An Introduction To Biomechanics
The foot is a highly complex system capable of producing balance and propulsion of
the body by these two opposing mechanisms. The foot is the foundation for the
whole body and its actions will effect other parts of the body, and indeed, its
function will be affected by numerous forces acting upon it. The two principle
components of foot function are the motions of pronation and supination:
Mobile adaptor (Pronation).
During this function it is capable of absorbing shock and adapting to any terrain: flat,
angled, rough, soft etc. whilst maintaining the equilibrium of the body.
Rigid lever (Supination).
During this function it is locked into a structure capable of stabilising, lifting and
propelling the body.
Frontal Plane Motion: Inversion and eversion with axis in sagittal and
Sagittal Plane Motion: Plantar and dorsiflexion with axis in frontal and
Transverse Plane Motion: Abduction and adduction with axis in frontal and sagittal
Ankle, subtalar, midtarsal, 1st & 5th rays have triplane axis with motion
perpendicular to the axis. Axis of motion is determined by the shape of the articular
surface and the position of the entire joint (axis of motion for the midtarsal joint is
dependent upon the relative position of the subtalar joint.)
A change in structure or function of one part of the foot to neutralise or
accommodate the effect of an abnormal force, resulting from a deviation in structure
or function of another part.
When the foot compensates for a deformity or abnormal force then the
compensation will occur in the plane that can produce the most motion in the same
plane as the deformity or abnormal force. This is the dominant plane for that
For example, when a sagittal plane deformity exists (Ankle Equinus deformity) then
to compensate the most effectively, the body needs to compensate within the
sagittal plane. This can be achieved by a combination of actions: flexion of the
knee, early heel lift, wearing a high heeled shoe, or by pronation of the subtalar joint
to unlock the midtarsal joint to allow motion around the oblique axis of the midtarsal
joint. All of these actions are compensation in the sagittal plane.
The Gait Cycle
HS FFL HL TO
0% 27% 67% 100%
Stance Phase of Gait Swing Phase of Gait
Contact 27% Midstance 40% Propulsive
0% 10 20 30 40 50 60 70 80 90 100
% of Gait Cycle
HS = Heel Strike FFL = Forefoot Loading
HL = Heel Lift TO = Toe Off
HSO = Heel Strike opposite foot.
Overview Of The Adaptive And Rigid Process
Of The Foot During The Gait Cycle.
At the contact phase of gait (heel strike) the motion of the foot is that of pronation.
This is seen by the eversion, abduction and dorsiflexion of the calcaneus. It is a
complex motion between the talus and the calcaneus (subtalar joint). Since the talus
is locked in the ankle mortise, the leg will internally rotate and can be seen by the
movement of the tibia backwards (best observed by the tibial malleolus). The result
of this pronation is the unlocking of the Midtarsal Joint and a very mobile foot. This
continues for approx 25% of the gait cycle at which point the foot must start to
become a rigid lever. The foot will then start to move in the direction of supination
(inversion, adduction and plantarflexion) but will remain in a pronated position.
The foot should reach its neutral position by 50% of the cycle (Midstance). This
continues to allow the midtarsal joint to become locked with all the metatarsals in
ground contact. The first ray will plantarflex to allow the hallux to provide the range
of motion required for propulsion. The plantarflexion of the first ray is vital to allow
the hallux to dorsiflex to 60. Failure of this mechanism will cause the hallux to
impinge and jam against the head of the first metatarsal - a condition known as
functional hallux limitus.
KEY LANDMARKS - During The Contact Phase
1. Subtalar joint pronation throughout. At end of the contact phase the foot
starts to supinate and continues throughout stance phase. Pronation only
normally occurs during contact phase (27% of cycle). Its function is to allow
the foot to adapt to forces from the ground (uneven ground etc). Pronation is
stopped mainly by the Posterior Tibialis muscle with Tibialis Anterior helping
to control the extent and rate of pronation.
2. The leg internally rotates during contact phase and externally rotates from
the end of the contact phase. This is linked to the motion of the subtalar joint
and can be seen by the movement of the tibia in a forwards direction (Tibial
malleolus becomes prominent from the front).
3. Metatarsus becomes fully loaded during contact period.
4. Vertical ground reaction forces peak for the first time by the end of contact.
Shared between the heel and metatarsal heads.
KEY LANDMARKS - During Midstance Phase.
1. Midstance commences at the end of the contact phase and shortly after toe
off of the opposite foot. It ends with heel lift and makes up the intermediate
40% of the cycle.
2. The subtalar joint continues to supinate and converts the foot into a rigid
lever. It moves out of a pronated position at the end of the contact phase and
into a supinated position before heel lift. The motion is a combined effect of
the calf muscles and leg rotation.
3. The leg externally rotates.
4. Vertical ground reaction forces decrease by up to 75% body weight it
increases again before heel lift. (The swinging of the leg is a main factor).
5. The opposite foot is in swing phase and should pass the weight bearing foot
before heel lift occurs.
KEY LANDMARKS - During Propulsive Phase.
1. The propulsive phase begins with heel lift and ends at toe off. It makes up
the final 33% of the gait cycle.
2. The subtalar joint supination continues to increase skeletal stability and
enhance the lever effects of the foot actions. The foot will pronate slightly just
prior to toe off. External leg rotation also continues.
3. Vertical ground reaction forces peak again by up to 25% of body weight upon
the metatarsals and toes.
4. Body weight is transferred from the lateral side of the foot to the medial side
and onto the opposite foot as it loads. The 5th metatarsal is free of weight at
5. The toes are generally not loaded during the other phases but may be active
during the toe off period.
Major Pathological Gait Defects.
Variance from the normal smooth locomotory function of gait can be associated with
a deformity in:
1. Osseous - congenital, developmental, metabolic,
2. Neurological - sensory, motor, spastic, paralytic.
3. Muscular, Soft Tissue - contractures, fibrosis, laxity, metabolic etc.
4. Functional - lack of coordination, neuromuscular.
Any gait where there is muscle paralysis or weakness (nerve involvement, muscle
pathology or a change in the osseous structure the muscle is associated with). The
person will attempt to bring the centre of gravity towards that muscle in stance
phase. A lack in muscle strength causes the person to use body weight to provide
the necessary force.
Gluteus Medius Lurch (Trendelenberg Gait)
Also seen in dislocated hip and muscular dystrophy. The trunk shifts over the side of
the weak muscle in stance phase to minimise the fall of the swing phase side of the
Gluteus Maximus Lurch (Hip Extensor)
The trunk lurches back on the stance phase side hyperextending.
Ankle Dorsiflexors (Drop foot Gait)
Produces a high knee lift to raise the foot clear of the ground.
Person tries to avoid pain associated with the ambulation. Often quick, short and
soft foot steps.
Spinal - proprioceptive pathways of the spine or brainstem are interrupted.
There is loss of position and motion sense. The person will walk with a
wide base of gait with foot slap at heel contact. Often watch feet as
Cerebella - coordinating functions of the cerebella are interfered with, so the
person tends to walk with a wide base of gait with an unsteady
irregular gait, even if watching feet.
Parkinsonian Gait (Festinating)
The trunk is bent forward, the legs and arms are stiff, with short shuffling steps
(chasing centre of gravity).
An unbalanced muscle action of certain muscle groups leads to deformity. Prime
example is “Scissor gait” - adduction and internal rotation of the hips with an
equinus of the feet and flexion of the knee.
Short Leg Gait.
Compensation may be combination of pronation of the long leg and supination of
the short leg. A pelvic drop and equinus may occur. With a pelvic drop the vertebral
column bends convexly to the short side and the shoulder drops on the long side
making the arm appear longer. If full joint extension is not possible then the limb
functions as a short limb with associated limp.
Leg Length Discrepancy Internal Leg Rotation
Long Leg Internal Leg
THE SUB TALAR JOINT AXIS
The commonly accepted angular values for the subtalar joint are 42 to the transverse
plane, 16 to the sagittal plane and 48 to frontal plane. It must be stressed that these are
average values and variances do occur. However, assuming that the axis is approximately
45 then every 1 of calcaneal motion (inversion/eversion) should equal 1 of transverse
leg rotation (internal & external leg rotation) in closed chain motion. Therefore, clinically a
high inclination angle of the axis (60) will produce more leg rotation than calcaneal motion
and, therefore, more postural problems. A low axis of motion will lead to more calcaneal
motion and more foot problems than postural ones.
Furthermore, the axis can be relatively abducted or adducted to the longitudinal axis of the
foot. The average position of the axis is from the posterior lateral aspect of the calcaneus
to the first metatarsal medial cuneiform joint. Therefore, a shift in the axis medially
(adducted) will allow forces across the forefoot and ground reaction force to produce a
strong pronatory force at the Subtalar joint. A shift laterally (abducted) will produce a
supinatory effect (Figure 1). Kirby (1) has defined non-weight bearing techniques to assess
the direction of the axis, but the DYNASTAT allows the assessment of the axis in the
weight bearing position.
Sketch showing the shift in subtalar axis - pronated
Interpretation of Results
When the axis of motion shows a medial shift, then the orthosis will require modification.
This can be achieved by medial and lateral additions to the positive cast to increase the
weight bearing area and increase the supinatory effect of the varus post. The supinatory
effect can also be increased by the medial heel skive technique or lateral calcaneal
tubercle addition. The principle of the Heel Skive technique is to remove Plaster of Paris
from the medial side of the positive to increase the amount of varus wedging, when the
plate material is pressed.
Lateral Calcaneal Tubercle Addition can be achieved when Plaster of Paris is added to the
lateral tubercle of the calacneus to create the same affect as the medial heel skive
technique. It is smoothed to blend with the cast and creates the necessary supinatory force
to control the Subtalar joint pronation.
Assessment of the inclination angulation of the calcaneus will provide information used in
the modification of the orthosis post to correct for the amount of “normal pronation”. When
there is a high inclination angle, more normal pronation is required and can be achieved
by increasing in the pronatory grind off on the post. Conversely, when there is a low
inclination angle, the pronatory grind off can be decreased. This inclination angle of the
calcaneus can be assessed by the DYNASTAT.
Sagittal Plane Scale showing 8 degrees inclination angle.
Common Foot Deformities
The following lists of conditions are not generally seen as single entities but as a
combination of pathologies.
Subtalar Varus (Calcaneal Varus)
An inverted position of the posterior aspect of the calcaneus relative to the lower third of
the tibia due to an incomplete derotation from its infantile position.
Note that the forefoot will be in the same alignment as the rearfoot, if there is no
forefoot to rearfoot deformity.
Measuring Rearfoot Varus with the DYNASTAT
Pronation of the subtalar joint to evert the calcaneus until the medial side of the rearfoot
and forefoot are in contact with the ground. The amount of pronation may be sufficient to
fully compensate or only partially compensate for the amount of subtalar varus.
Dorso medial bunion.
Lesion of 5th toe, Digiti Quiniti Varus (Tailor’s bunion).
Plantar callus under 2nd MTPJ but possible under 3rd & 4th MTPJ depending on extent of
Haglund’s deformity - Heel bump due to movement of heel against shoe.
In relaxed calcaneal stance position the calcaneus may appear:
a) inverted if limited subtalar joint motion,
b) vertical if subtalar varus only deformity or
c) everted if other pronatory deformity present.
Leg fatigue and nocturnal leg cramp.
Low back pain or fatigue.
Lateral ankle sprains.
Cast the foot in the subtalar joint neutral position and post the rear of the orthosis with the
degrees of subtalar varus. Allow for normal pronation by medial grind off of the post. The
degree of grind off should be related to the extent of sagittal plane motion of the
Posts of over 6 are uncommon and not well tolerated without modifications and/or
extensions to the cast or orthosis i.e. medial and lateral cast expansion.
When the rearfoot pathology is excessive, then other posting techniques will be required.
This can be achieved by the Kirby medial heel grind technique or by lateral calcaneal
An osseous deformity of the forefoot as a result of failure of the head and neck of the talus
to derotate from the infantile position. The plane of the lesser metatarsals is inverted to a
bisection of the posterior aspect of the calcaneus when the subtalar joint is in its neutral
position and the midtarsal joint is fully pronated.
Pronation occurs at the subtalar joint to allow the medial side of the forefoot to reach the
ground. This will cause the rearfoot to evert and unlock the midtarsal joint.
Pronation will occur during the stance phase of gait and often into the swing phase. The
extent of pronation is limited by the range of motion at the subtalar joint.
Measuring Forefoot Varus with the DYNASTAT
Hallux abducto valgus or hallux limitus/rigidus depending upon the forefoot angulation,
relative length of first metatarsal and shape of the metatarsal head.
Overlapping toes especially 2nd.
Lesion 5th toe.
Plantar keratoma and callus under 2nd MTPJ but also possibly 3rd & 4th.
Genu valgum from internal leg rotation (knock Knees).
Calcaneal spurs, Heel pain syndrome, Plantar fascia pain.
Everted calcaneus with medial bulging of the talar head. Lateral cuboid break and
abducted forefoot relative to the rearfoot with midtarsal joint involvement.
Generalised forms of metatarsalgia.
Chronic low back pain.
Inferior calcaneal bursitis or plantar fasciitis.
Cast the foot with the subtalar joint in its neutral position and the midtarsal joint fully
pronated. Make an orthosis to provide a post to support the medial side of the foot and
produce a heel bisection that is vertical.
This can be achieved intrinsically or extrinsically. Intrinsic posting requires the addition of
plaster of Paris to the medial side of the positive cast and smoothed and reduced laterally
to create the medial wedge. When the plate is moulded the post will be created within the
plate. The traditional extrinsic post is achieved by adding the posting material to the plate
to the required angle.
Forefoot Valgus and Plantarflexed 1st Ray (rigid)
An osseous deformity of the forefoot in which the plane of the lesser metatarsals is everted
relative to the bisection of the posterior aspect of the calcaneus, when the subtalar joint is
in its neutral position and the midtarsal joint is fully pronated. The everted appearance of
the forefoot may be due to the plantarflexed position of the 1st ray (2 - 5 in same plane) or
the complete rotation of all metatarsals (2 - 5 everted).
The 1st ray will not dorsiflex to the level of the other metatarsals when the subtalar joint is
A plantarflexed 1st ray may be caused by:
1. Congenital torsional deformity of the head of the talus resulting in the everted forefoot.
2. Neurological conditions.
3. Congenital plantarflexion of the 1st ray.
4. Trauma or surgery.
Measuring Forefoot Valgus with the DYNASTAT
The parallel plane to the ground of the metatarsals is achieved by ankle joint dorsiflexion
and subtalar joint supination (to invert the forefoot).
Large keratoma under the 1st MTPJ and/ or 5th MTPJ due to supination.
Relaxed calcaneal stance position will show inverted calcaneal bisection with high arch
Chronic lateral ankle sprains.
Leg and thigh fatigue.
Lateral knee pain.
General shock absorption symptoms.
Neutral subtalar joint cast with midtarsal joint fully pronated to produce orthosis with
forefoot post on lateral side to prevent subtalar joint supination. This can also be achieved
intrinsically or extrinsically.
Plantarflexed 1st Ray (flexible)
A first metatarsal whose neutral position is below the plane of the other lesser metatarsals
and can be dorsiflexed to the same plane, by a force applied to its plantar surface.
This foot type is often confused with a forefoot valgus deformity because of its position off
the ground. It also masks other conditions of forefoot varus and subtalar varus by
plantarflexing to provide stability to the foot through a tripod stance (rearfoot, 5th MTPJ
and 1st MTPJ).
Compensation involves pronation of the subtalar joint to unlock the midtarsal joint and
allow the 1st ray to dorsiflex to the level of the lesser metatarsals. When there is a forefoot
varus deformity the midtarsal joint will also supinate to bring the plane of the lesser
metatarsals to the ground and dorsiflex the 1st ray.
With Subtalar Varus
Dorso medial bunion.
Lesion 5th toe.
Plantar callus under 2 MTPJ and or 3 & 4 MTPJ with possible keratoma.
Leg fatigue and nocturnal leg cramp.
Low back pain or fatigue.
Lateral ankle sprains.
With Forefoot Varus
Hallux abducto valgus / limitus.
Lesion 5th toe.
Overlapping toes commonly 2nd.
Plantar callus of 2 MTPJ and or 3 &4 MTPJ with deep keratoma.
Chronic low back pain.
Inferior calcaneal bursitis.
The main aim is to control the foot around its neutral position and establish a 1st MTPJ
against the ground.
A neutral cast of the foot to produce an orthosis with rearfoot at neutral and the forefoot
with either a forefoot bar post (2 - 5) or a forefoot varus post with 1st ray cut out.
Forefoot Valgus Post
This shows how the lateral border of the foot is brought into ground contact at the same
time as the medial border. The effects of the valgus deformity have been corrected.
A failure of the ankle joint to achieve a dorsiflexed position of at least 10 degrees past the
vertical when the subtalar joint is in neutral.
The subtalar joint pronates to allow the oblique axis of the midtarsal joint to produce
dorsiflexion within its range of motion. Often the midtarsal joint will be subluxed and an
early heel lift during gait may produce a bouncing gait.
These are dependent on the extent of compensation occurring in the foot. It is possible for
the compensation to occur during gait by an early heel lift seen by the bouncing type of
gait. It can also be compensated for by abduction of the feet or flexed or hyperextended
Abducted forefoot on rearfoot with bulging of the talar head.
Severe hallux subluxation and deformity.
Lesions on dorsum of toes.
Over and under lapping of toes.
Plantar callus with severe keratoma of 2nd MTPJ but possibly 3rd or 4th MTPJ.
Low inclination angle of the calcaneus.
Increase in posterior muscle bulk.
Severe postural symptoms.
Determine if the cause is neurological, traumatic or due to a restriction in motion from bone
block or short muscle.
Active physiotherapy to produce sustained dorsiflexion of the foot for at least 30 seconds
with the foot adducted.
Assess gait pattern and knee function. The heel of the opposite known weight bearing foot
should pass the weight bearing foot before heel lift. Compensation may occur by knee
flexion or early heel lift.
Orthosis may accommodate the deformity and can be successful as functional orthosis if
combined with physiotherapy.
Other treatments may be required:
Stretching - Tendon Achilles or Hamstrings.
Surgical lengthening of structures proximally or at Tendo Achilles.
Therapy Complications of Orthosis.
Complaint Causes Considerations
Discomfort at lateral heel 1. Seat too narrow 1. New plate with lateral heel expansion
or along border of plate 2. Restricted STJ motion on cast.
3. Rearfoot over posted 2. Remove rearfoot post or check
4. Valgus post required pronatory grind off.
at forefoot 3. Post forefoot.
5. Forefoot posted with 4. Reduce density or rigidity of post /
excessive varus post. plate.
6. Equinus deformity not 5. Recast with pronated Subtalar Joint.
controlled. 6. Remake device to “root plate” and
7. Poor grinding of plate ensure appropriate grinding.
along lateral edge.
Pain under metatarsals 1. Device too long or not 1. Shorten device.
finished (bevelled). 2. Reduce / remove forefoot post to
2. Forefoot incorrectly allow first metatarsal to plantarflex.
posted. 3. Apply forefoot extension of shock
Medial arch pain 1. Equinus deformity. 1. Raise heel and increase stretch
2. Internal rotation still programme.
excessive. 2. Change to less corrective devices.
3. Forefoot or rearfoot 3. Extend rearfoot post medially.
post incorrect. 4. Consider change in posting
4. No pronatory grind off. technique at rearfoot.
Pain in distal heel 1. Equinus deformity. As above.
2. Casts taken in Check casts have straight lateral border.
Knee pain 1. Check forefoot & 1. Change posts.
- Medial rearfoot post 2. Use Flexible device.
(excessive). 3. Check inclination angle of calcaneus
2. Genu recurvatum. and pronatory grind off.
- Lateral 1. Check rearfoot post - 1. Change rearfoot post to allow
limiting normal pronation. Use softer post material
pronation. or increase grind off.
2. Forefoot Valgus 2. Post forefoot.
- Posterior 1. Equinus deformity. 1. Reduce control or add heel lift.
2. Consider stretching programme.
- Anterior 1. Excess post or no 1. Change posts to correct angle, softer
normal pronation. post or increase grind off.
2. Equinus deformity. 2. Heel raise.
3. Supinated cast. 3. Recast.
4. Consider Chondromalasia patella
and treat accordingly.
Tendon Achilles pain 1. Supinated device. 1. Remake and check neutral STJ
2. Rearfoot over posted. position.
3. Equinus deformity 2. Heel raise.
Plantar heel pain 1. Supinated cast. 1. Remake device and check posting
2. Equinus deformity. and flexibility.
3. Forefoot post incorrect. 2. Raise heel.
Pain around 1st MTPJ 1. Reduced ROM at joint - 1. Check length of device and posting.
Functional hallux 2. 1st ray cut out or rocker bar
2. Tibial sesamoid
General leg muscle pain 1. Anterior shin splints 1. Check post angle for correctness.
2. Posterior shin splints 2. May require forefoot post.
3. Post modification techniques may be
Secondary back pain Over control or no shock Reduce posts or change to flexible
Shoe conditions 1. Heel slippage. 1. Change footwear to deeper heel
2. Heel counter break seat.
down. 2. Medial wear consider neuromotor
defect or internal rotation.
3. Lateral wear - Over posted rearfoot
or Forefoot Valgus deformity