Impairment of posturo-kinetic co-ordination during initiation of forward oriented stepping movements in parkinsonian patients.

In order to differentiate between a specific impairment affecting gait initiation and a non-specific deficit in the postural adjustment which occurs prior to any forward oriented stepping movement, 3 forward oriented movements (FOMs), performed by a group of parkinsonian patients and a group of healthy age-matched subjects, were compared in the present study. These FOMs all consisted of initiating 1 step, but differed in their respective planning characteristics. The first consisted of initiating normal walking. The second consisted of initiating a single step, while the third was a visually guided task, consisting of placing the foot just behind a mark on the ground. In all 3 FOMs, the postural phase, i.e., the time elapsing between the initial shift of the center of pressure (CP) and the onset of the first step, was significantly longer in the patients than in the healthy subjects, whereas the duration of the subsequent movement phase, i.e., that of the first step, was within the same range in both groups. The horizontal reaction forces that led to a forward center of gravity (CG) acceleration during the postural phase were markedly reduced in the patients in all 3 FOMs, and the maximal velocity of the iliac crest marker, which corresponds approximately to that of the CG, decreased significantly in the patients. In addition, the length of the first step was significantly shorter in the patients than in the healthy subjects, in all 3 FOMs. The EMG pattern differed significantly between the patients and the healthy subjects; the amplitudes of the early tibialis anterior (TA) and vastus lateralis (VL) activations often decreased and were unilateral rather than bilateral. In addition, the gastrocnemius medialis (GM) burst associated with foot lift-off at the end of the postural phase was either absent or greatly reduced, thus suggesting that the co-ordination between the preparatory postural adjustment of the whole body and the actual stepping movement was impaired. The present results suggested that the lengthening of the postural phase is a common deficit in all FOM tasks in parkinsonian patients and is due to the impaired production of the requisite propulsive forces providing the forward acceleration of the CG. Consequently, a shortening of the first step length occurs. However, the step length is reduced less in the FOM tasks which provide some information about the goal of the first step (single step, visually guided step) than in a normal walking task, during which such information is missing. This suggests that although the stepping movement can be improved with the aid of any sensory cue about the end of the step in patients with Parkinson's disease, the postural phase will always be prolonged whichever FOM task they perform.