Comparison of motor strategies in sit-to-stand and back-to-sit motions between healthy and Alzheimer's disease elderly subjects.

We studied the kinematics of shoulder displacement during sit-to-stand and back-to-sit in 6 healthy elderly subjects and six elderly subjects with mild to moderate Alzheimer's disease in order to elucidate the impact of Alzheimer's disease on motor planning and control processes. During sit-to-stand, Alzheimer's disease subjects reduced their forward displacement and started their upward displacement earlier than healthy elderly subjects. Furthermore, shoulder path curvatures were more pronounced for upward compared with downward displacement in healthy elderly group, in contrast with Alzheimer's disease group. Temporal analysis found that: 1) for both groups, profiles of velocity of sit-to-stand and back-to-sit showed two peaks corresponding respectively to forward/upward and to downward/backward displacements, 2) peaks of velocity were almost comparable between the two groups, 3) duration of sit-to-stand was shorter than duration of back-to-sit in the two groups and 4) duration of sit-to-stand and back-to-sit was shorter in Alzheimer's disease group than in healthy elderly group. However, dissimilarities were observed for transition and deceleration phases during sit-to-stand, and for acceleration and transition phases during back-to-sit, between the two groups. Interestingly, while sit-to-stand and back-to-sit differed in healthy elderly subjects during transition and deceleration phases, such a difference was not observed for Alzheimer's disease subjects. So, our study showed that invariant spatio-temporal movement parameters in the two groups differed, while non-invariant parameters did not, and suggests that higher level motor process of whole body motions are affected by Alzheimer's disease, while lower level motor features remain intact.