Sensorimotor postural control in healthy and pathological stance and gait

Postural control during standing and walking is an inherently
unstable task requiring the interaction of various biomechanical,
sensory, and neurophysiological mechanisms to shape stable patterns
of whole-body coordination that are able to counteract postural
disequilibrium. This thesis focused on the examination of central
aspects of the functional roles of these mechanisms and the modes
of interaction between them. A further aim was to determine the
conditions of dynamic stability for healthy standing and walking
performance as well as for certain balance and gait disorders. By
studying movement fluctuations in the walking pattern it could be
demonstrated that dynamic stability during walking depends on gait
speed and is differentially regulated for the medio-lateral and the
fore-aft walking planes. Stability control in the fore-aft walking
plane exhibits attractor dynamics typical for a dynamical system.
Accordingly, the most stable pattern of movement coordination in
terms of minimal fluctuations in the order parameter (i.e., the
relative phase between the two oscillating legs) can be observed at
the attractor of self-paced walking. Critical fluctuations occur at
increasingly non-preferred speeds, indicating a loss of dynamic
gait stability close to the speed boundaries of the walking mode.
Moreover, stability control during slow walking is critically
dependent on sensory feedback control, whereas dynamic stability
during fast walking relies mainly on the smooth operation of
cerebellar pacemaker regions. Disturbances of sensory and
cerebellar locomotor control in certain gait disorders could be
further linked to a loss of dynamic gait stability, in particular
an increased risk of falls. Furthermore, this thesis examined
alterations in the sensorimotor postural control scheme that may
trigger the experience of subjective imbalance and vertigo in the
conditions of phobic postural vertigo and visual height
intolerance. Both conditions are characterized by an inadequate
mode of balance regulation featuring increased levels of open-loop
balance control and a precipitate integration of closed-loop
sensory feedback into the postural control scheme. This inadequate
balance control strategy is accompanied by a stiffening of the
anti-gravity musculature and is elicited by specific influences of
attention and sensory feedback control. The findings of this thesis
contribute to the understanding of central sensorimotor mechanisms
involved in the control of dynamic postural stability during
standing and walking. They further provide relevant information for
the differential diagnosis and fall risk estimation of certain
balance and gait disorders.