Trunk control in unstable sitting posture during functional activities in healthy subjects and patients with multiple sclerosis

OBJECTIVE: To examine trunk stability in unstable sitting posture in 2 different functional activities. DESIGN: A randomized crossover design. SETTING: Rehabilitation center in Italy. PARTICIPANTS: Ten healthy subjects and 10 patients with multiple sclerosis. INTERVENTIONS: Subjects were seated on an unstable support surface. A task in which the subjects had to keep their trunk as stable as possible was compared with tasks in which they had to track an object with the head or grasp an object.Main outcome measures Angular displacement and mean absolute angular velocity in the anteroposterior and mediolateral planes of the support surface. RESULTS: Angular displacement showed differences between patients and healthy subjects both in the sagittal plane (P<.0001) and frontal plane (P<.002). Velocity of angular displacement differentiated between the groups in both planes (P<.0001). Differences between velocity of angular displacement in different tasks were also detected (P<.0001). Correlation coefficients between sitting balance scores and laboratory measures were low and not statistically significant. In the healthy subjects, instability in the frontal plane correlated positively with the subjects' weight and height. CONCLUSIONS: Both patients and healthy subjects had more difficulty with frontal plane stability than with sagittal plane stability. Angular velocity for a given task showed the greatest difference between the groups and between postural challenge tasks within a group. Patients were more unstable than healthy subjects during head movements in the frontal plane; conversely, arm movements produced larger angular displacement, especially in the sagittal plane.