Hypotonia refers to abnormally diminished muscle tone in children that causes reduction in muscle strength, decreased activity tolerance, delayed motor skill development, hypermobile joints and increased flexibility.
This disorder is usually associated with many different conditions related to the central nervous system, neuromuscular system, genetics, connective tissue or metabolic disturbances; Down’s syndrome remains the most common congenital condition to be associated with hypotonia.
The biomechanical implications of hypotonia involve postural misalignments such as a rounded shoulder posture, inadequate balance causing a tendency to lean against external support, loss of stability and abnormal functioning of foot and ankle musculature.
Clinical assessment of hypotonia firstly involves making a clear distinction between a central or peripheral origin – the prevalence of sufficient muscle strength indicates a central origin and muscle weakness signifies a peripheral origin.
This becomes challenging especially in the assessment of ‘floppy’ infants since no objective measurement methods exist; clinicians have to rely on the observance of antigravity movements of the infant and conduct manual testing of muscle strength using different manoeuvres.
Reus et al. sought to develop a method to measure muscle strength in hypotonic infants by testing 17 infants with Prader-Willi syndrome (commonly associated with hypotonia) and comparing the results to the muscle strength in normal infants.
The authors observed a significant correlation between muscle strength and motor performance in children with Prader-Willi syndrome, making it a determining factor for skill acquisition especially in the case of antigravity movements; diminished muscle strength was established as a cause of motor development delay in hypotonic infants.
The efficacy of orthotic implementation in conjunction with gross motor therapy was analysed in detailed by Charmayne Ross and Susan Shore, from the Department of Physical Therapy at Azusa Pacific University.
For this purpose, the authors recruited twenty-five children with hypotonia and flexible flat feet who were receiving gross motor therapy twice a week for motor delay accompanied by low muscle tone, lower limb weakness, gait deficits, and balance impairments; none of the children had previously worn orthoses.
The addition of orthoses to treatment modalities was found to have significantly modified the arch index as compared to gross motor therapy alone which was successful in improving gait parameters such as mean stride length, cadence and velocity.
The resultant elevation in the medial longitudinal arch and subsequent foot supination was suggestive of a change in the soft tissues of the foot over the 6 months of wearing orthoses namely, the joints and ligaments assuming the correct position over time to enhance foot stability and muscle control during the gait cycle.
The global physical characteristics of the child such as height, weight and overall hypermobility need to be considered in the manufacture of the orthotics, as these are more related to gait parameters than specific biomechanical measurements like foot parameters.
In an effort to improve the quality of life of these children and to prevent surgical interventions at a later stage, conservative approaches in the form of customised foot orthotics such as MASS4D® along with physical and/or occupational therapy treatment need to be adopted early on to improve the physical development of the hypotonic child.
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Repetitive plantarflexion can lead to pain and mechanical limitation in the posterior ankle joint which is known as posterior ankle impingement syndrome. This pathology commonly occurs in ballet dancers and football players.