The foot core system: A new paradigm for understanding intrinsic foot muscle function

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.

Full text

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 ‘core’of 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 multifidus 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.
4–7
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) finally discussing future research directions. Our
overall goal was to propose a new paradigm by
which to view foot function, assessment and
treatment.
Editor’s choice
Scan to access more
free content
Review
McKeon PO, et al.Br J Sports Med 2015;49:290. doi:10.1136/bjsports-2013-092690 1of9
group.bmj.com on August 22, 2015 - Published by http://bjsm.bmj.com/Downloaded from

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 reflects 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-
defined 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 defined 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-defined medial longitudinal arch.
17
The modern human body (ie, Homo), especially in musculoskel-
etal design, reflects 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
springs’in 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 flattening 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
significant role in controlling load distribution under the foot as well
as augmenting the flexor 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 superfi-
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 ‘control’and ‘capacity’components. 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 figure 1. These concepts as they relate to the ankle
and foot were first 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 configuration 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 flexibly adapting to load
changes during dynamic activities (see figure 2). This half dome
has been thought to be predominantly supported by passive
structures including the plantar aponeurosis (see figure 3A) and
plantar ligaments (see figure 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 figure 5). While there are intrinsic muscles on both the
dorsal and plantar aspects, the plantar intrinsic muscles are most
Review
2 of 9 McKeon PO, et al.Br J Sports Med 2015;49:290. doi:10.1136/bjsports-2013-092690
group.bmj.com on August 22, 2015 - Published by http://bjsm.bmj.com/Downloaded from

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 first two layers have
muscle configurations which align with the medial and lateral
longitudinal arches of the foot whereas the deeper layers config-
ure more so with the anterior and posterior transverse arches
(see figure 6A–E). 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 configuration, 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,
32–35
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 figure 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.
40–44
The sensory contributions of the
intrinsic foot muscles remain less clear. Based on the intrinsic
foot muscles’anatomical and biomechanical configuration, 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
flexibility 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.
Review
McKeon PO, et al.Br J Sports Med 2015;49:290. doi:10.1136/bjsports-2013-092690 3 of 9
group.bmj.com on August 22, 2015 - Published by http://bjsm.bmj.com/Downloaded from

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
flexion, navicular drop during standing increased significantly 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 specific conditions such as diabetic neuropathy
48
and
claw toes.
49
Assessments for these conditions have largely been
related to diminished toe flexion 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 ‘direct’and ‘indirect’evaluations of intrinsic muscle func-
tion.
31
Direct evaluations have focused on assessment of toe
flexion strength while indirect evaluations include imaging tech-
niques and EMG to estimate intrinsic foot muscle function.
Tests focusing on toe flexion strength are inherently limited
by the inability to conclusively separate the contributions of the
intrinsic and extrinsic toe flexor 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 flexion, 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 patient’s 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
patient’s 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.
Review
4 of 9 McKeon PO, et al.Br J Sports Med 2015;49:290. doi:10.1136/bjsports-2013-092690
group.bmj.com on August 22, 2015 - Published by http://bjsm.bmj.com/Downloaded from

Figure 5 Insertions of the extrinsic
foot muscle tendons on the plantar
surface of the foot. (A) The insertions
of the flexor digitorum longus, flexor
hallucis longus and peroneus longus
are depicted. Note the longitudinal
alignment of the flexor 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 figure 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) flexor digitorum brevis, (3) abductor digiti minimi, (4) quadratus
plantae (note its insertion into the flexor digitorum tendon), (5) lumbricals (note their origin from the flexor digitorum longus tendon), (6) flexor
digiti minimi, (7) adductor hallucis oblique (a) and transverse (b) heads, (8) flexor hallucis brevis, (9) plantar interossei, (10) dorsal interossei and
(11) extensor digitorum brevis.
Review
McKeon PO, et al.Br J Sports Med 2015;49:290. doi:10.1136/bjsports-2013-092690 5 of 9
group.bmj.com on August 22, 2015 - Published by http://bjsm.bmj.com/Downloaded from

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 fine 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 superfi-
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
confirm the location of the indwelling electrode. Kelly et al
36
reported the ability to assess the activation of the abductor hal-
lucis, flexor 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 fine 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
specific 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 deficits have been identified
with ultrasonography.
48
As with surface EMG, the superficial
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
specific 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 flexion
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 flexor hal-
lucis longus and flexor digitorum longus muscles. Recently, the
‘short foot exercise’has been described as a means to isolate
contraction of the plantar intrinsic muscles
45 54 55
(figure 7).
The foot is ‘shortened’by using the intrinsic muscles to pull the
first 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 Hodges’concept
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 flexed 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, flexor 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-
nificant increases in great toe flexion 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
Review
6 of 9 McKeon PO, et al.Br J Sports Med 2015;49:290. doi:10.1136/bjsports-2013-092690
group.bmj.com on August 22, 2015 - Published by http://bjsm.bmj.com/Downloaded from

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 benefits.
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 significant reduction in the foot length
(measured radiographically from the anterior aspect of the
calcaneus to the first 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-
nificant 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 significantly improved when standing bare feet
as opposed to in thin socks.
66
This suggests that the thin socks
filter 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
fitness 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 findings?
▸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 figures 2–7. POM is the guarantor; however, the decision to
publish was agreed on by all authors and contributors. The final version of the
manuscript has been agreed on by all authors.
Competing interests None.
Provenance and peer review Not commissioned; externally peer reviewed.
REFERENCES
1 Kibler WB, Press J, Sciascia A. The role of core stability in athletic function. Sports
Med 2006;36:189–98.
2 Bergmark A. Stability of the lumbar spine. A study in mechanical engineering. Acta
Orthop Scand Suppl 1989;230:1–54.
3 Leetun DT, Ireland ML, Willson JD, et al. Core stability measures as risk factors for
lower extremity injury in athletes. Med Sci Sports Exerc 2004;36:926–34.
4 Ferber R, Noehren B, Hamill J, et al. Competitive female runners with a history of
iliotibial band syndrome demonstrate atypical hip and knee kinematics. J Orthop
Sports Phys Ther 2010;40:52–8.
5 Milner CE, Ferber R, Pollard CD, et al. Biomechanical factors associated with tibial
stress fracture in female runners. Med Sci Sports Exerc 2006;38:323–8.
6 Noehren B, Davis I, Hamill J. ASB clinical biomechanics award winner 2006
prospective study of the biomechanical factors associated with iliotibial band
syndrome. Clin Biomech (Bristol, Avon) 2007;22:951–6.
7 Noehren B, Hamill J, Davis I. Prospective evidence for a hip etiology in
patellofemoral pain. Med Sci Sports Exerc 2013;45:1120–4.
8 Wearing SC, Smeathers JE, Urry SR, et al. The pathomechanics of plantar fasciitis.
Sports Med 2006;36:585–611.
9 McPoil TG, Martin RL, Cornwall MW, et al. Heel pain—plantar fasciitis: clinical
practice guidelines linked to the international classification of function, disability,
and health from the orthopaedic section of the American Physical Therapy
Association. J Orthop Sports Phys Ther 2008;38:A1–18.
10 Bowring B, Chockalingam N. A clinical guideline for the conservative management
of tibialis posterior tendon dysfunction. Foot (Edinburgh, Scotland)
2009;19:211–17.
11 Winters M, Eskes M, Weir A, et al. Treatment of medial tibial stress syndrome:
a systematic review. Sports Med 2013;43:1315–33.
12 Brewer RB, Gregory AJ. Chronic lower leg pain in athletes: a guide for the
differential diagnosis, evaluation, and treatment. Sports Health 2012;4:121–7.
13 Crompton RH, Vereecke EE, Thorpe SK. Locomotion and posture from the common
hominoid ancestor to fully modern hominins, with special reference to the last
common panin/hominin ancestor. J Anat 2008;212:501–43.
14 Susman RL, Stern JT Jr, Jungers WL. Arboreality and bipedality in the Hadar
hominids. Folia Primatol (Basel) 1984;43:113–56.
15 Ward CV, Kimbel WH, Johanson DC. Complete fourth metatarsal and arches in the
foot of Australopithecus afarensis. Science 2011;331:750–3.
16 Harcourt-Smith WE, Aiello LC. Fossils, feet and the evolution of human bipedal
locomotion. J Anat 2004;204:403–16.
17 Bennett MR, Harris JW, Richmond BG, et al. Early hominin foot morphology based
on 1.5-million-year-old footprints from Ileret, Kenya. Science 2009;323:1197–201.
18 Bramble DM, Lieberman DE. Endurance running and the evolution of Homo. Nature
2004;432:345–52.
19 Alexander RM. Energy-saving mechanisms in walking and running. J Exp Biol
1991;160:55–69.
20 Ker RF, Bennett MB, Bibby SR, et al. The spring in the arch of the human foot.
Nature 1987;325:147–9.
21 Reeser LA, Susman RL, Stern JT Jr. Electromyographic studies of the human foot:
experimental approaches to hominid evolution. Foot Ankle 1983;3:391–407.
22 Caravaggi P, Pataky T, Gunther M, et al. Dynamics of longitudinal arch support in
relation to walking speed: contribution of the plantar aponeurosis. J Anat
2010;217:254–61.
23 Basmajian JV, Stecko G. The role of muscles in arch support of the foot. J Bone
Joint Surg Am 1963;45:1184–90.
24 Wang WJ, Crompton RH. The role of load-carrying in the evolution of modern body
proportions. J Anat 2004;204:417–30.
25 Panjabi MM. The stabilizing system of the spine. Part I. Function, dysfunction,
adaptation, and enhancement. J Spinal Disord 1992;5:383–9; discussion 97.
26 Hodges PW. Core stability exercise in chronic low back pain. Orthop Clin North Am
2003;34:245–54.
27 Jam B. Evaluation and retraining of the intrinsic foot muscles for pain syndromes
related to abnormal control of pronation. [October 10, 2013]. http://www.aptei.
com/articles/pdf/IntrinsicMuscles.pdf
28 Gray H, Standring S, Ellis H, et al.Gray’s anatomy : the anatomical basis of clinical
practice 39th edn. New York: Elsevier Churchill Livingtone, 2005.
29 McKenzie J. The foot as a half-dome. Br Med J 1955;1:1068–9.
30 Ridola C, Palma A. Functional anatomy and imaging of the foot. Ital J Anat Embryol
2001;106:85–98.
31 Soysa A, Hiller C, Refshauge K, et al. Importance and challenges of measuring
intrinsic foot muscle strength. J Foot Ankle Res 2012;5:29.
32 Fiolkowski P, Brunt D, Bishop M, et al. Intrinsic pedal musculature support of the
medial longitudinal arch: an electromyography study. J Foot Ankle Surg
2003;42:327–33.
33 Headlee DL, Leonard JL, Hart JM, et al. Fatigue of the plantar intrinsic foot muscles
increases navicular drop. J Electromyogr Kinesiol 2008;18:420–5.
34 Drewes LK, Beazell J, Mullins M, et al. Four weeks of short foot exercises affect
lower extremity function, but not alignment, in patients with lower extremity
injuries. J Athl Train 2008;43:S105.
35 Mulligan EP, Cook PG. Effect of plantar intrinsic muscle training on medial
longitudinal arch morphology and dynamic function. Man Ther 2013;
18:425–30.
36 Kelly LA, Kuitunen S, Racinais S, et al. Recruitment of the plantar intrinsic foot muscles
with increasing postural demand. Clin Biomech (Bristol, Avon) 2012;27:46–51.
37 Mann R, Inman VT. Phasic activity of intrinsic muscles of the foot. J Bone Joint Surg
Am 1964;46:469–81.
38 Mann RA, Hagy JL. The function of the toes in walking, jogging and running. Clin
Orthop Relat Res 1979(142):24–9.
39 Cheung JT, Zhang M, An KN. Effect of Achilles tendon loading on plantar fascia
tension in the standing foot. Clin Biomech 2006;21:194–203.
40 Eils E, Nolte S, Tewes M, et al. Modified pressure distribution patterns in walking
following reduction of plantar sensation. J Biomech 2002;35:1307–13.
41 Hoch MC, McKeon PO, Andreatta RD. Plantar vibrotactile detection deficits in
adults with chronic ankle instability. Med Sci Sports Exerc 2012;44:666–72.
42 Kavounoudias A, Roll R, Roll JP. The plantar sole is a ‘dynamometric map’for
human balance control. Neuroreport 1998;9:3247–52.
43 Kavounoudias A, Roll R, Roll JP. Foot sole and ankle muscle inputs contribute
jointly to human erect posture regulation. J Physiol 2001;532:869–78.
44 McKeon PO, Hertel J. Diminished plantar cutaneous sensation and postural control.
Percept Motor Skills 2007;104:56–66.
45 Janda V, Vavrova M, Hervenova A, et al. Sensory motor stimulation. In:
Liebenson C. ed Rehabilitation of the spine: a practitioner’smanual. 2nd edn.
Lippincott Williams & Wilkins, 2006.
46 Hiemstra LA, Lo IK, Fowler PJ. Effect of fatigue on knee proprioception: implications
for dynamic stabilization. J Orthop Sports Phys Ther 2001;31:598–605.
47 Boucher JA, Abboud J, Descarreaux M. The influence of acute back muscle fatigue
and fatigue recovery on trunk sensorimotor control. J Manipulative Physiol Ther
2012;35:662–8.
48 Severinsen K, Obel A, Jakobsen J, et al. Atrophy of foot muscles in diabetic patients
can be detected with ultrasonography. Diabetes Care 2007;30:3053–7.
49 Bus SA, Maas M, Michels RP, et al. Role of intrinsic muscle atrophy in the etiology
of claw toe deformity in diabetic neuropathy may not be as straightforward as
widely believed. Diabetes Care 2009;32:1063–7.
50 Chang R, Kent-Braun JA, Hamill J. Use of MRI for volume estimation of tibialis
posterior and plantar intrinsic foot muscles in healthy and chronic plantar fasciitis
limbs. Clin Biomech (Bristol, Avon) 2012;27:500–5.
51 Andreassen CS, Jakobsen J, Ringgaard S, et al. Accelerated atrophy of lower leg
and foot muscles—a follow-up study of long-term diabetic polyneuropathy using
magnetic resonance imaging (MRI). Diabetologia 2009;52:1182–91.
52 Cameron AF, Rome K, Hing WA. Ultrasound evaluation of the abductor hallucis
muscle: reliability study. J Foot Ankle Res 2008;1:12.
53 Teyhen DS, Gill NW, Whittaker JL, et al. Rehabilitative ultrasound imaging of the
abdominal muscles. J Orthop Sports Phys Ther 2007;37:450–66.
54 Greenman P. Exercise principles and prescription. In: Butler J. ed Principles of
manual medicine. 2nd edn. Baltimore: Williams & Wilkins, 1996:449–525.
55 Sauer LD, Beazell J, Hertel J. Considering the intrinsic foot musculature in
evaluation and rehabilitation for lower extremity injuries. Athl Train Sports Health
Care 2011;3:43–7.
56 Black M. Pilates for Feet: Pilates-Pro.com; [10 October 2013]. http://www.
pilates-pro.com/pilates-pro/2009/3/24/pilates-for-feet.html
57 Jung DY, Koh EK, Kwon OY. Effect of foot orthoses and short-foot exercise on the
cross-sectional area of the abductor hallucis muscle in subjects with pes planus: a
randomized controlled trial. J Back Musculoskelet Rehabil 2011;
24:225–31.
58 Lynn SK, Padilla RA, Tsang KK. Differences in static- and dynamic-balance task
performance after 4 weeks of intrinsic-foot-muscle training: the short-foot exercise
versus the towel-curl exercise. J Sport Rehabil 2012;21:327–33.
59 Rothermel S, Hale S, Hertel J, et al. Effect of active foot positioning on the outcome
of a balance training program. Phys Ther Sport 2004;5:98–103.
60 Sauer LD, Saliba SA, Ingersoll CD, et al. Effects of rehabilitation incorporating short
foot exercises on self-reported function, static and dynamic balance in chronic ankle
instability patients. J Athl Train 2010;45:S67.
Review
8 of 9 McKeon PO, et al.Br J Sports Med 2015;49:290. doi:10.1136/bjsports-2013-092690
group.bmj.com on August 22, 2015 - Published by http://bjsm.bmj.com/Downloaded from

61 Robbins SE, Hanna AM. Running-related injury prevention through barefoot
adaptations. Med Sci Sports Exerc 1987;19:148–56.
62 An KN, Linscheid RL, Brand PW. Correlation of physiological cross-sectional areas of
muscle and tendon. J Hand Surg (Edinburgh, Scotland) 1991;16:66–7.
63 Brüggemann G, Potthast W, Braunstein B, et al.Effect of increased mechanical
stimuli on foot muscles functional capacity. American Society of Biomechanics
Annual Meeting. Cleveland, OH, 2005. http://www.asbweb.org/conferences/2005/
pdf/0553.pdf (accessed 10 Oct 2013).
64 Maurer C, Mergner T, Bolha B, et al. Human balance control during cutaneous
stimulation of the plantar soles. Neurosci Lett 2001;302:45–8.
65 Meyer PF, Oddsson LI, De Luca CJ. The role of plantar cutaneous sensation in
unperturbed stance. Exp Brain Res 2004;156:505–12.
66 Shinohara J, Gribble P. Five-toed socks decrease static postural control
among healthy individuals as measured with time-to-boundary analysis.
2009 American Society of Biomechanics Annual Meeting. State College,
PA, 2009. http://www.asbweb.org/conferences/2009/952.pdf (accessed 10 Oct
2013).
67 Rose W, Bowser B, McGrath R, et al.Effect of footwear on balance. American
Society of Biomechanics Annual Meeting. Long Beach, CA, 2011. http://www.
asbweb.org/conferences/2011/pdf/344.pdf (accessed 10 Oct 2013).
Review
McKeon PO, et al.Br J Sports Med 2015;49:290. doi:10.1136/bjsports-2013-092690 9 of 9
group.bmj.com on August 22, 2015 - Published by http://bjsm.bmj.com/Downloaded from

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
http://bjsm.bmj.com/content/49/5/290
Updated information and services can be found at:
These include:
Material
Supplementary
DC1.html
http://bjsm.bmj.com/content/suppl/2014/03/21/bjsports-2013-092690.
Supplementary material can be found at:
References #BIBLhttp://bjsm.bmj.com/content/49/5/290
This article cites 58 articles, 8 of which you can access for free at:
service
Email alerting box at the top right corner of the online article.
Receive free email alerts when new articles cite this article. Sign up in the
Collections
Topic Articles on similar topics can be found in the following collections
(246)Editor's choice (108)BJSM Reviews with MCQs
Notes
http://group.bmj.com/group/rights-licensing/permissions
To request permissions go to:
http://journals.bmj.com/cgi/reprintform
To order reprints go to:
http://group.bmj.com/subscribe/
To subscribe to BMJ go to:
group.bmj.com on August 22, 2015 - Published by http://bjsm.bmj.com/Downloaded from