Should the instructions issued to the subject in traditional static posturography be standardised?

The postural ability of a subject is usually evaluated through the observation of the centre of pressure parameters obtained through posturography. These parameters are known to be sensitive to various factors and standards have consequently been proposed for data acquisition and analysis. A factor usually not taken into due consideration but likely to influence the postural exam is the kind of standing posture (e.g. natural or immobile) a subject is instructed to maintain. This study aimed at investigating whether instructions issued in a traditional static posturographic test influence its outcome and hence should be considered in the standardisation of the posturography protocols. Two groups of young healthy subjects were each issued one of two common instructions, "stand quietly" or "stand as still as possible", by means of projected instructions. Differences between the two groups were investigated for commonly calculated centre of pressure parameters. All these parameters, but the mean frequency, were significantly different, with variations in the range between 8% (mean velocity) and 71% (confidence circle area). These results suggest that instructions given to the subjects strongly influence the outcome of posturography and should, hence, be standardised.