Changes in centre of pressure of ground reaction forces prior to rapid arm movement in normal subjects and patients with cerebellar ataxia.

This study was designed to investigate the biomechanics of anticipatory postural responses. In particular the aim was to determine whether the centre of pressure of ground reaction forces moves in anticipation of an expected upper limb movement, and if so to determine the pattern (s) of centre of pressure change in normal subjects, and patients with cerebellar ataxia. Twenty-eight healthy, normal subjects (aged 30-64 years) and ten cerebellar ataxic patients (aged 30-80 years) each stood with feet parallel and 6 cm apart on a force platform. The subjects elevated their right arm to the horizontal as quickly as possible in response to a visual stimulus. The centre of pressure of ground reaction forces was recorded for 1 s: 600 ms before and 400 ms after initiation of arm movement. In 96% of normal subjects there were consistent patterns of change in the location of centre of pressure prior to the beginning of the rapid self-initiated arm movement. In 79% of subjects, there was a posterior shift of centre of pressure prior to arm movement. Ninety-six per cent of subjects exhibited a preparatory shift in centre of pressure in the lateral direction, most frequently to the left (75%). These patterns were repeatable both within and between subjects. There was a bimodal distribution in the onset time of the preparatory centre of pressure excursions, with initiation occurring at ⩽ 150 ms and ⩾ 300 ms prior to arm acceleration. Cerebellar ataxic subjects exhibited marked between-subject and within-subject variability in the patterns of centre of pressure excursion, including absence of preparatory excursions (in three subjects).