Postural compensations to the potential consequences of instability: kinematics

Previous work has shown that central set can significantly affect postural control. Fear of falling, which is based on the perceived risk of injury as a result of instability, may act as a contributing source of central set. This study addressed an essential component of fear of falling by manipulating central set and altering postural threat. Eight healthy male subjects (23 +/- 5.67 years) were exposed to five repeated external perturbations under conditions which modified the consequences of a loss of balance: a non-threatening environment in which subjects stood al ground lever (LO), and a threatening environment (HI) in which they stood at the edge of an elevated platform (0.76 +/- 0.05 m). Two questions were of interest: How is the centre of mass regulated when there is an increased threat to a loss of balance? Is the ability to habituate to repeated perturbations affected by an increased threat to a loss of balance? The results demonstrated that the strategy for postural control is influenced by environmental constraints that alter postural threat, and that habituation to repeated perturbations occurs independent of the potential consequences of instability. Two subjects stepped following the initial perturbations delivered in the LO condition. When the potential consequences of a loss of balance were more severe, seven of the eight subjects enforced tighter control over centre of mass (COM) kinematics. Specifically, the COM starting position was more posterior, the range of COM displacement was significantly reduced and the time to peak COM velocity occurred significantly earlier. Habituation of the postural response was demonstrated in both conditions as a progressive increase in COM displacement with repeated perturbations. These findings contribute to a better understanding of the effects of central set on postural control and provide insight into how postural control is managed by individuals who are concerned with the consequences of a loss of balance. (C) 1997 Elsevier Science B.V.