Altered muscle tone is a common problem that wheelchair prescribers face when assessing for wheelchairs and seating. Altered muscle tone can negatively impact on function and affect the way a user interacts with their wheelchair equipment.
What is muscle tone?
“a state of tension that is maintained continuously - minimally - even when relaxed - and which increases in resistance to passive stretch”(Dictionary of Sports and Exercise Science (2008).
We need normal muscle tone for postural stability as muscles work in surrounding joints to stabilise body segments to enable function. This tone enables us to maintain certain positions for longer periods without fatigue. When we increase the muscle tone by activating or contracting those muscles either side of a joint (co-contraction), we can achieve greater stabilisation.
Low muscle tone
Reduced muscle tone is also known as hypotonia and can occur in two ways. Either as a congenital condition e.g. benign congenital hypotonia or more commonly, it is seen as a generalised symptom in cerebral palsy or damage to the spinal cord, in which the muscles in the trunk and legs below the level of the injury do not receive any motor signals. The person can appear ‘floppy’ and unable to maintain an upright functional seated position. Individuals with low tone require seating that offers greater support of body segments:
Tilt in space is a feature that is commonly employed for wheelchair users to assist with low tone as tilting the user backwards allows gravity to assist rather than decrease postural stability. Manual chairs that offer tilt in space are the BREEZY Cirrus G5 and ZIPPIE TS. Powered chairs that offer powered tilt are the Q700 series, Q500 series and Q400 series.
High muscle tone
Elevated or increased muscle tone limits joint movement and is characterised by muscles that are stiff and difficult to move. It occurs when there is overload of signals to the muscles and can occur when the brain or spinal cord have been damaged. It is also known as hypertonicity or hypertonia. There are two types; spastic and rigid hypertonia. In spasticity, the tone increases with increased speed and movement of a limb through the joint range and is felt as increased resistance as the limb is extended or straightened. This is common in cerebral palsy. Rigidity is increased muscle tone that is not dependent on direction or speed of movement. Commonly the limb is extremely difficult to move and common in conditions such as acquired or traumatic Brain Injury or Parkinson’s disease.
Seat cushions need to have variable contouring that can match the user's leg and pelvic position and be changed in response to altered body position due to an increase in hypertonicity. JAY cushions with variable contouring that can be easily altered to increase or support the pelvis and lower limbs are the JAY J2, JAY J3 and JAY GS.
Some users can present with mixed tonal patterns and this can prove challenging for the prescriber as scoliosis and pelvic asymmetry are common. Cerebral palsy is the most common condition where this occurs and requires seat cushions to be able to accommodate abnormal pelvic position but also offer increased postural support. Cushions used in this instance are the JAY Fusion, JAY J2, and JAY GS. Backrests are often needed to correct or accommodate trunkal asymmetry (scoliosis and rotation) with the use of lateral thoracic supports. JAY Go and JAY J3 backs are commonly used in these cases.