The influence of postural threat on the control of upright stance

In order to utilize static posturography as a tool with which to diagnose p athological balance disorders, it is necessary to understand the contributi on of psychological factors such as fear of falling. In this study we exami ned kinetic and kinematic parameters during quiet stance of eight young hea lthy adults standing under three conditions which posed increasing levels o f postural threat. Participants were required to stand quietly, both with a nd without visual information and with normal or reduced vestibular informa tion for 2-min periods at each of three surface conditions: ground level (l ow threat), 81 cm above the ground completely surrounded by a support surfa ce (medium threat) and 81 cm above ground at the edge of the surface thigh threat). In addition to calculating displacement and frequency measures for movements of centre of pressure (COP) and centre of mass (COM), a measure of stiffness was derived from the difference between COP-COM signal. Mean E MG activity was recorded bilaterally from anterior and posterior muscles of the lower and upper leg. A stiffening strategy was adopted when individual s stood under high threat conditions involving significant changes in kinem atic, kinetic and EMG variables. The A-P stiffness constant increased signi ficantly (27.5%) for the high threat compared to low threat condition, inde pendent of vision or vestibular information. Changes in stiffness were acco mpanied by: (1) backward shift of the mean A-P position of COP and COM, (2) increased mean power frequency and decreased amplitude displacement of the COP and (3) decreased amplitude displacements of the COM. Of the significa nt changes observed in mean EMG activity with increased postural threat, on ly tibialis anterior was significantly correlated with increased stiffness, while activity in soleus, medial gas trocnemius and rectus femoris was hig hly correlated with mean position of COP.