Children with cerebral palsy and powerful whole body extensor spasms find sitting in a rigid seat uncomfortable and sometimes painful due to the large forces they apply to their constraints. They are usually unable to speak and communication is difficult. The spasms affect every aspect of their lives. This thesis describes the genesis of a new functional dynamic seat for children with severe whole body extensor spasms, and the novel method used to design it. This novel seat technology is known as ‘Whole Body Dynamic Seating’. The thesis describes the clinical need this seat addresses, and the design and technology context in which this research takes place. The user evaluation, observation, measurement, analysis and reasoning that led to a successful seat design are described in detail. Children with cerebral palsy sometimes have whole body spasms that mean they cannot be seated in conventional static seating that positions a child in a fixed posture. For this research the children were classified as functioning at Chailey Sitting Ability Level 1 and Gross Motor Functional Classification System Level V. Such children spend much of their time being held by a person, or lying on a mat, bed or pad. This results in difficulty with social engagement and physical functioning, particularly in school. This research created a seat that such children could sit in, providing a comfortable and functional seat for use in a home or school classroom environment. This seat was designed with the direct and essential involvement of disabled children, their parents, therapists, teachers and carers. The work is part of a larger programme of research into seating and support technology that will enhance a child’s ability to gain functional movement and communication skills that can be employed to enable the child’s free self expression and social participation. The research investigated means of supporting children with whole body extensor spasms through a progressive iterative method utilizing direct user evaluation of a series of prototypes incrementing in complexity and fidelity towards a fully functional physical seat. An iterative method was used to design, build and evaluate three dynamic seats. This method incorporated two new approaches to prototyping developed for the research programme in response to difficulties encountered in designing dynamic systems for children with highly complex neuromotor disability. Soft and Semi-soft prototyping and evaluation methods provided essential feedback on dynamic seating concepts that guided proposed solutions, without requiring costly and time-consuming manufacture. Video was used to create a record of the children’s movements and responses for subsequent analysis. Instrumentation was built into the seats to enable direct objective measurement of the reaction forces and seat movement caused by extensor spasms. This thesis presents several unique features created through this research programme: 1. Independent and virtually hinged anatomical dynamic thigh supports; 2. Independent anatomical dynamic foot supports; 3. A virtually hinged dynamic back support; 4. An anatomical dynamic head support concept. The final Whole Body Dynamic Seat was child-centred in its functionality and aesthetic design, and was favourably commented upon by parents, children and school staff. Use of the new dynamic seating by three children (including one from a previous work programme) showed that children with severe whole body extensor spasms can be seated comfortably. The children also demonstrated gains in physical and social function as a result of using the dynamic seats. The two fully independent dynamic seats made advances in comfort over static seating for children with whole body extensor spasms. One of the children especially liked the seat and resisted being put back into his usual seating. An adult with severe cerebral palsy and extensor spasms evaluated a dynamic foot support concept and reported very significant reductions in spasticity and pain, and gains in physical function. The Whole Body Dynamic Seats showed gains in postural symmetry and in hand and head function over the usual static seats when used by the children with spasms. These gains were reported by staff during long term evaluations and measured specifically during the final evaluation. Two children learned to control the movement of seats in which they were sat, and were able to control their posture and use that control to carry out functions such as switch pressing. Such learning through the use of dynamic seating by children with severe dystonic cerebral palsy and whole body extensor spasms has not previously been documented. The seats did not just affect the children - school staff were affected too. School staff working around the children in the dynamic seats were observed to be more inclusive towards the children, and to expect more interaction from them. The ability of the children to move altered staff expectations of their ability to participate and communicate. This new seating has improved the quality of life of the children that use it. Future implementation of this technology in commercially produced seating offers the possibility of similar gains to many more severely disabled children who are currently less comfortable and less functional than they need to be.

High-tone extensor thrusts, or involuntary muscle contractions experienced by many children with cerebral palsy, can cause problems that are not addressed by current seating systems. This thesis is concerned with the development of a dynamic seating system to better accommodate individuals who exhibit high-tone extensor thrusts. The first part of the thesis is focused on obtaining a general understanding of extensor thrusts from a mechanical perspective. To achieve this goal, an analytical dynamic model of a human subject undergoing an extensor thrust on a rigid chair is created. This model is validated experimentally, and inferences about the nature of extensor thrusts are made from the simulation and experimental results. A Dynamic-Hingeback Seating System which allows the occupant to lean back during an uncontrolled extensor thrust is developed. This system is capable of maintaining seatback rigidity during an intentionally-induced episode, thereby enabling the occupant to communicate or interact with his/her environment. The design of this system is influenced by the results obtained from the rigid seat study, as well as by numerical simulation results gathered with a commercial dynamic simulation software package (Working Model 2D). The improved seatback performance is characterized through experimentation. Alternative dynamic seating systems are considered. The important features of each of these systems are identified, and the desired motion of the system occupant during an extensor thrust is verified through Working Model simulations.