It is a prospective, single-center pilot study comparing two medical devices evaluated using SCED (Single Case Experimental Design) ABAB methodology: NETTI DYNAMIC chair in dynamic mode (intervention group; phase B) versus the same chair in static mode (control group; phase A). The subject will be his/her own control conditions: Hyperextension Spasms conditions: Involuntary Movements studyType: OBSERVATIONAL observationalModel: CASE_ONLY timePerspective: PROSPECTIVE count: 5 type: ESTIMATED name: Observation measure: Forward slippage in static mode (PHASE A) measure: Forward slippage in static mode (PHASE A) measure: Forward slippage in static mode (PHASE A) measure: Forward slippage in static mode (PHASE A) measure: Forward slippage in dynamic mode (PHASE B) measure: Forward slippage in dynamic mode (PHASE B) measure: Forward slippage in dynamic mode (PHASE B) measure: Forward slippage in dynamic mode (PHASE B) measure: Peak force in static mode (PHASE A) measure: Peak force in static mode (PHASE A) measure: Peak force in static mode (PHASE A) measure: Peak force in static mode (PHASE A) measure: Peak force in dynamic mode (PHASE B) measure: Peak force in dynamic mode (PHASE B) measure: Peak force in dynamic mode (PHASE B) measure: Peak force in dynamic mode (PHASE B) measure: Maximum downward force in static mode (PHASE A) measure: Maximum downward force in static mode (PHASE A) measure: Maximum downward force in static mode (PHASE A) measure: Maximum downward force in static mode (PHASE A) measure: Maximum downward force in dynamic mode (PHASE B) measure: Maximum downward force in dynamic mode (PHASE B) measure: Maximum downward force in dynamic mode (PHASE B) measure: Maximum downward force in dynamic mode (PHASE B) measure: Patient's comfort in static mode (PHASE A) measure: Patient's comfort in static mode (PHASE A) measure: Patient's comfort in static mode (PHASE A) measure: Patient's comfort in static mode (PHASE A) measure: Patient's comfort in dynamic mode (PHASE B) measure: Patient's comfort in dynamic mode (PHASE B) measure: Patient's comfort in dynamic mode (PHASE B) measure: Patient's comfort in dynamic mode (PHASE B) measure: Caregiver's comfort in static mode (PHASE A) measure: Caregiver's comfort in dynamic mode (PHASE B) measure: Caregiver's comfort in dynamic mode (PHASE B) measure: Caregiver's comfort in dynamic mode (PHASE B) measure: Number of repositionings required in static mode (PHASE A) measure: Number of repositionings required in static mode (PHASE A) measure: Number of repositionings required in static mode (PHASE A) measure: Number of repositionings required in static mode (PHASE A) measure: Number of repositionings required in dynamic mode (PHASE B) measure: Number of repositionings required in dynamic mode (PHASE B) measure: Number of repositionings required in dynamic mode (PHASE B) measure: Number of repositionings required in dynamic mode (PHASE B) measure: Discomfort perceived by the caregiver in static mode (PHASE A) measure: Discomfort perceived by the caregiver in static mode (PHASE A) measure: Discomfort perceived by the caregiver in static mode (PHASE A) measure: Discomfort perceived by the caregiver in static mode (PHASE A) measure: Discomfort perceived by the caregiver in dynamic mode (PHASE B) measure: Discomfort perceived by the caregiver in dynamic mode (PHASE B) measure: Discomfort perceived by the caregiver in dynamic mode (PHASE B) measure: Discomfort perceived by the caregiver in dynamic mode (PHASE B) measure: Safety as perceived by the caregiver/carer in static mode (PHASE A) measure: Safety as perceived by the caregiver/carer in static mode (PHASE A) measure: Safety as perceived by the caregiver/carer in static mode (PHASE A) measure: Safety as perceived by the caregiver/carer in static mode (PHASE A) measure: Safety as perceived by the caregiver/carer in dynamic mode (PHASE B) measure: Safety as perceived by the caregiver/carer in dynamic mode (PHASE B) measure: Safety as perceived by the caregiver/carer in dynamic mode (PHASE B) measure: Safety as perceived by the caregiver/carer in dynamic mode (PHASE B) measure: Average of peak forces exerted on the wheelchair's backrest in static mode (PHASE A) measure: Average of peak forces exerted on the wheelchair's backrest in static mode (PHASE A) measure: Average of peak forces exerted on the wheelchair's backrest in static mode (PHASE A) measure: Average of peak forces exerted on the wheelchair's backrest in static mode (PHASE A) measure: Average of peak forces exerted on the wheelchair's backrest in dynamic mode (PHASE B) measure: Average of peak forces exerted on the wheelchair's backrest in dynamic mode (PHASE B) measure: Average of peak forces exerted on the wheelchair's backrest in dynamic mode (PHASE B) measure: Average of peak forces exerted on the wheelchair's backrest in dynamic mode (PHASE B) measure: Average of peak forces exerted on the wheelchair's seat in static mode (PHASE A) measure: Average of peak forces exerted on the wheelchair's seat in static mode (PHASE A) measure: Average of peak forces exerted on the wheelchair's seat in static mode (PHASE A) measure: Average of peak forces exerted on the wheelchair's seat in static mode (PHASE A) measure: Average of peak forces exerted on the wheelchair's seat in dynamic mode (PHASE B) measure: Average of peak forces exerted on the wheelchair's seat in dynamic mode (PHASE B) measure: Average of peak forces exerted on the wheelchair's seat in dynamic mode (PHASE B) measure: Average of peak forces exerted on the wheelchair's seat in dynamic mode (PHASE B) measure: Sex measure: Age measure: Height measure: Weight measure: Body mass index sex: ALL minimumAge: 16 Years stdAges: CHILD stdAges: ADULT stdAges: OLDER_ADULT hasResults: False