Thursday, November 06, 2008

The abductory twist

During gait, in a number of patients, just as the heel comes off the ground, the foot often abducts. There is no doubt this happens and its called an abductory twist.

What is interesting about it, is that two different biologically plausible and theoretically coherent theories can be used to explain it:

1. Foot pronates beyond midstance--> internal rotation moment to leg; the opposite leg is swinging forward and rotating the pelvis --> externally rotation moment to leg on ground --> conflict between proximal external rotation and distal internal rotation moments -- initially the pronated foot causing the internal rotation moment wins the battle and foot does not resupinate to accommodate that proximal external rotation moment ..... eventually as heel comes off ground, friction between the ground and foot can no longer no longer resist the external rotation moment coming from above --> abductory twist

2. As heel starts to come off the ground a functional hallux limitus kicks in (for whatever reason); as the body has to move forward over the first MPJ, it can do so by a number of mechanisms; one of these is to abduct the foot to roll off the medial side of the blocked first MPJ --> abductory twist

This is a classic example of how a fact (the presence of an abductory twist during gait) can be explained by two competing theories (this is what make clinical biomechanics fun).

Lets wee what others think on the abductory twist.

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