R. J. St George, V. S. Gurfinkel, J. Kraakevik, J. G. Nutt, F. B. Horak

Upright stance in humans requires an intricate exchange between the neural mechanisms that control balance and those that control posture; however, the distinction between these control systems is hard to discern in healthy subjects. By studying balance and postural control of a participant with camptocormia - an involuntary flexion of the trunk during standing that resolves when supine - a divergence between balance and postural control was revealed. A kinematic and kinetic investigation of standing and walking showed a stereotyped flexion of the upper body by almost 80 degrees over a few minutes, and yet the participant's ability to control their center of mass within their base of support and to compensate for external perturbations remained intact. This unique case also revealed the involvement of automatic, tonic control of the paraspinal muscles during standing and the effects of attention. Although strength was reduced and MRI showed a reduction in muscle mass, there was sufficient strength to maintain an upright posture under voluntary control and when using geste antagoniste maneuvers or "sensory tricks" from visual, auditory and haptic biofeedback. Dual-tasks that either increased or decreased the attention given to postural alignment would decrease, or increase the postural flexion, respectively. The custom-made, 'twister' device that measured axial resistance to slow passive rotation revealed abnormalities in axial muscle tone distribution during standing. The results suggest that the disorder in this case was due to a disruption in the automatic, tonic drive to the postural muscles and myogenic changes were secondary.