Biomechanical dysfunction is most commonly caused by two factors:

1. The alignment of the tibia - generally the tibia exhibit a 4-6° angle to the ground (slight bow leg) allowing the foot to strike the ground laterally together with ground reaction forces.
2. Biomechanical dysfunction factors - osseous conditions (anomalies) can affect the body by changing our true gait or causing our biomechanical structure to wear unevenly.

Excess pronation is the symptomatic compensatory factor to these anomalies.

Excess Pronation

Generally 2 main causes:

1. Angle of the tibia combined with a requirement of the foot to make contact with the ground in gait;
2. Factors which affect the foot biomechanically such as internal tibial rotation, internal tibial torsion, medial biomechanical forces on the talonavicular area causing subtalar joint pronation and midtarsal join pronation.

Features that usually accompany subtalar joint and midtarsal joint pronation:

• internal tibial rotation
• calcaneal eversion
• lowering and elongation of the arch
• excess lower limb strain medially
• medial knee pain
• displacement of the talus on the calcaneus medially
• increased weight bearing over the 1st MPJ
• excess upper limb compensation laterally

How Does Tibial Varum Cause Excess Pronation?

Our feet were designed to be able to traverse a range of different types of terrain, however in our modern society man has covered the ground with cement and hard surfaces.

The normal lower limb compensates when approaching the ground by pronating excessively at the subtalar joint in an effort to gain contact with the ground.

When we walk on soft ground or sand the ground compacts under the arch and creates a natural arch support.  Excess pronation is a compensation caused by our normal tibial varum angle when the foot strikes the ground and the hard ground does not give way to accommodate.  Many common biomechanical conditions can occur throughout our entire body.

The Tibial Varum Element and Ground Reaction Forces

The average lower limb on hard surfaces gains ground contact by collapsing inwards (everting).

Notice in the picture above the foot sinks into the soft sand on the lateral side (outside) and the sand compacts under the foot on the medial side (inside).  It is this action that creates the correct surface for the foot to walk on in nature, when we walk on hard surfaces we need to "change the ground to suit our feet".  ICB Orthotics are designed to do this by having intrinsic correct angles built into their design to prevent excess pronation - just like mother nature intended!

Ground Reaction Force Element

Isaac Newton said "For every action there is an opposite and equal reaction".  Newton's 3rd law indicates that downward stress will naturally receive an opposite and equal response OR "accommodative reaction" by the ground as when one walks on the sand at the beach.

Further sports stress studies show that when Newton's law W=mg is applied and forward motion is increased, then the biomechanical downward stress and ground reaction forces multiply dramatically with increased compensatory outcomes to the body.

Note:  when excessive downward force is applied, if the ground reaction force is more than that which is applied by the body mechanics, then the body must give way and collapse - just like the photo of finger above.  This same principle can be applied to the lower limb with the resultant outcome being excessive pronation, ie. more pronation than the body allows in the foot structure (approximately 4°).

The tibial varum angle together with the ground reaction force is the major element of compensatory "excess pronation" - on unnatural hard, flat surfaces the body seeks to control the correct biomechanical alignment and changes our structure to accommodate to the ground.

Biomechanical Deformity Factors

Biomechanical factors in the body can adversely affect our structure and the way that we compensate to the environment.  Factors such as:

1. Internal tibial torsion - twisted bone from the head of the tibia to the malleolus can affect knees, back and upper structure by compensatory elements.
2. Internal tibial rotation - rotation of the bone will have a direct relationship to wear and tear at the knee joint and when combined with soft tissue elements affecting the femur and hip biomechanical dysfunction its resultant effects will be obvious.
3. Medial biomechanical forces - on the talonavicular area causing subtalar joint pronation and midtarsal joint pronation.

Elements That Effect the Gait Cycle

The skeletal structure is interdependent on the soft tissues of the body (ligaments/muscles/tendons) and these can have a contributing biomechanical effect.

Tibialis anterior is the primary foot and ankle dorsiflexor and in the contact phase of gait decelerates the forefoot.  When this is under tractional stress due to compensatory excess pronation in an attempt to gain ground contact, tendonitis can be experienced and is commonly referred to as anterior shin splints and has a definite correlation to anterior compartment syndrome.

Studies carried out by ICB Gait & Posture Clinics have established anecdotal evidence to support that there is a direct relationship to pronation - medial and anterior shin splints and lateral shin splints with supination (see International College of Biomechanics Courses).

Tibialis posterior muscle inverts and plantarflexes the foot and is most active in midstance during gait as it prevents the foot from everting past the neutral position.

Tibial Varum Element and its Effect on Excess Pronation

During the gait cycle the foot strikes laterally and the foot takes on an inverted position at heel strike.  A common fallacy is that it is not good for shoes to be worn on the later (outside) of the heel on shoes... this is actually natural, however excessive wear is unnatural (What to Look For When Buying New Shoes).

The heel strike phase of gait is commonly represented by:

• supination at the subtalar joint
• inversion of the calcaneus
• external tibial rotation
• abduction and dorsiflexion of the talus over the calcaneus

Pronation occurs as the tibia internally rotates and adducts the talus over the calcaneus.  As this occurs, subtalar joint pronation occurs.

The transverse distance increases and the vertical distance decreases because the calcaneus everts and allows the talus to plantarflex, compensatory action takes place biomechanically and the knees rotate inward, there is an anterior shift to the pelvis and an increased lordotic (sway back) to the lower back with resultant kyphotic compensation to the thoracic region and at the same time an anterior shift of the head to maintain equilibrium of the skeletal frame.  The foot elongates and the arch is lowered putting stress on the fascias of the foot (see Conditions - Plantarfasciitis).

Excess pronation as explained above causes loss of transverse arch together with rotation of the phalanges and an unlocking of the arch structure with resultant bone instability and increased tractional force on the soft tissues of the foot.

Content & Images © 2006 Footsteps Orthotics Pty Limited