by Shelley Mannel, PT, C/NDT 

Any rocket needs a great deal of force to push off the surface of the earth.  And when I think about the spectrum of my clinical practice, all my kids with movement challenges have difficulty has difficulty accomplishing this same task.  They are like a rocket without enough fuel – they have difficulty pushing off the surface in any position.  We call this a generating a ground reaction force.  And without this force, compensations will be required to keep from crashing back to earth.  In sitting, I see my clients collapsed into a slumped posture, holding their head up with their hands, wrapping their feet around the legs of the desk, constantly changing position looking for a new stabilization strategy. Even if I ask them to place their feet on the ground, they can only manage it for a very short time.  In standing they hyperextend their knees, collapse into hip and knee flexion, internally rotate at their hips and collapse at the medial arches of their feet.  I’m sure you can identify more biomechanical compensations you see when your clients are trying to stay upright against gravity.

But what might be the difficulty with the generation of a ground reaction force?  Well as usual, there may be more than one system involved.

The vestibular component: The powerful vestibular system serves to stabilize our gaze but it also a large impact on our postural control, by virtue of its input to the  postural tone of the anti-gravity extensors of the trunk, it’s impact on the development of anti-gravity flexion and its involvement in our postural adjustments.  Dysfunction in registration or processing of vestibular input can manifest in decreased vestibular input to anti-gravity muscle tone,  central stability and/or postural adjustments (1).

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