Cerebral palsy (CP) is the most common physical disabilities in childhood, affecting 2 to 2.5 per 1000 live-births worldwide and about 1.3 per 1000 live-births in Hong Kong. It is a non-progressive lesion of the developing central nervous system affecting the control of movements and postures in the children. Children with CP are usually classified using the Gross Motor Function Classification System (GMFCS) according to age-specific gross motor functional abilities and reliance on assistive devices (such as walking aids or wheeled mobility). It is an ordinal grading system of five levels (I to V), in which self-initiated movements, such as sitting, standing or walking, are described in relation to different age groups. Children of levels I and II can generally walk without aids while children of level III can walk with aids for short distances but usually choose wheeled mobility in community settings. Children at level IV have limited motor ability in assisted standing, stepping, and transfers. Children classified as level V are completely dependent for transportation in a wheelchair and lack antigravity postural control. Numerous studies have shown that children with CP, regardless their GMFCS levels, demonstrate problems in their postural control in sitting and standing, which in turn affect their function and participation in daily life. The main deficits in postural control in sitting for children with CP are lack of ability to recruit direction-specific muscles in more severe children (GMFCS levels IV and V) or inability to fine-tune the degree of muscle contraction according to specific situations. Excessive contraction of antagonists is a common compensatory strategy. These ineffective strategies to compensate their balance in sitting or standing positions will affect the children's daily function such as desk work at school, transfers and ambulation.

Postural control involves controlling the body's position in space for stability and motility. Good postural control around the trunk or 'trunk control' in short, allows an individual to perform different tasks in an upright and vertical posture without losing the balance. The control of trunk to maintain balance in either sitting or standing positions is usually tested in the following three conditions: static balance, anticipatory balance and reactive balance. Assessment of trunk control in children is commonly part of a motor assessment such as Bruininks-Osteretsky Test of Motor Proficiency. The main limitation of these motor assessments is that the trunk is considered a one unit during a motor act, despite that there are different segments in the spine. Preliminary findings in very young infants have shown that infants with lumbar control had better movement quality in reaching actions when compared to those with thoracic control only. It seems that specific outcome measures on trunk control are necessary to demonstrate improvement in movement quality like this. There are specific outcome measures assessing balance, an indication of trunk control in recovering stability after internal or external perturbations such as Pediatric Reach Test (PRT), Pediatric Balance Scale, and Segmental Assessment on Trunk control (SATCo). It has been found very few of the commonly used outcome measures on balance have covered assessing all three aspects of balance as mentioned above. Most of these outcome measures require the children to be able to sit independently with or without hand support, making it unsuitable for children with CP of GMFCS levels IV and V. Furthermore, most of these outcome measures have the same limitation in considering the trunk as one whole unit, not consisting of different regions. Among all the commonly used assessment of trunk control, SATCo is a recently refined assessment on segmental trunk control in children. The child's trunk control is examined by progressively reducing the trunk support from the shoulder girdle to the pelvis in order to assess the head control, upper thoracic, mid-thoracic, lower thoracic, upper lumbar, lower lumbar and full trunk control. The trunk control is tested under 3 different conditions in sitting: to maintain the posture (static control), during voluntary head movements (active control) and recovery of trunk control after a disturbance of balance by a nudge (reactive control). (Butler et al, 2010) The preliminary results of the SATCo showed a high inter-rater reliability (ICC ≥ 0.8) and moderate to good correlations with other established motor assessments (r from 0.65 to 0.88). Besides assessing the trunk in different segments, SATCO is one of the rare ones being able to assess reactive balance. It has been recommended that to increase the comprehensiveness in assessing the balance mechanism in children with CP, researchers can use the SATCo in combination of outcome measures assessing more dynamic sitting balance, such as the PRT.

Interactive computer play (ICP) has become popular in rehabilitation for people with motor impairments, including children with CP. During an ICP, a child interacts and plays with virtual objects in a computer-generated environment, through the use of a computer console or platform and the software, allowing the child to control the games through certain body movements. Because the computer games are fun and enjoyable, the child will repeat the required body movements in numerous times so as to get a high score in the game without losing the interest. These numerous repetitions (essentially mass practice) and feedback from the game scores (knowledge of performance and results) are important in motor learning and enhancing neuroplasticity. Hence, ICP may be a feasible way to improve trunk control in children with CP, in addition to the conventional therapies. Up to date, there are six published studies using ICP or virtual reality in improving balance in children with CP.. From these 6 published studies, promising though conflicting results were found whether ICP is effective in improving trunk control in children with CP. The main limitations of these studies are the overall low levels of study quality, small sample sizes and heterogeneous population groups under the umbrella term CP. In fact, similar conclusions for future studies with larger sample sizes and rigorous study designs have been echoed from the four recent systematic reviews on the topic.

One of possible reasons for the conflicting conclusion on the use of ICP in rehabilitation for children with CP may be lack of sensitive outcome measures to capture the specific changes in the trunk control post-intervention. The above-mentioned studies were either using general motor assessments or outcome measures with unknown psychometric properties to assess the posture of the participants. Besides, when the child received the ICP intervention in a standing position, compensatory strategies, due to uncontrolled degrees of freedom in lower limbs, might be used to maintain their trunk control in this more challenging position than a sitting position. These compensatory strategies may in turn, water down the overall effect of the ICP intervention on the trunk control. Most of these studies examined only one or two aspect of the trunk control as testing the static or anticipatory balance of the children. The commercial gaming consoles, such as Nintendo Wii Fit, are not specifically developed for children with CP to elicit targeted movements to overcome their impairment and the software may not be sensitive enough to translate the subtle movement changes to an increase in the game scores as well to an adequate challenge for these children.

Tymo (Tyromotion, Austria, www.tyromotion.com) is a wireless force plate detecting the movement of the centre of pressure, either in a sitting or standing position. Force and weight distribution can be measured using the software provided with the Tymo in these two positions. During the assessment, the user will move the trunk forward, backward and sideways as far as they can. The amplitude of the force and weight distribution between the two sides of the body will be recorded. This information will be used when the user uses the Tymo as a balance training module, in which the user will move the trunk forward, backward and sideways to participate in a computer game. For example, the user will move their trunk in different directions so as to manoeuvre a basket to catch the falling apples. The difficulties of the game will be adjusted according to the information provided during the assessment so that the user needs to move their body accordingly in different directions without losing their balance. This equipment is specially designed for rehabilitation for people with movement disorders, hence unlike the commercial games, there are no flashing lights or sudden noises during the games.

20 children with CP or similar physical disabilities will be recruited from 2 special schools for children with physical disabilities in Hong Kong. Informed consent will be sought from the parents or guardians before the study will commence. After the initial assessment, all study children will be randomly allocated into a control group or treatment group (10 children in each arm) using a 'draw card from the hat'. The randomization will be separately done in each school. The children in the treatment group will receive training on their trunk control using the Tymo in sitting 4 times per week for 20 minutes per session. The treatment will last for 6 weeks. All study children will continue their usual therapies at school but will be asked to stop playing any interactive computer play, which is designed to train balance such as Nintendo Wii balance board, during the study period. As there is no strong research evidence that commercial computer games are effective in improving the balance for children with CP and this type of commercial computer play is not a routine training exercises for balance at the schools, it is believed that discontinuing this type of 'training' for 6 weeks would not cause detrimental effect on the balance of the study children.

All the assessment will be carried out by a blinded assessor, who is not aware of the group allocation of the children. All the children will be assessed at the beginning, and 3, 6 and 12 weeks after the commencement of the intervention.

All children will be calibrated using the Tymo in a static sitting position in each treatment session by the research staff and the amplitude of the force and weight distribution generated by the child between the two sides of the body will be recorded. This information will be used to set up the Tymo as a training module (the intervention) by the software, in which the child will move the trunk forward, backward and sideways to participate in a computer game in sitting. The child will choose which game they want in each treatment session and they have to stay on the same game for at least 10 minutes before changing to another game. The child will have a maximal choice of 2 computer games in each treatment session. All children will start at a medium level of difficulty. Only if the child is unable to score any point in 3 trials in a row, the difficulty level will be reduced to one level down. Similarly, if the child scores full points in 3 trials in a row, the difficulty level will be increased to one level up. The progression of the intervention will then be closely monitored by the research staff. A logbook will be used for each child to record the level of difficulty in each treatment session, the number of treatment sessions, duration of each session and general comments on the child in each session during the intervention phase by a research assistant.