Biomechanics of the normal and abnormal foot

Abstract

The foot is an engineering marvel that allows the body to perform many physical activities over a wide variety of terrain with remarkable efficiency. The functions of the foot and the lower extremity are biomechanically integrated; thus normal lower-extremity function requires normal foot function and vice versa. Because the subtalar joint is the main pedal joint allowing the triplanar translation of motion between the foot and lower extremity, normal subtalar joint function is critical to normal foot and lower-extremity function. This article provides an overview of the interrelationships between foot and lower-extremity function and mechanically based pathology of the foot and lower extremity, with an emphasis on the subtalar joint.

Full text

Biomechanics of the Normal and
Abnormal Foot
KEVIN A. KIRBY, DPM, MS*
The foot is an engineering marvel that allows the body to perform many
physical activities over a wide variety of terrain with remarkable effi-
ciency. The functions of the foot and the lower extremity are biomechan-
ically integrated; thus normal lower-extremity function requires normal
foot function and vice versa. Because the subtalar joint is the main
pedal joint allowing the triplanar translation of motion between the foot
and lower extremity, normal subtalar joint function is critical to normal
foot and lower-extremity function. This article provides an overview of
the interrelationships between foot and lower-extremity function and me-
chanically based pathology of the foot and lower extremity, with an em-
phasis on the subtalar joint. (J Am Podiatr Med Assoc 90(1): 30-34,
2000)
The human foot is an engineering marvel. It is de-
signed to allow a bipedal animal, the human, to walk,
run, and jump efficiently over many types of support-
ing surfaces without pain or injury. It is subjected to
impact forces from contact with the ground and
must therefore be able to work with the rest of the
body to absorb or dissipate the stresses on the struc-
tural components of the foot and lower extremity.
The foot is also subjected to large magnitudes of ro-
tational forces—or moments—during weightbearing
activities. It must be able to handle the moments that
result from ground-reactive force in such a way that
the joints of the foot and lower extremity accelerate
and decelerate smoothly and efficiently during the
activity being performed. Because the foot, lower ex-
tremity, and rest of the body must all function in con-
cert during weightbearing activities, the lower ex-
tremity and body will not function normally if the
foot does not also function normally. Furthermore,
the foot must remain functional for the lifetime of
the individual, or pain or disability will result.
The Normal Foot
Nearly 3 decades ago, Root et al
1
published their "Bio-
physical Criteria for Normalcy," which listed eight cri-
* Assistant Clinical Professor, Department of Podiatric Bio-
mechanics, California College of Podiatric Medicine. Mailing
address: 2626 N St, Sacramento, CA 95816.
teria representing the ideal structure for the foot and
lower extremity. For many years, podiatric physicians
have been taught that the structure of an individual's
foot and lower extremity must meet all eight of these
criteria in order to be considered "normal."
Unfortunately, very few individuals have feet and
lower extremities that can be considered "normal"
under the relatively strict definition proposed by Root
et al.
1
As the criteria for normalcy established by Root
et al are too restrictive to be practical for this discus-
sion, the author will define the normal foot and lower
extremity as those that function normally during gait,
have no history of significant trauma or surgery, and
currently have no pain or significant deformity.
Normal and Abnormal Function of the
Foot During Standing
During bipedal standing, each foot supports approxi-
mately one-half of the individual's body weight. In
order for the body to maintain balance in bipedal
standing, a plumb line dropped from a person's cen-
ter of mass must fall within an imaginary area that
lies under or between the feet. The individual uses
the muscles of the lower extremity to maintain bal-
ance by applying varying magnitudes of force on the
ground with the plantar aspect of the foot to adjust
for any tendencies for the center of mass to migrate
outside the balance area between the two feet.
25
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Journal of the American Podiatric Medical Association

In a normal foot during bipedal standing, the gas-
trocnemius muscle is intermittently active to main-
tain the center of mass somewhat anterior to the
ankle joint axis (Fig. 1). This muscle exerts a plan-
tarflexion moment across the ankle joint axis to
counteract the dorsiflexion moment from the center
of mass being positioned anterior to the ankle joint
axis. Individuals with normal feet will also intermit-
tently contract the other extrinsic muscles of the foot
in order to maintain their center of mass in the bal-
ance area between their feet.
5
'
6
A foot that functions the most normally during
walking does not exhibit a neutral position of the
subtalar joint during bipedal standing. In 15 years of
clinical observations, the author has noted that feet
that are in the subtalar joint neutral position during
standing are likely to exhibit symptoms or be associ-
ated with clinical findings indicating the presence of
an abnormal supination moment acting across the
subtalar joint axis. The author also considers feet
that have the subtalar joint either in or within 1° to 2°
of the maximally pronated position during standing
to be abnormal owing to the lack of available prona-
Figure 1. Normal relaxed bipedal stance. The center
of mass of the body is positioned anterior to the ankle
joint axis, which causes a dorsiflexion moment across
the ankle joint axis. In order for the body to maintain
equilibrium across the ankle joint axis and stay bal-
anced while in this position, the gastrocnemius mus-
cle must contract to exert a plantarflexion moment
across the ankle joint axis.
tion range of motion for shock absorption. The feet
that the author has observed to function the most
normally during gait have subtalar joints that are ap-
proximately midway between the maximally pronat-
ed and neutral positions.
In order for the subtalar joint to be maintained ap-
proximately midway between the neutral position
and the maximally pronated position, there must be a
balance of pronation and supination moments acting
across the subtalar joint axis. Whenever there are
equal magnitudes of pronation and supination mo-
ments acting across the subtalar joint axis during
standing, rotational equilibrium of the subtalar joint
exists.
7
Rotational equilibrium occurs across an axis
of rotation only when the sums of the moments act-
ing in both directions of rotation are exactly equal to
each other.
8
The most common example of rotational
equilibrium in the subtalar joint is the relaxed cal-
caneal stance position, in which the subtalar joint is
in a static position, with no rotational velocity.
7
The author has clinically measured the location of
the subtalar joint axis in relation to the plantar aspect
of the foot in well over a thousand normal and abnor-
mal feet. It is apparent from these observations that
the position of the subtalar joint axis in relation to the
plantar weightbearing surface of the foot is of prime
importance in determining the balance of supination
and pronation moments acting across the subtalar
joint in standing and other weightbearing activities.
7
' ^
During relaxed bipedal standing, the subtalar joint
axis of a normal foot exits through the posterosupero-
lateral aspect of the calcaneus posteriorly and lies
approximately over the first metatarsal head area an-
teriorly (Fig. 2). With the subtalar joint axis in this
positional relationship to the plantar aspect of the
foot, the ground-reactive force acting on the medial
calcaneal tubercle causes a supination moment
across the subtalar joint axis, and the ground-reac-
tive force acting on the lateral midfoot and lateral
metatarsal heads causes a pronation moment across
the subtalar joint axis. In addition, muscular tension
and ligamentous tension on the bones of the foot
cause supination and pronation moments across the
subtalar joint. A normal foot will have a balance of
these pronation and supination moments acting
across the subtalar joint axis during standing so that
the subtalar joint is resting approximately midway
between the neutral and the maximally pronated po-
sition of the subtalar joint axis.
7
'
913
Any number of structural deformities can cause
medial deviation of the subtalar joint axis; in medial
deviation, the subtalar joint axis is medially translat-
ed or internally rotated compared with the normal
subtalar joint axis location (Fig. 2). As a result, the
Volume 90 • Number 1 • January 2000
31

Medially Deviated
STJ Axis
Normal STJ
Axis Location
Laterally Deviated
STJ Axis
Figure 2. When the subtalar joint (STJ) axis is in its normal position (center), it passes through the posterolateral
aspect of the calcaneus and anteriorly over the first metatarsal head. When it is medially deviated (left), there are
excessive pronation moments acting across it. When it is laterally deviated (right), there are excessive supination
moments acting across it. The position of the STJ axis in relation to the plantar aspect of the foot is an important
determinant of the function of the foot during standing, walking, and other weightbearing activities.
pronation moments are increased and the supination
moments are decreased by the action of ground-reac-
tive force on the plantar structures of the foot and by
the actions of ligamentous and muscular tension on
the bones of the foot. A foot with a medially deviated
subtalar joint axis is likely to be maximally pronated
at the subtalar joint during both standing and other
weightbearing activities owing to the increase in
pronation moments and the decrease in supination
moments acting across the subtalar joint axis.
7
-
913
Lateral deviation of the subtalar joint axis is
caused by structural deformities in which the subtalar
joint axis is laterally translated or externally rotated
compared with the normal subtalar joint axis loca-
tion (Fig. 2). In a foot with a laterally deviated subta-
lar joint axis, the pronation moments are decreased
and the supination moments are increased; this re-
sults in a net increase in supination moment.
7
-
913
These subtle but measurable changes in spatial lo-
cation of the subtalar joint axis in relation to the
plantar aspect of the foot cause most foot and lower-
extremity biomechanical pathology seen in cases of
abnormal feet.
7
-
913
Positional changes of as little as 2
mm in the subtalar joint axis location in relation to
the plantar aspect of the foot may alter the balance
of moments acting across the subtalar joint axis so
significantly that a normal foot may begin to function
abnormally and develop biomechanical pathology.
Normal and Abnormal Function of the
Foot During Walking
During walking, the foot functions as the base of an
inverted pendulum where the body's center of mass
is transferred from posterior to anterior over the sta-
tionary foot.
2
This process is repeated for the con-
tralateral foot and then repeated over and over again
during gait. Because the most energy-efficient method
of moving a body from one point to another is to
move its center of mass along as straight a line as
possible with as constant a velocity as possible, the
foot must allow for this smooth transfer of the center
of mass over the stationary foot. In this way, energy
is conserved, and the work associated with walking
is kept to a minimum.
8
In addition to facilitating the relatively smooth
transfer of the center of mass from one point to an-
other, the foot also acts as a terrain adapter, a shock
absorber, and a rigid lever during walking. Initially,
during the contact and early midstance phases, the
subtalar joint pronates. The increased range of mo-
tion of the midtarsal and intertarsal joints occurring
32
Journal of the American Podiatric Medical Association

with subtalar joint pronation allows the foot to adapt
to uneven terrain.
r>
In addition, subtalar joint prona-
tion allows for shock absorption. The linearly direct-
ed momentum of the body striking the ground at heel
strike is converted into an angular momentum (ie, in-
ternal rotation) of the whole lower extremity. The
ability of subtalar joint pronation to act as a "momen-
tum converter" during the contact phase of walking
is critical in dissipating the shock of impact on the
structural components of the body. Inadequate sub-
talar joint pronation during the contact phase com-
monly leads to impact-related injury or dysfunction.
13
During the late midstance phase of walking in the
normal foot, as the center of mass is now moving
ahead of the stationary foot, the tibia, femur, and
pelvis should all be externally rotating in relation to
the ground; this allows the contralateral limb to ad-
vance smoothly ahead of the stance phase limb. In
order for the necessary external rotation of the lower
extremity to occur during late midstance, either the
foot must slide in an external rotation direction in
relation to the ground or the subtalar joint must
supinate.
5
'
14
~
16
Either inadequate supination motion or
pronation motion of the subtalar joint during late
midstance will lead to the common gait finding of ab-
ductory twist at the instant of heel lift. In addition,
the abnormal gait finding of late midstance pronation
of the subtalar joint is probably responsible for many
biomechanically induced pathologies such as plantar
fasciitis, intermetatarsal neuroma, hallux limitus, hal-
lux valgus, sesamoiditis, and low-back pain.
13
After heel lift in a normal foot, the subtalar joint
should be supinating because of the subtalar joint su-
pination moment generated by contraction of the
gastrocnemius and soleus muscles.
5
-
14
>
16
Supination
motion of the subtalar joint causes the talar head to
rotate externally and translate in a superolateral di-
rection in relation to the anterior calcaneus; this, in
turn, results in the talonavicular joint being posi-
tioned more superiorly and less medially in relation
to the calcaneocuboid joint. The more vertical align-
ment of the talonavicular joint to the calcaneocuboid
joint, which results directly from subtalar joint supi-
nation, gives the foot a greater structural potential to
resist the powerful dorsiflexion moments that occur
across the midtarsal joint as a result of ground-reac-
tive force acting on the plantar metatarsal heads dur-
ing propulsion. Therefore, the foot becomes a more
rigid and efficient lever as a direct result of positional
changes of the midtarsal joint caused by closed-ki-
netic-chain subtalar joint supination.
5
'
16
>
17
If subtalar joint supination does not occur during
propulsion, the extrinsic and intrinsic muscles of the
foot must contract more forcefully to prevent the
forefoot from excessively dorsiflexing on the rear-
foot. If the midtarsal or intertarsal joints are unstable
owing to subtalar joint pronation, propulsion by the
lower extremity will be relatively inefficient, as the
foot may be unable to generate sustained plantar
force on the ground without experiencing wasted
motion at these joints.
5
Detailed kinematic and kinetic analysis of the
human body during walking reveals an amazing se-
ries of carefully orchestrated events that allows an in-
dividual to move efficiently from one point to another
with a minimum of energy. Of utmost importance is
the proper timing and appropriate movements of the
foot in relation to the ground and the lower extremi-
ty. A thorough analysis of walking biomechanics
clearly demonstrates that without normal foot func-
tion, the rest of the lower extremity and body cannot
possibly function normally during walking. There-
fore, those health practitioners with the greatest
knowledge of the intricate biomechanics of the foot
and lower extremity will be the most successful in
treating the multitude of mechanically related foot
and lower-extremity pathologies that can occur.
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