A major problem for human standing posture is a high centre of gravity (COG) maintained over a relatively small base of support. The body, therefore, has a high potential energy, leading to priority of equilibrium control during almost all motor tasks including quiet standing. Research on postural control has focused mainly on two types of study. One type evaluates balance with respect to external conditions. Unexpected external disturbances reveal centrally programmed stereotyped postural responses. Afferent feedback also influences posture when the initial setting is disturbed. The second type evaluates postural adjustments to anticipated internal disturbances of balance and reveals feedforward postural adjustments (for review, see Oddson, 1990;Dietz, 1992). By feedforward, we mean that the controller predicts an external input or behaves using higherorder processing rather than simple negative feedback of a variable. Fewer studies have dealt with the problem of maintaining balance during quiet relaxed stance. Hellebrandt (1938) introduced the concept of the stretch reflex strategy, or 'geotropic reflex' where the shift of the COG constantly stimulated stretch afferents of postural muscles that then contracted reflexively. This strategy was questioned since the angular motion at the ankle was less than necessary to elicit a stretch reflex (Kelton & Wright, 1949). Subsequent studies supported a central organization of posture that did not regulate the ankle angle or muscle length but a more global parameter such as the position of the COG (Gurfinkel et al. 1980;Dietz, 1992). Thus, the system strategy replaced the stretch reflex strategy whose role was then limited to Journal of Physiology (1999) 1. We studied quiet stance investigating strategies for maintaining balance. Normal subjects stood with natural stance and with feet together, with eyes open or closed. Kinematic, kinetic and EMG data were evaluated and cross-correlated. 2. Cross-correlation analysis revealed a high, positive, zero-phased correlation between anteroposterior motions of the centre of gravity (COG) and centre of pressure (COP), head and COG, and between linear motions of the shoulder and knee in both sagittal and frontal planes. There was a moderate, negative, zero-phased correlation between the anteroposterior motion of COP and ankle angular motion. 3. Narrow stance width increased ankle angular motion, hip angular motion, mediolateral sway of the COG, and the correlation between linear motions of the shoulder and knee in the frontal plane. Correlations between COG and COP and linear motions of the shoulder and knee in the sagittal plane were decreased. The correlation between the hip angular sway in the sagittal and frontal planes was dependent on interaction between support and vision. 4. Low, significant positive correlations with time lags of the maximum of cross-correlation of 250-300 ms were found between the EMG activity of the lateral gastrocnemius muscle and anteroposterior motions of the COG and COP during normal stance. Narrow ...

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