Balance assessment in children with cerebral palsy; methods for measuring postural stability

Abstract: The most common way to measure postural stability is to examine the displacement of the center of pressure (CoP). But some scientists claim that the center of mass (CoM) is what really indicates the sway of the whole body, since the body is a multi-joint system. Many previous studies of human balance have targeted groups with diffrent kinds of balance impairments. In a recent study C. Lidbeck investigated factors influencing standing in children with bilateral spastic cerebral palsy (BSCP). The conclusion of that study was that the crouched position, that is common with this kind of disability, was not found to be related to strength and not entirely related to the degree of their motor disorders. In this thesis a number methods were chosen to assess the postural stability of children with BSCP, using both the CoP and the CoM. The hypothesis was that the different methods would show different aspects of the children's balance impairment. Also, the influence of visual stimuli on the crouching position was examined. The long term aim is that the results may contribute to a deeper understanding of the balance disturbances that often accompany this group of children. 16 children with BSCP (GMFCS level I-III) and 20 typically developing (TD) children were included in the study. Data was collected, before the start of this project, using two force plates and an eight-camera 3D motion analysis system with passive markers. The children performed three different standing tasks during 30 seconds each; quiet standing, blindfolded and an attention-task. Five methods were chosen (based on previous literature) and implemented in Matlab to examine the postural stability of the two groups during the three tasks. Result shows that all methods used can clearly distinguish between the balance in the BSCP group and the TD group. When comparing the quiet standing task with the blindfolded task in the BSCP group, there were some significant results from the statistical evaluation (P$<$0.05). The result from several of the methods indicated that the children of this group have better postural stability when blindfolded, which is not in agreement with previous literature. In contrast, one method using the total mean velocity indicated that the postural stability decreased. During the attention-task, the methods disagreed with each other, implying a change in balance strategy in the BSCP group that was different from the TD group. Four methods are suggested for future studies, two using the CoP and two using the CoM. These four methods highlighted different aspects of the data and in combination they may provide a bigger picture of the postural stability of children with BSCP. Even though there were no significant difference in the vertical displacement of the CoM between the BSCP and the TD group, the CoM was slightly elevated during the attention-task in the BSCP group. In the TD group the CoM was lowered during the same task. This indicates that the children with BSCP in this study straighten up a bit when they can focus on something outside of their own body.