Page 71 - Handbook of Electronic Assistive Technology

58 HANDBOOK OF ELECTRONIC ASSISTIVE TECHNOLOGY

inhibitory neuron within the spinal cord. Disturbances in muscle tone occur when there
is an imbalance in the excitatory and inhibitory input to motor neurons, which is caused
by damage to the spinal cord and/or the CNS. This damage then results in an imbalance
between messages from the nervous system to the muscles causing altered excitability of
this stretch reflex (Britton, 2004).
Commonly seen disturbances in muscle tone include:

Hypotonia or low muscle tone, where there is too little muscle tone at rest and muscle
and joints may feel ‘floppy’. This is due to an abnormally low resistance to stretch as
a result of too much inhibition in the stretch reflex arc. This may result in instability
and lack of control of movement. For example, a person with low muscle tone in
their trunk may have problems sitting upright for any length of time and may adopt a
slouched posture (Fig. 3-4).
Hypertonia or high muscle tone is where there is too much tension in the muscles
and an abnormally strong resistance to stretch as a result of too much excitability in
the stretch reflex arc. The muscle may seem tense and contracted even when resting.
Spasticity is a condition where muscles are resistant to rapid stretching and results
in muscle hypertonia and involuntary muscle contraction (Lance, 1990; Barnes,
2001; Ibuki and Bernhardt, 2007). Often it presents in specific muscle groups and can
result in abnormal movement patterns and posturing. This can be seen in a common
pattern of spasticity in the upper limb following a stroke (Fig. 3-5).
Dystonia is a movement disorder in which involuntary sustained or intermittent
muscle contractions cause twisting and repetitive movements, abnormal postures or
both (Sanger et al., 2010).

FIGURE 3-4 Child sitting on a box with a slouched posture demonstrating low muscle tone (Pountney et al�, 2000)�