In France, stroke is the third leading cause of death and the first cause of acquired disability in adults. Approximately 130,000 people suffer a stroke each year. On the other hand, since the frequency of stroke is age-related, the aging of the population suggests that the number of people with stroke will increase in the coming years.

One of the causes of disability in stroke patients is postural disturbances. These postural disturbances cause a greater risk of falls and are a source of loss of autonomy for these patients. In standing position, during evaluation on a force platform, these postural disturbances can be characterized by postural asymmetry which results in a greater displacement of the center of pressure on the lesion side and thus a greater percentage of support on the lower limb non-paretic (Weight-Bearing Asymmetry (WBA)). Today, the mechanisms of balance disorders in standing position are better understood. Indeed, in addition to sensory and motor deficits, spatial cognitive disorders also contribute to these postural disturbances, particularly in right brain damage stroke. Many authors agree on a localization of spatial cognition and in particular the mental elaboration of the representation of the body in space according to the different types of spatial frame at the level of the right cerebral hemisphere. This would be the reason why patients with right brain damage have a more precarious and time-consuming balance to rehabilitate than patients with lesions located in the left hemisphere.

Postural disturbances can also result in a disturbance of balance in the sitting position. The persistence of these disorders in a sitting position is a poor prognosis for the acquisition of transfers, standing and walking. To date, the mechanisms of these postural disturbances in the seated position in stroke patients have not been fully described. Indeed, many discrepancies are found in the literature. Although it appears that for a majority of authors, stroke patients have greater sitting asymmetry than healthy subjects, not all of them are unanimous. Moreover, among these authors highlighting this asymmetry in the sitting position, some note a more pronounced lateral plane while others find a more pronounced imbalance in the antero-posterior plane. Apart from the motor and sensory deficit, the postural asymmetry found in the sitting position could also be due to a spatial cognitive disorder. The authors Au-Yeung et al showed a more pronounced deviation in patients with a localized stroke in the right hemisphere. This result in relation to the relationship between posturography data from the sitting position and the postural vertical suggests the involvement of spatial cognition in balance disorders in the sitting position. However, since the authors Van Nes et al did not find this result in their study, this hypothesis remains to be confirmed.

Unlike standing posture disorders, which are commonly assessed by force platforms in rehabilitation, sitting posture disorders benefit from few instrumental measurement tools outside clinical measurement scales. In addition, in the literature, a very wide variety of evaluation methods by instrumental measures are proposed and not validated. In some cases, the patient was placed directly on the force platform, others chose to sit the patient on a chair positioned on the platform. Initially, sensor pad are usually used to adapt the bases of patients with pressure ulcers but they may be useful for quantifying the postural base. However, the involvement of the head and trunk in the sitting posture is well documented in the literature and the addition of an assessment of the trunk and head therefore seems essential. To our knowledge, no author has studied sitting balance disorders in a quantified way by taking into account both the posture of the trunk associated with the head and a measurement of postural asymmetry.