Position of the Subtalar Joint Axis
and Resistance of the Rearfoot to
Supination
Craig Payne, DipPod, MPH*
Shannon Munteanu, BPod(Hons)*
Kathryn Miller, BPod*
Determination of the position of the subtalar joint axis is being more
widely used clinically to facilitate the prescription of foot orthoses and
the understanding of foot function, but clinical determination of the axis
has not been widely investigated. The aim of this study was to determine the relationship between clinical determination of the subtalar joint
axis and the amount of force needed to supinate the foot. The transverse plane position of the subtalar joint axis was determined in 47 subjects. The sagittal plane orientation of the subtalar joint axis was determined using the relative amounts of forefoot adduction and abduction
obtained when the rearfoot was supinated and pronated. The amount of
force needed to supinate the foot was measured using a device designed to measure resistance to supination. The only two parameters
that were correlated to supination resistance of the rearfoot were body
weight (r = 0.52) and the perpendicular distance from the fifth metatarsal head to the subtalar joint axis (r = 0.59). The model on which determination of the subtalar joint axis is based may not be valid, but it might
help determine how much force is needed to supinate a foot using foot
orthoses. (J Am Podiatr Med Assoc 93(2): 131-135, 2003)
The rearfoot complex of the foot has been the subject of a great deal of biomechanical research because it is widely believed to have a significant effect
on foot function and its dysfunction is related to a
range of lower-limb pathologic conditions. The axis
of rotation of the subtalar joint has received a considerable amount of attention.1-9 Traditionally, the
axis of rotation of the subtalar joint was modeled as
a hinge joint, and it has been described as having an
orientation to all three body planes. The most commonly recognized model of the axis is oriented 16°
from the sagittal plane and 42° from the transverse
plane. However, these values represent the mean
*Department of Podiatry, School of Human Biosciences,
La Trobe University, Victoria, Australia.
Corresponding author: Craig Payne, DipPod, MPH, Department of Podiatry, School of Human Biosciences, La Trobe
University, Victoria 3086, Australia.
found in research on cadavers,1, 4 and there is great
variation from individual to individual. It is assumed
that the axis can vary in its angulation to any of the
body planes and can be translated in any of the body
planes. For example, in the transverse plane, the subtalar joint axis could be more medially or laterally
deviated from the population mean relative to the
sagittal plane and more medially or laterally translated relative to the population mean in the transverse
plane (Fig. 1). This variability is thought to have clinical implications.
Variations assumed to occur in this hinge model
of the subtalar joint axis have been thought to be responsible for different amounts of calcaneal inversioneversion relative to leg internal-external rotation10-13
and different locations of injury14 and have been related to knee loads in cyclists.15 Variations in the orientation of the assumed subtalar joint axis have been
Journal of the American Podiatric Medical Association • Vol 93 • No 2 • March/April 2003
131
A
B
C
Figure 1. Assumed possible variations in the subtalar joint axis (STJA) in the transverse plane. A, A normal foot;
B, a foot with a medially located axis; C, a foot with a laterally located axis. The pronation moments generated by
ground reaction forces (GRFs) are greater in the foot with a medially deviated STJA (B), and the supination moments generated by GRFs are greater in the foot with a laterally deviated STJA (C). (From Kirby KA: Rotational
equilibrium across the subtalar joint axis. JAPMA 79: 1, 1989. Reprinted by permission.)
theoretically linked to different aspects of prescribing foot orthoses.16, 17
Although variations in subtalar joint axis position
have implications for foot function, lower-limb injury, and prescription of foot orthoses, the more sophisticated laboratory techniques for determining this
position3, 18 are not suitable for routine clinical use.
Kirby19 described a nonweightbearing method for
clinically determining the transverse plane position
of the subtalar joint axis. Phillips and Lidtke5 developed Kirby’s method further to give a three-dimensional position of the axis. However, given the complexity of the kinematics of the rearfoot complex and
differences between open- and closed-chain function,
it is not clear whether the techniques described are
actually measuring the position of the subtalar joint
axis or a rearfoot complex axis that is a composite of
the talocalcaneal, talocrural, calcaneocuboid, and
talonavicular joints. The morphologic features of the
subtalar joint have a large effect on the directions of
these motions, but during clinical determination of
the axis, significant movement is occurring at the
talonavicular joint, which would also affect the location of the assumed axis position. It may be more appropriate to consider clinical determination of the
axis as being the location of a nonweightbearing rearfoot complex axis,2 but the axis is widely referred to
132
in the literature as the subtalar joint axis, and so this
term will be used here.
Kirby16 proposed use of the medial heel skive to increase the supinatory moment from foot orthoses
when the subtalar joint axis has a more medial location. Similarly, an assumed increased magnitude of
supination moment can also be obtained with the inverted technique20 or DC wedge type21 of foot orthosis. These types of devices are thought to apply a
greater amount of force in the decreased surface area
of the foot that is medial to the joint axis when the
axis is in a more medial location (Fig. 1B). A problem
of the inverted device is that its exact prescription
seems to be based on trial and error.22 If the transverse plane position of the subtalar joint axis can be
related to the amount of force needed to supinate the
rearfoot, more accurate prescription of foot orthoses
and, therefore, better prognoses may be achieved.
The aim of this project was to determine the relationship between the position of the subtalar joint axis
and the amount of force needed to supinate the foot.
Methods
Subjects were recruited from the undergraduate student population at La Trobe University, Victoria, Australia. Individuals with a history of lower-limb trauma
March/April 2003 • Vol 93 • No 2 • Journal of the American Podiatric Medical Association
or surgery that interfered with motion of the rearfoot
were excluded. Age, gender, and body weight were
recorded for each subject.
The transverse plane position of the subtalar joint
axis was determined by two experienced clinicians
(C.P. and S.M.) independently using the method originally described by Kirby.19 The line representing this
axis was marked on the plantar surface of the foot
with a water-based ink pen so that the line could be
transferred to a footprint. Subjects then stepped onto
an inked footprint system so that the footprints could
be recorded. The inked mat was covered with a clear
sheet of plastic so that the inked line on the plantar
surface of the foot could be recorded. A pin was used
to pierce the overhead transparency so that the orientation of the axis could be transferred to the inked
footprint. To obtain a measurement for the axis position, a bisection of the midline of the foot was drawn
on the footprint by bisecting the center of the posterior aspect of the calcaneus and the center of the second toe. Three measurements were taken to describe
the axis position (Fig. 2). The angle that the axis
made with the foot bisection was measured. The distance between the axis and the lateral aspect of the
fifth metatarsal head was measured along a line
drawn perpendicular to the axis. The distance between the posterior aspect of the calcaneus and the
point where the axis bisected the midline was also
measured. The distance from the fifth metatarsal
head and the distance from the posterior aspect of
the foot were normalized by dividing by the foot
length. Foot length was measured from the footprint.
The means of the two clinicians’ measurements were
used for the calculations. Intraclass correlation coefficients (ICCs) were used to determine the interobserver reliability of the measurements of the axis position
taken by the two clinicians. As a check of reliability
of the measurements taken from the footprints, another experienced clinician (K.M.) repeated the measurements on 15 of the footprints.
To measure the amount of force needed to supinate the rearfoot, a supination resistance device (Fig.
3) was constructed based on the manual supination
resistance test described by Kirby and Green.23 This
device allowed the subjects to stand in their normal
angle and base of gait. A 25-mm-wide piece of nonstretchable woven fabric (similar to seat belt material)
was fixed to the base of the platform lateral to the foot
in the region of the calcaneocuboid joint. It passed
under the foot medial to the talonavicular joint and
then proximally to be attached to a pulley system
and a force gauge (Mecmesin, Horsham, England)
that measured the maximum force. The pulley system was used to apply a force to just overcome the
A
B
C
D
E
Figure 2. Example of a footprint showing the long
axis of a foot (line A) and the subtalar joint axis (line
B). Three measurements were taken for analysis: C
is the perpendicular distance from the subtalar joint
axis to the lateral aspect of the fifth metatarsal head;
D, the angle the subtalar joint axis makes with the
long axis of the foot; and E, the distance from the
point where the subtalar axis bisects the long axis of
the foot to the center of the heel.
inertia needed to invert the calcaneus as the investigator observed a bisection that was placed on the
posterior aspect of the calcaneus. Participants were
instructed to stand relaxed and not to assist the in-
Figure 3. The supination resistance device used to measure the amount of force needed to supinate the foot.
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133
vestigator. If any assistance was detected (eg, muscle
contraction), the test was repeated. The test was repeated ten times for each subject, with the mean
used for analysis. For 13 subjects, the supination resistance test was repeated 1 week later to determine
the repeatability of the device.
The Pearson correlation coefficient (r) was used
to determine the relationship between the amount of
force needed to supinate the foot and body weight,
the transverse plane position of the subtalar joint
axis, and the sagittal plane position of the subtalar
joint axis. Type (2,1) ICCs were used to determine
the reliability of the measurements.
Results
A total of 47 subjects (25 women and 22 men) were
recruited for the study (mean ± SD age, 22.3 ± 5.3
years; weight, 69.2 ± 16.9 kg). The mean ± SD transverse plane position of the subtalar joint axis was
offset 8.9° ± 4.2° from the midline of the foot, 67 ± 17
mm between the fifth metatarsal head and the joint
axis, and 77 ± 48 mm from the posterior aspect of the
heel and the point where the subtalar joint axis bisects the line representing the midline of the foot.
Because both of the latter parameters would be affected by differences in foot size, they were normalized by dividing by foot length (determined from the
ink footprint): a value of 0.434 ± 0.178 was obtained
for the analysis of the distance from the fifth metatarsal, and a value of 0.473 ± 0.407 was obtained for
the distance from the posterior aspect of the calcaneus to the bisection. The mean ± SD of the ten trials
for supination resistance was 167 ± 71.6 N (range, 66
to 438 N).
The ICCs (95% confidence intervals [CIs]) for the
comparison between the two clinicians using the three
measurements of the subtalar joint axis were 0.86
(0.67–0.97) for the angle of the axis, 0.81 (0.69–0.92)
for the intersection of the axis and long axis of the
foot, and 0.79 (0.65–0.86) for the distance to the fifth
metatarsal. The measurements taken on 15 of the footprints were repeated by a different clinician, and the
ICCs (95% CIs) of these measurements were 0.98
(0.92–0.99), 0.98 (0.91–0.99), and 0.97 (0.89–0.99), respectively. The repeated measurements 1 week later
on 13 subjects for supination resistance had an ICC of
0.95 (95% CI, 0.88–0.98).
The correlations between the axis measurements
and supination resistance were as follows: axis
angle, r = 0.118 (P = .49); distance from the posterior
heel, r = –0.29 (P = .59); distance from the fifth metatarsal head, r = 0.59 (P = .02); and body weight,
r = 0.52 (P = .001).
134
Discussion
The results of this study show that of those variables
measured, body weight and the perpendicular distance between the measured subtalar joint axis and
the lateral aspect of the fifth metatarsal head are associated with greater force needed to supinate the
foot in static stance. It was not surprising that body
weight was a factor in the amount of force needed to
supinate the foot, as the greater the body weight, the
more force assumed to be needed. The greater force
needed to supinate the foot when the subtalar joint
axis has more of the foot lateral to it (Fig. 1B) is likely to result from a greater lever arm and moment
causing the pronatory force.24 When a foot requires a
higher magnitude of force to supinate it, this might indicate that a larger posterior tibial muscle force or larger supination moment from a foot orthosis is required.
Kirby19 used a simple hinge model to describe the
significance of differences in orientation of the subtalar joint axis for foot function. The clinical method
described in the article by Kirby was used to determine the subtalar joint axis in the present study. This
model is a nonweightbearing open-chain model that
has not been shown to be representative of the weightbearing closed-chain situation in dynamic function.
In the nonweightbearing position, calcaneal motion
on the talus clearly represents subtalar joint motion,
but in the weightbearing situation the motions of the
ankle, subtalar, and midtarsal joints have a complex
interrelationship. Dynamically, the rearfoot functions
in a more complicated way than a simple hinge
model,3, 8, 9, 18 and the axis moves through different
positions during the stance phase of gait. To the authors’ knowledge, the relationship of the clinical determination of the subtalar joint axis based on the
simple hinge model to the kinematic function of the
rearfoot complex has not yet been investigated to
validate the clinical methods described by Kirby19
and used in the present study. Motion about the subtalar joint is better described as a bundle of instantaneous axes.2, 8, 9
This research has potential clinical implications
despite the validity issues regarding the model on
which the subtalar joint axis is based. Kirby16 suggests the use of foot orthoses that provide greater
supinatory moment when the assumed axis is located more medially. This is assumed to be provided by
orthoses with a medial heel skive16 or a DC wedge21
or an inverted device.20, 22 These suggestions, based
on clinical experience, are consistent with the research presented here in that subjects with a more
medially located subtalar joint axis needed more
force to supinate the foot in static stance using the
March/April 2003 • Vol 93 • No 2 • Journal of the American Podiatric Medical Association
supination resistance device. Additional research
generating kinematic data is still needed to further
validate these concepts, but they may be clinically
applicable when making decisions about prescribing
foot orthoses.
Conclusion
This study found an experimental correlation between the position of a nonweightbearing subtalar
joint axis as described by Kirby19 and the amount of
force needed to supinate the foot in static stance.
The technique of clinically determining the axis position can be useful in helping clinicians decide how
much supinatory force may be needed from foot orthoses.
10.
11.
12.
13.
14.
15.
16.
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