BIOMECHANICS
Biomechanics: The study of the anatomical principles of movement.
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In order to prescribe custom orthotics, a Biomechanical Assessment of the individual must be performed. After discussing the patient's symptoms, the following factors are assessed:
1. FOOT ANATOMY
The musculoskeleal podiatrist has a thorough understanding of the anatomy of motions of the lower limbs. In each foot alone there are 126 muscles and ligaments, 33 joints and 26 bones (plus 2 sesamoid bones under the 1st MPJ). The foot has been designed to withstand great stresses and yet contains some of the smallest and most delicate bones in the human body. Simply put, there are 4 basic arch types, the percentage of the population with each being:
5% Flat Foot - typically no arch contour at all, characterised by well developed muscle in the plantar area of the foot structure.
85% Pronated Foot - minimum arch shape, arch collapses due to ligamental laxity and the effects of gravity.
1% Normal Foot - appearance of the perfect arch contour.
9% High Arched Foot - highly defined footprint with least amount of foot on the ground, population usually a high supinator with minimal shock absorption.
2. FOOT MOTIONS
Plantar flexion and dorsiflexion are motions in the sagittal plane. Adduction and abduction are motions in the transverse plane.
Inversion and Eversion are motions in the frontal plane.
Pronation = abduction dorsiflexion eversion
Supination = adduction plantarflexion inversion
Both pronation and supination are triplane motions.
Structural Positions:
Knock Kneed Position - Genu valgum is the medical name for knock knees. Knock knees has to do with how the knee lines itself up with the hip and ankle. In a normal standing position, if someone's knees are touching, then their ankles will also touch. Excess pronation causes an anterior tilt to the pelvis and resultant knock kneed position.The typical gait pattern is circumduction - requiring that the patient swing each leg outward whilst walking to avoid striking the planted limb with the moving limb. The mechanics of gait are compromised and with the significant angular deformity, a high degree of anterior and medial knee pain are common, reflecting the pathological strain on the knee and its patello-femoral extensor mechanism.
Bow Legged Position - Genu varum is the medical name for bow legged position. This condition is recognisable by an abnormal out-curving of the legs, which results in a gap between the knees on standing. A certain degree of bowing is normal in small children, but persistence into adult life, or later development of this deformity results from abnormal growth of the epiphysis or arthritis. Average tibial varum angle in adults should be between 4-6° to the vertical.
Everted Calcaneus - Calcaneal Valgum
Inverted Calcaneus - Calcaneal Varum
Forefoot Adductus
Hallux Abductus
Equinus - Toe Walker
3. GAIT ANALISYS
Simply put, "gait" is the way in which a person or animal walks. When pain, paralysis or tissue damage occurs, abnormal gait is the result. Loss of motor control will also result in a gait disorder. There are two major abilities essential to walking. The first, equilibrium, is the ability to assume an upright posture and maintain balance. Locomotion, the ability to initiate and maintain rhythmic stepping, is the second. However, although these two abilities are essential, there are many additional contributing factors involved. The musculoskeletal system must provide intact bones and well functioning joints as well as adequate muscle strength. Normal gait requires the proper functioning of the musculoskeletal system and the nervous system.
Three Fundamental Stages of Gait:
Contact Stage - The contact stage begins with heel strike on the lateral border of the calcaneus. The tibia internal rotation occurs to the tibia as the foot pronates at the subtalar joint through the contact phase. Contact is made by the 5th metatarsal with the ground as the foot continues rolling medially until the metatarsals become fully loaded at the conclusion of the contact phase. The contact phase is designed to convert the foot into a mobile adaptor and shock absorbing mechanism.
Midstance Stage - The midstance phase converts the foot from a mobile adaptor into a rigid lever. The tibia externally rotates during this phase and the foot supinates at the subtalar joint preparing the foot for propulsive phase.
Propulsive Phase - Heel lift commences the propulsive phase. The subtalar joint should approach the neutral position just prior to heel lift. It is at this point that the forefoot and the rearfoot lock together to enable effective toe-off. Supination continues during toe-off with the resultant external tibial rotation.
4. BIOMECHANICAL ABNORMALITIES
Biomechanical dysfunction is most commonly caused by two factors:
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.
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 has generally 2 main causes: Angle of the tibia combined with a requirement of the foot to make contact with the ground in gait;
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).
When walking on sand, 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". Custom orthotics are designed to do this by having intrinsic correct angles built into their design to prevent excess pronation - just like mother nature intended! Find out what some of our hundreds of satisfied clients have to say about their orthoses.
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. 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:
Internal tibial torsion - twisted bone from the head of the tibia to the malleolus can affect knees, back and upper structure by compensatory elements.
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.
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 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.
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 PronationDuring 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.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 plantar fasciitis).
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.
