Page 74 - Handbook of Electronic Assistive Technology

Chapter 3 • Functional Posture 61

CNS with information about the tension of the muscle and whether it is in a state of
relaxation or contraction (Purves et al., 2004).
The vestibular system: This provides the CNS with information about the position
and movement of the head with respect to gravity and inertia forces. There are two
types of receptors:
• Semicircular canals, which sense angular acceleration of the head such as in
imbalance trips and falls.
• The otoliths, which sense linear position and acceleration and mainly respond to
slow head movements such as during postural sway (Saladin, 2011).
The visual system: The visual field and pathway are important regulators of postural
control. Visual input provides information regarding the position and motion of the
head with respect to surrounding objects. It helps to fixate the position of the head
and upper trunk in space, primarily so that the centre of mass of the trunk maintains
balance (Hansson et al., 2010).
Feedforward and Feedback Mechanisms

The human body, in its structure, is inherently unstable and while maintaining balance
and equilibrium are important, a static stable position is of relatively little use for function.
A dynamic postural control system is required to make continual and adequate adjust-
ments and meet functional demands. In response to ever-changing conditions, these
adjustments are known as anticipatory postural adjustments and compensatory postural
adjustments and work on a feedforward and feedback basis (Massion et al., 2004).
Anticipatory postural adjustments: a feedforward system – Postural adjustments are
anticipated and predicted and anticipatory forces are provided to minimise the expected
disturbances.
Compensatory postural adjustments: a feedback system – Disturbances in posture are
detected by the sensory systems and are corrected immediately by postural reflex mecha-
nisms called righting reactions (Fig. 3-6).
In summary, postural control is a complex multisystem and dynamic mechanism influ-
enced by:

• The integrity of the nervous system.
• Sensory processing.
• Length-associated changes in muscles.
• Selective control of muscle for posture and movement.
• Bone and joint formation.
• Biomechanical forces.
• The environment.
• The complexity of task.

Children and adults who have a developmental, neurological, motor or body structural
impairment may therefore experience difficulties with their postural control.