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I posed this question to a group of students last week ... lets see what response we get here:

Is the tibialis anterior an invertor or evertor of the foot?

 

Invertor: it resists plantarflexion at heel strike, and pronation in the swing phase :eek:

 

Anyone else? Agree or disagree :confused:

 

It depends on the orientation of the axis of the subtalar joint...
If the axis is deviated medial enough, then it will start to act as an everter, though you would not expect it to be very strong-
Have I guessed correctly the trick question???

 

Hi Craig

You got it .... all the anatomy text books are wrong.

The muscle can both be an invertor and evertor, depending on where the STJ axis is.

If the axis is at its textbook mean (which no one actually has!!) of 16 degrees from the sagittal plane, then yes the insertion of tib ant is just medial to the axis, so has a lever arm to be an invertor (or resist pronation). If the axis is more medially deviated, then the insertion of tib ant is actualy lateral to the axis, so it then becomes an evertor of the subtalar joint.

The correct answer is it can be both.

 

I have a great drawing that shows this problem with the anatomy textbooks regarding the function of the anterior tibial muscle, as Craig mentions, that I used in my lecture on foot orthosis theory and function in Melbourne last week. It is actually fairly common to see that a foot which is maximally pronated will also have the subtalar joint axis passing through it (i.e. anterior tibial muscle will only produce compression force at the STJ axis) or will have the STJ axis passing medial to the anterior tibial tendon (i.e. anterior tibial muscle will produce a STJ pronation moment). Understanding this and discussing it are best done by using the following terms:
rotational position of STJ axis : (e.g. either maximally pronated, 2 degrees supinated from maximally pronated, 3 degrees supinated from neutral or maximally supinated), and
spatial location of STJ axis: the actual location of the STJ axis within space, relative to the anatomical structures of the foot (i.e. both its angle relative to the cardinal body planes and where it pierces both posteriorly and anteriorly through the calcaneus and talus).
Here's the reference for a more in depth discussion (Kirby KA: Subtalar joint axis location and rotational equilibrium theory of foot function. JAPMA, 91:465-488, 2001).