for 5 months in shoes that
lacked any support to the arch and rearfoot. They reported sig-
nicant increases in the cross-sectional area of many of these
muscles. Further studies are needed to determine whether
strength and cross-sectional area gains of the foot core muscles
lead to a reduction in running-related injuries.
Another advantage of being completely barefoot is the
increase in sensory input received from the plantar surface of
the foot. Sensory input has long been recognised for its
importance in postural stability and dynamic gait pat-
terns.
43 64 65
In a study of single leg standing, postural stability
was found to be signicantly improved when standing bare feet
as opposed to in thin socks.
66
This suggests that the thin socks
lter out important sensory input that assists us with our static
stability. This sensory input appears to be important to
dynamic stability as well. In a recent study of single leg land-
ings, dynamic stability was improved when landing in the bare-
foot condition compared to a minimal running shoe and a
traditional running shoe.
67
In fact, stability progressively
increased with decreasing amount of footwear support. These
studies highlight the potential importance of sensory input to
the function of the foot. Therefore barefoot activities, in safe
environments, should assist in improving foot function.
However, it should be noted that individuals without normal
sensation should avoid barefoot activities.
SUMMARY
In summary, we hope we have increased the awareness of the
importance of the foot core, making up our arch, to overall
foot function. We have presented evolutionary evidence that
the foot core system developed in response to the increased
demands of load carriage and running. Admittedly, there is
much we do not know about the intricacies of our foot
mechanics. However, advancements in dynamic imaging such
as biplanar videoradiography will further enhance our under-
standing of normal and abnormal foot kinematics. Clearly, a
stronger foot is a healthier foot. To this end, we are suggesting
a paradigm shift in the way we think about treating the foot.
Current clinical guidelines include the use of foot orthotic
devices for heel pain and plantar fasciitis, but lack any refer-
ence to strengthening of the foot. While temporary support
may be needed during the acute phase of an injury, it should
be replaced as soon as possible with a strengthening pro-
gramme just as would be carried out for any other part of the
body. Therefore, we believe that more attention needs to be
focused on static and dynamic foot core function in rehabilita-
tion programmes. These concepts can be extended to organised
tness programmes as well. Our feet were designed with the
strength for unsupported endurance walking and running.
Unfortunately, adding permanent support to the foot, as
opposed to strengthening the foot core, is the current standard
of care.
We would like to suggest that perhaps it is time for the
Decade of the Foot. This type of attention to a largely ignored,
but critical, part of our body might help to raise awareness of
the amazing function of our feet and their underappreciated
potential for improvement.
What are the new ndings?
The foot core system is comprised of interacting
subsystems that provide relevant sensory input and
functional stability for accommodating to changing
demands during both static and dynamic activities. The
interaction of these subsystems is very similar to the
lumbopelvic core system.
The plantar intrinsic foot muscles within the active and
neural subsystems play a critical role in the foot core system
as local stabilisers and direct sensors of foot deformation.
Assessment of the foot core system can provide clinical
insight into the ability of the foot to cope with changing
functional demands.
Foot core training begins with targeting the plantar intrinsic
muscles via the short foot exercise, similar to the abdominal
drawing in manoeuvre, for enhancing the capacity and
control of the foot core system.
Figure 7 The short foot manoeuvre
is depicted. Note in the relaxed foot
(left) the resting length of the foot
(top image with solid black line). In
the contracted position (right), note
the change in foot length (dashed line)
due to the short foot contraction
drawing in the foot (arrows) from the
relaxed condition (solid black line).
Review
McKeon PO, et al.Br J Sports Med 2015;49:290. doi:10.1136/bjsports-2013-092690 7 of 9
group.bmj.com on August 22, 2015 - Published by http://bjsm.bmj.com/Downloaded from
Acknowledgements The authors would like to thank Tom Dolan, MS, the
medical illustrator in this report. Mr Dolan is a medical illustrator and multimedia
developer within the Department of Academic Technology at the University of
Kentucky. He is an outstanding anatomical artist whose contributions to this review
have been critical to the presentation of the foot core paradigm.
Contributors POM, JH and ID developed the concept for this manuscript. DB
contributed the Evolution of the Human Foot section and Tom Dolan was the
medical illustrator for gures 27. POM is the guarantor; however, the decision to
publish was agreed on by all authors and contributors. The nal version of the
manuscript has been agreed on by all authors.
Competing interests None.
Provenance and peer review Not commissioned; externally peer reviewed.
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understanding intrinsic foot muscle function
The foot core system: a new paradigm for
Patrick O McKeon, Jay Hertel, Dennis Bramble and Irene Davis
doi: 10.1136/bjsports-2013-092690
2015 49: 290 originally published online March 21, 2014Br J Sports Med
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... The underdevelopment of these structures in the human ancestor Australopithecus provides further support that Homo sapiens further evolved to run. 1,66 Additionally, the intrinsic muscles of the foot are also important in running when compared to walking. Electromyographic (EMG) activity in the planar intrinsic muscles is greater in running than in walking and greater in higher load activities. ...
... In quadrupeds, these muscles are nearly or completely absent. 66 This is likely because human running requires careful balance on uneven surfaces as well as during single leg stance. ...
... These structures have been likened to the abdominal and paraspinal muscles of the lumbopelvic-hip core. 66 The function of the intrinsic muscles as a group includes providing support to the arches of the foot, 33,40,71 being responsive to applied loads, 64,65 acting in synergy with the extrinsic muscles of the foot, and serving to regulate how forces are transmitted during gait, posture, and balance activities such as single leg stance. 50,94 Weakness or atrophy of the intrinsic muscles have been associated with several pathologic conditions including pes cavus and Charcot-Marie-Tooth, 20,36 hammer toe and claw toe deformities, 27,74 plantar fasciitis, 18,66 hallux valgus, 28,52,97 and pes planus. ...
Article
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Distance runners represent a unique patient population. The cyclic activity associated with distance running leads to a high incidence of injury. Gait patterns, the extrinsic and intrinsic muscles of the foot and ankle, foot strike pattern, shoe wear considerations, alignment, and orthotics are also all important considerations that must be considered by the treating provider. The purpose of this work is to review relevant functional anatomy, recent studies on gait patterns in running, orthotics, and theory on how the body moves through space during running in order to better equip the clinician to treat long distance runners.
... The fascia and ligaments of the plantar are important structures that maintain the dome structure of the foot. However, maintaining the stability of the medial longitudinal arch (MLA) during gait and controlling the movement of the foot rely on the intrinsic and extrinsic foot muscles (McKeon et al., 2015). Weak foot muscles cannot provide sufficient support to MLA during dynamic movement, resulting in repeated strains of the plantar fascia, which in turn, increase the risk of plantar injuries . ...
... However, foot core exercise alone may fail to meet the functional requirements of the arch under dynamic conditions (e.g., running, jumping.), while FFS training alone may increase the risk of running-related injury because the foot structure cannot support the rapid increase in load caused by the sudden change in strike pattern (Ridge et al., 2013). A more appropriate training program would be a combined intervention to enhance both the intrinsic and extrinsic foot muscles (i.e., active subsystem) of the foot core system, which can simultaneously improve foot function in static posture and dynamic activities (McKeon et al., 2015). Accordingly, previous gait retraining studies have begun to incorporate foot core exercise to reduce the risk of injury in participants when the strike pattern suddenly changes (Warne et al., 2014;Deng et al., 2020;Zhang et al., 2021). ...
... The 12-week gait retraining combined with foot core exercise significantly increased the normalized standing arch height in the INT group, thus corroborating the finding of McKeon et al. (McKeon et al., 2015). Moreover, the improvement reached a moderate effect size (Cohen's d = 0.55), which was superior to previous studies with no significant changes in arch height after standalone interventions (Lynn et al., 2012;Miller et al., 2014). ...
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Objective: This study aims to explore the effects of a 12-week gait retraining program combined with foot core exercise on arch morphology, arch muscles strength, and arch kinematics. Methods: A total of 26 male recreational runners with normal arch structure who used rear-foot running strike (RFS) were divided into the intervention group (INT group) and control group (CON group) (n = 13 in each group). The INT group performed a 12-week forefoot strike (FFS) training combined with foot core exercises. The CON group did not change the original exercise habit. Before and after the intervention, the arch morphology, as well as the strength of hallux flexion, lesser toe flexion, and the metatarsophalangeal joint (MPJ) flexors were measured in a static position, and changes in the arch kinematics during RFS and FFS running were explored. Results: After a 12-week intervention, 1) the normalized navicular height increased significantly in the INT group by 5.1% (p = 0.027, Cohen's d = 0.55); 2) the hallux absolute flexion and relative flexion of the INT group increased significantly by 20.5% and 21.7%, respectively (p = 0.001, Cohen's d = 0.59; p = 0.001, Cohen's d = 0.73), the absolute and relative strength of the MPJ flexors of the INT group were significantly improved by 30.7% and 32.5%, respectively (p = 0.006, Cohen's d = 0.94; p = 0.006, Cohen's d = 0.96); 3) and during RFS, the maximum arch angle of the INT group declined significantly by 5.1% (p < 0.001, Cohen's d = 1.49), the arch height at touchdown increased significantly in the INT group by 32.1% (p < 0.001, Cohen's d = 1.98). Conclusion: The 12-week gait retraining program combined with foot core exercise improved the arch in both static and dynamic positions with a moderate to large effect size, demonstrating the superiority of this combined intervention over the standalone interventions. Thus, runners with weak arch muscles are encouraged to use this combined intervention as an approach to enhance the arch.
... Plantar fasciitis is the most common musculoskeletal condition of the foot in the general population and in the running community [1][2][3]. Mounting evidence suggests that it is caused by repetitive tensile loads applied to the plantar aponeurosis due to excessive deformation of the foots longitudinal arch [2][3][4][5]. Excessive deformation of the arch is facilitated by weakness of the intrinsic foot muscles [2][3][4]6,7]. Modern footwear use is associated with weaker intrinsic foot muscles [7,8] a greater prevalence of flat feet [9] and changes to the shape of the foot that are not seen in habitually barefoot or minimally shod populations [7,10,11]. ...
... The coordinated action of the neural, passive and muscular subsystems is thought to underpin healthy movement that does not overload any one tissue [4,5]. Failure in any one of these subsystems is likely to disrupt coordinated movement. ...
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Background: Foot characteristics and running biomechanics in shod populations are associated with the aetiology of plantar fasciitis, the most common musculoskeletal disease of the foot. Previous Case reports have demonstrated improvements in the symptoms of plantar fasciitis after a period of barefoot running on grass. Methods: Recreational runners with symptomatic plantar fasciitis were prospectively enrolled into a 6-week grass based barefoot running programme. Duration of symptoms, previous management and current pain scores (NRS, VAS) were recorded at entry. Daily pain scores were recorded during the 6-week period and 12 weeks from entry to the programme. Results: In total, 20 of 28 patients (71.4%) enrolled were included in the analysis. Relative to the entry point, pain at 6-weeks was lower (2.5 ± 1.4 vs. 3.9 ± 1.4, p < 0.001) and pain at the 12-week point was lower (1.5 (1.8), p = 0.002). 19 out of 20 patients had improved at week-6 (mean ± SD % change in pain score, −38.8 ± 21.5%) and at week-12 (median (IQR) % change in pain score, −58.3 (34.8) %). Conclusion: Barefoot running on grass improved pain associated with plantar fasciitis at the 6-week and 12-week follow up points. This type of barefoot running has the ability to improve symptoms whilst allowing patients to continue running, the intervention may also address some impairments of the foot associated with plantar fasciitis.
... The human foot is a complex static/dynamic structure that holds the body's weight [1]. The foot's core system consists of three subsystems -neural, active and passive -which provide the stability and flexibility that are indispensable for everyday life. ...
... The foot's core system consists of three subsystems -neural, active and passive -which provide the stability and flexibility that are indispensable for everyday life. Proper functioning of the foot depends on appropriate shape and tension of two transverse and five longitudinal arching and morphological structures [1]. An efficient core system is essential for the proper functioning of the foot, allow it to adapt to changing structure and load conditions [2]. ...
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Background The purpose of this study, conducted within the framework of the project entitled ‘Let’s get the kids moving’, was to determine the associations between children’s longitudinal and transverse foot arch and their weight, gender and age. Method The pro-health initiative ‘Let’s get the kids moving’ was created by researchers of the Medical University and the Run for Health Foundation. A total of 655 children (51.5% boys) aged 7 to 10 years from primary schools in south-western Poland participated in the study. The mean age of the subjects was 8.7 ± 0.8 years. In all the children, we assessed anthropometric measurements (weight and height) and the longitudinal and transverse arch of the foot under the load of their weight. A two-dimensional foot scanner (Sensor Medica, Italy) was used to examine the plantar part of the children’s feet. Results The data collected from the 655 subjects revealed that excessive weight predisposed them to less longitudinal and transverse arching. The foot shape was not differentiated by gender or age. Conclusions Screening school-aged children’s footprints can detect abnormalities in the shape of children’s feet early on, which allows for early diagnosis of functional or structural flatfoot in children.
... Our finding of increased PF strain during FFS supported earlier findings which suggested that unaccustomed FFS may place more demands on the PF. Therefore, strengthening their foot muscles was required to reduce the risk of plantar fasciitis (Kogler et al., 2001;McKeon et al., 2015). In addition, we found that the PF shortened in RFS at the first 10% of the stance phase, but sustained elongation during this period in FFS. ...
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Accurately obtaining the in vivo motion of the medial longitudinal arch (MLA), first metatarsophalangeal joint (MTPJ), and plantar fascia (PF) is essential for analyzing the biomechanics of these structures in different running strike patterns. Most previous studies on the biomechanics of the MLA, first MTPJ, and PF have been based on traditional skin-marker–based motion capture, which cannot acquire the natural foot motion. Therefore, this study aimed to 1) describe the movement of the MLA, first MTPJ, and PF during running by using the high-speed dual fluoroscopic imaging system (DFIS) and 2) explore changes of the in vivo kinematics of the MLA and first MTPJ, and the length of the PF during the stance phase of running with different foot strike patterns. Fifteen healthy male runners all of whom ran with a regular rearfoot strike (RFS) pattern were required to run with forefoot strike (FFS) and RFS patterns. Computed tomography scans were taken from each participant’s right foot for the construction of 3D models (the calcaneus, first metatarsal, and first proximal phalanges) and local coordinate systems. A high-speed DFIS (100 Hz) and 3D force platform (2,000 Hz) were used to acquire X-ray images of the foot bones and ground reaction force data during the stance phase of running (3 m/s ± 5%) simultaneously. Then, 3D-2D registration was used to obtain the in vivo kinematic data of the MLA and first MTPJ and the length of the PF. When compared with RFS, in FFS, 1) the range of motion (ROM) of the medial/lateral (5.84 ± 5.61 mm vs . 0.75 ± 3.38 mm, p = 0.002), anterior/posterior (14.64 ± 4.33 mm vs . 11.18 ± 3.56 mm, p = 0.010), plantarflexion/dorsiflexion (7.13 ± 3.22° vs . 1.63 ± 3.29°, p < 0.001), and adduction/abduction (−3.89 ± 3.85° vs . −0.64 ± 4.39°, p = 0.034) motions of the MLA were increased significantly; 2) the ROM of the anterior/posterior (7.81 ± 2.84 mm vs . 6.24 ± 3.43 mm, p = 0.003), superior/inferior (2.11 ± 2.06 mm vs . −0.57 ± 1.65 mm, p = 0.001), and extension/flexion (−9.68 ± 9.16° vs . −5.72 ± 7.33°, p = 0.018) motions of the first MTPJ were increased significantly; 3) the maximum strain (0.093 ± 0.023 vs . 0.075 ± 0.020, p < 0.001) and the maximum power (4.36 ± 1.51 W/kg vs . 3.06 ± 1.39 W/kg, p < 0.001) of the PF were increased significantly. Running with FFS may increase deformation, energy storage, and release of the MLA and PF, as well as the push-off effect of the MTPJ. Meanwhile, the maximum extension angle of the first MTPJ and MLA deformation increased in FFS, which showed that the PF experienced more stretch and potentially indicated that FFS enhanced the PF mechanical responses.
... Another study argues that the intrinsic plantar foot muscles within the active and neural subsystems play a critical role in the central foot system as local stabilizers and direct sensors of foot deformation (McKeon et al., 2015). Therefore, it is imperative to strengthen the muscular-ligamentous structures of the foot, especially the plantar ones, which are essential in maintaining the natural plantar arch of the foot and its stability together with the ankle joint. ...
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... ➔ support and stability [4][5][6], ➔ increasing stride length and energy conversion [2,7,8], ➔ reducing the contact collision [9], and ➔ absorbing the impact by forceful plantarflexion [10,11]. ...
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Medial tibial stress syndrome (MTSS) is a common exercise-induced leg injury among athletes and military personnel. Several treatment options have been described in the literature, but it remains unclear which treatment is most effective. The objective of this systematic review was to assess the effectiveness of any intervention in the treatment of MTSS. Published or non-published studies, reporting randomized or non-randomized controlled trials of any treatment in subjects with MTSS were eligible for inclusion. Treatments were assessed for effects on pain, time to recovery or global perceived effect. Computerized bibliographic databases (MEDLINE, CENTRAL, EMBASE, CINAHL, PEDro and SPORTDiscus) and trial registries were searched for relevant reports, from their inception to 1 June 2012. Grey literature was searched for additional relevant reports. The Cochrane Risk of Bias Tool was used to appraise study quality of randomized clinical trials (RCTs) whereas the Newcastle Ottawa Scale was used to appraise non-randomized trials. The 'levels of evidence', according to the Oxford Centre for Evidence-Based Medicine, addressed the impact of the assessed trials. Two reviewers independently performed the search for articles, study selection, data extraction and appraised methodological quality. Eleven trials were included in this systematic review. All RCTs revealed a high risk of bias (Level 3 of evidence). Both non-randomized clinical trials were found to be of poor quality (Level 4 of evidence). RCTs, studying the effect of a lower leg brace versus no lower leg brace, and iontophoresis versus phonophoresis, were pooled using a fixed-effects model. No significant differences were found for lower leg braces (standardized mean difference [SMD] -0.06; 95 % CI -0.44 to 0.32, p = 0.76), or iontophoresis (SMD 0.09; 95 % CI -0.50 to 0.68, p = 0.76). Iontophoresis, phonophoresis, ice massage, ultrasound therapy, periosteal pecking and extracorporeal shockwave therapy (ESWT) could be effective in treating MTSS when compared with control (Level 3 to 4 of evidence). Low-energy laser treatment, stretching and strengthening exercises, sports compression stockings, lower leg braces and pulsed electromagnetic fields have not been proven to be effective in treating MTSS (level 3 of evidence). None of the studies are sufficiently free from methodological bias to recommend any of the treatments investigated. Of those examined, ESWT appears to have the most promise.
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A specific training program emphasizing the neuromuscular recruitment of the plantar intrinsic foot muscles, colloquially referred to as "short foot" exercise (SFE) training, has been suggested as a means to dynamically support the medial longitudinal arch (MLA) during functional tasks. A single-group repeated measures pre- and post-intervention study design was utilized to determine if a 4-week intrinsic foot muscle training program would impact the amount of navicular drop (ND), increase the arch height index (AHI), improve performance during a unilateral functional reaching maneuver, or the qualitative assessment of the ability to hold the arch position in single limb stance position in an asymptomatic cohort. 21 asymptomatic subjects (42 feet) completed the 4-week SFE training program. Subject ND decreased by a mean of 1.8 mm at 4 weeks and 2.2 mm at 8 weeks (p < 0.05). AHI increased from 28 to 29% (p < 0.05). Intrinsic foot muscle performance during a static unilateral balancing activity improved from a grade of fair to good (p < 0.001) and subjects experienced a significant improvement during a functional balance and reach task in all directions with the exception of an anterior reach (p < 0.05). This study offers preliminary evidence to suggest that SFE training may have value in statically and dynamically supporting the MLA. Further research regarding the value of this exercise intervention in foot posture type or pathology specific patient populations is warranted.
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Striding bipedalism is a key derived behaviour of hominids that possibly originated soon after the divergence of the chimpanzee and human lineages. Although bipedal gaits include walking and running, running is generally considered to have played no major role in human evolution because humans, like apes, are poor sprinters compared to most quadrupeds. Here we assess how well humans perform at sustained long-distance running, and review the physiological and anatomical bases of endurance running capabilities in humans and other mammals. Judged by several criteria, humans perform remarkably well at endurance running, thanks to a diverse array of features, many of which leave traces in the skeleton. The fossil evidence of these features suggests that endurance running is a derived capability of the genus Homo, originating about 2 million years ago, and may have been instrumental in the evolution of the human body form.
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Objectives. To compare the effectiveness of a traditional balance training program and a balance training program that emphasizes foot positioning, in improving postural control.Design. Randomized control study using a pre-post design.Setting. A laboratory setting.Participants. Forty-five healthy college students participated. Sixteen completed a traditional balance training program (TRAD), 14 completed a training program emphasizing foot positioning (POS), and 15 received no intervention (CONT). Subjects in the TRAD and POS group performed balance training on their dominant lower extremity for 4 weeks.Main outcome measure. Center of pressure excursion velocity (COPV) assessed during single leg quiet standing on a force plate during eyes open and eyes closed trials on both the trained and untrained limbs.Results. The TRAD group improved substantially more than did the POS or CONT groups. Improvements in COPV measures were seen in the TRAD group for both the trained and untrained limbs. The most substantial improvements occurred on the trained leg in the eyes closed condition.Conclusion. Traditional balance training appears to be more effective than balance training emphasizing active foot positioning in healthy individuals. Bilateral improvement in balance for the TRAD group suggests a central nervous system control of postural control.
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Purpose: Patellofemoral pain (PFP) is the leading cause of knee pain in runners. Proximal and distal running mechanics have been linked to the development of PFP. However, the lack of prospective studies limits establishing a causal relationship of these mechanics to PFP. The purpose of this study was to prospectively compare running mechanics in a group of female runners who went on to develop PFP compared with healthy controls. It was hypothesized that runners who go on to develop PFP would exhibit greater hip adduction, hip internal rotation, and greater rear foot eversion. Methods: Four hundred healthy female runners underwent an instrumented gait analysis and were then tracked for any injuries that they may have developed over a 2-yr period. Fifteen cases of PFP developed, which were confirmed by a medical professional. Their initial running mechanics were compared to an equal number of runners who remained uninjured. Results: We found that female runners who developed PFP exhibited significantly greater hip adduction (P = 0.007). No statistically significant differences were found for the hip internal rotation angle (P = 0.47) or rear foot eversion (P = 0.1). Conclusions: The finding of greater hip adduction in female runners who develop PFP is in agreement with previous cross-sectional studies. These results suggest that runners who develop PFP use a different proximal neuromuscular control strategy than those who remain healthy. Injury prevention and treatment strategies should consider addressing these altered hip mechanics.
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