Postural stability was measured using a force balance platform (Bertec Corp., Columbus, OH), which measured the coordinates of the center of foot pressure (CoP) during upright stance in both anteroposterior (AP) and mediolateral (ML) directions. The platform was connected to a personal computer and set at a sampling frequency of 1000 Hz.
Participants were asked to remove their shoes and stand still with their arms by their sides, feet parallel and 15 to 20 cm apart, on the center of the platform. Study participants wore their habitual distance spectacle prescription (if required) rather than a best corrected refraction to avoid any potential limitations a trial frame might have on the peripheral field of view. As the fixation target was adjusted to participant eye level, multifocal spectacle wear was permitted; there was no head flexion or downward eye gaze that might otherwise impair balance.
24 Stability was examined under two visual test conditions, which were presented in a randomized order: both eyes open, on the firm surface of the platform (Fi); or both eyes open, on a foam-rubber surface (Fo) placed on top of the platform. The purpose of standing on the foam-rubber surface was to disrupt the somatosensory contribution to balance control. During upright standing, mechanoreceptors on the soles of the feet detect displacements of skin indentation, providing afferent information that facilitates appropriate corrective changes in lower limb muscle activity to maintain balance.
25 The use of a foam surface disperses foot pressure, thus affecting the reliability of information received from these cutaneous receptors. Foam has been used extensively as a method of disrupting the somatosensory contribution to balance, and it correlates well with dynamic (i.e., moving platform) posturography methods in isolating the vestibular input to postural control.
26–28 Postural stability was measured under two situations: (1) quiet stance, and (2) whilst performing a mental arithmetic task. Details are described below. Participants were instructed to rest on a chair for at least 3 minutes between tasks to minimize fatigue. Each participant had a 30-second familiarization period under each test condition and situation (i.e., Fi, Fo, quiet stance, mental arithmetic) prior to the start of the test procedure.