Axial synergies in parkinsonian patients during voluntary trunk bending

During upper trunk movements, the axial kinematic synergies (opposite movements of upper and lower segments) preserve the balance by minimizing the antero-posterior center of gravity (CG) shift due to the movement. Forward and backward upper trunk movements were analyzed in a population of parkinsonian patients (PD) that were subject to falling, in order to determine whether an impaired control of the kinematic synergies might explain the falling. Ten PD (stage III-IV of the Hoehn and Yahr classification; Hoehn MM, Yahr MD. Parkinsonism: onset, progression and mortality. Neurology 1967;17;427-432) were compared to seven age-matched control subjects (CS). Kinematic analysis and force platform recordings were carried out. Principal Component (PC) analysis was performed to measure the coupling between hip, knee and ankle joint angles during the movement. (1) In both PD and CS, the first principal component (PC1) was found to account for 98% or more of the joint angles changes, which indicates that there exists a strong coupling between the angles during the movement; however, the part of the movement not accounted for by PC1 was twice as high in PD as in CS. (2) The intertrial variability between the angle ratios was about twice as high in PD as in CS. (3) The absolute value of the antero-posterior CG shift occurring during the movement significantly increased in PD in the case of backward movements, both fast and slow. (4) As a high correlation was found between actual CG shift and its estimation based on the observed interjoint coordination, the increased CG shift in PD was related to unproper set of ratios between joint angles. It was concluded that the control of the kinematic synergy is preserved on the whole in PD, with an increased variability and unproper set of the ratios between joint angular changes. This may lead to CG shifts to beyond the support surface, especially in backward bending. Copyright 1998 Elsevier Science B.V.