Are there anticipatory segmental adjustments associated with lower limb flexions when balance is poor in humans?

For a leg raising task performed in a sagittal plane, it has been shown that body balance instability can suppress anticipatory postural adjustments (APAs). The aim of this study was to determine whether the global (centre of mass) postural adjustments were replaced by local (segmental) ones, which were compensating each other and resulting in a lack of global APAs. Six healthy subjects must perform a lower limb flexion from two initial postures, corresponding to a bipedal (Fbu) and an unipedal (Fuu) stance. Kinematics of postural adjustments were recorded with accelerometers. The results showed that in Fbu the kinematics have large durations of APAs, contrary to Fuu where there are none. They showed also that during the voluntary movement the magnitudes of the segmental postural accelerations were equal or superior in Fuu than in Fbu on the anteroposterior and lateral axes, where balance is poor. Also while, on the contrary, the magnitudes are reversed on the vertical axis. These results suggest that firstly: (1) the absence of APAs can correspond to a strategic response for weak postural base configuration and secondly; (2) the local postural accelerations, depending to the axes, are linked to two different functions: to maintaining the balance and to performing the focal movement.