A number of lower limb pathomechanical entities or states cause a variation from the assumed normal function that result in compensations. These compensations are assumed to increase the risk for tissue damage.

Compensation – an abnormal structural or functional change in one part of the body as an attempt to neutralise an abnormal position or function of another part
– may not necessarily be pathologic – but may increase the risk for tissue damage

Abnormal and abnormal compensation

Theorems of compensation of the foot* (Southerland in Valmassy) :
1) The heel and the forefoot will always attempt to purchase the ground unless prevented by limitation of motion
2) The rearfoot will always attempt to compensate to perpendicular with the weightbearing surface unless influenced by some proximal or distal force
3) If driven to valgus more than two degrees beyond perpendicular by some proximal or distal force, the rearfoot will “fall through” to its end range of motion in the direction of eversion
4) If the rearfoot is not able to purchase with normal dorsiflexion of the ankle joint, the midtarsal joints will offer additional dorsiflexion to the foot by unlocking and maximally pronating against a maximally everted rearfoot.
5) The forefoot loads under the lateral column and compensates under the medial column. Therefore, the forefoot compensates primarily in the direction of inversion from its fully loaded neutral position.
6) The order of compensation of the forefoot is:
1) Midtarsal joint longitudinal axis to end range of motion
2) Early subtalar joint mobilisation
3) Midtarsal joint oblique axis supination
4) Late subtalar joint supination
7) The metatarsal parabola will generally splay in a predictable direction as the foot goes through the motions of supination to pronation. In the ‘normal’ foot, the fourth metatarsal remains fixed in supination or pronation

*These theorems are not based on research evidence and many are open to dispute, but do underpin a lot of traditional understanding of foot function.

Laing et al (1997) consider the following as important concepts:
• subtalar joint function influences the range of motion of distal joints
• primary compensation will take place in the nearest joint whose largest component of movement is in the same plane as that affected by the pathology
• the severity of the resulting pathology is directly related to the time in the gait cycle when the primary joint compensates
• the level of compensation will depend on the range of motion at the primary compensation site and the degree of abnormality
• severity will also be affected by weight, occupation, footwear and extrinsic influences such as abnormalities located more proximally in the lower limb

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