Down syndrome is a genetic disorder that results from a chromosomal abnormality causing a number of physical and cognitive anomalies (Blackman, 1997). Children with Down syndrome typically have low muscle tone (hypotonia), short stature, and intellectual disability. A number of health problems may accompany this disorder such as congenital heart disease, visual deficits, and lowered resistance to infection. Instability between the first two cervical vertebrae, called atlantoaxial subluxation, occurs in 10 to 12% of children with Down syndrome; however, only 1 to 2% display symptoms (Ali, Al-Bustan, Al-Busairi, Al-Mulla, & Esbaita, 2006). X-rays of the neck are often recommended to rule out this condition because, in rare instances, pressure on the head and neck may result in damage to the spinal cord (Blackman, 1997).
Although gross motor delays are typical in children with mental retardation, the degree of delay is greater in Down syndrome than in retardation due to other factors (Fidler, Hepburn, Mankin, & Rogers, 2005). The impact of these delays on the child is great. In a study of 5- to 7-year-old children with Down syndrome, Volman, Visser, and Lensvelt-Mulders (2007) found that motor ability was a much better predictor of functional performance than mental ability. Typically developing children begin to walk without support anywhere from 9 to 15 months; children with Down syndrome begin to walk on average about a year later than typically developing children (Ulrich, Ulrich, Angulo-Kinzler, & Yun, 2001). In general, the age of acquisition of developmental milestones is late and the range of ages in which skills may be acquired is broad (Vicari, 2006). The gap between the age of acquisition of skills between children with Down syndrome and typically developing children becomes greater as motor complexity increases: Children with Down syndrome require more time to learn complex tasks (Palisano et al., 2001).
Lack of trunk rotation, variability, and poor balance characterize the quality of movement in children with Down syndrome. It is felt that these problems are caused by low muscle tone and limited coactivation around the joints (Lauteslager, Vermeer, & Helders, 1998), resulting in poor stability at the shoulders and hips and leading to limited ability to shift weight. In infancy, resistance against gravity is minimal and range of motion is greater than in typically developing children. Children with Down syndrome tend to use atypical posture in static positions, such as sitting, in which the legs tend to be widely abducted providing a wide base and eliminating the need for weight shift. In sitting, children with Down syndrome tend to avoid rotating the trunk to retrieve objects; instead, they may lean far forward with a rigid trunk or scoot in the sitting position to move toward a desired object.
Atypical movement patterns are often observed when these children move from one position to another. Movement often occurs in straight planes, with limited trunk rotation. For example, whereas the typical child moves from sitting to a modified side-sitting position to the hands-and-knees position, a child with Down syndrome is more likely to vault straight forward over the legs into the hands-and-knees position (Lauteslager et al., 1998; Vicari, 2006). In walking, children with Down syndrome are more likely to have their legs widely abducted and use lateral trunk movements to achieve weight shift for much longer periods than typically developing children do. Few children with Down syndrome develop the mature counterrotation and arm swing present in most typically developing 6-year-old children.
It may be hypothesized that although the movement patterns of young children with Down syndrome are efficient based on their musculoskeletal features, the stereotypical movement patterns they display may result in further delays in the future. For example, when typically developing children move from sitting to hands and knees, they get practice with trunk rotation and have better developed equilibrium reactions, more strength, and more variability of movement to prepare them for higher level skills, such as walking. Children with Down syndrome often do not have the same range of experiences in sitting and do not gain the same degree of control before learning to walk; therefore, the walking pattern is more restricted. To allow the child to experience greater variability of movement, intervention is often focused on tone building and facilitation of coactivation, weight shift, and rotation in movement activities.
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