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Control of medio-lateral stability during rapid voluntary step initiation with preferred and non-preferred leg: is it symmetrical?

Postural stability during stepping initiation (SI) is controlled in advance via medio-lateral (ML) ‘anticipatory postural adjustments’ (APA) (Nissan and Whittle 1990). The literature reviewed shows that in studies on SI in healthy subjects, the leg that initiates stepping is either self-chosen by subjects (it is therefore the preferred one) or is imposed by the experimenter. In both the cases, subjects initiated stepping with the same leg throughout the experiment. If control of ML postural stability is analysed when one leg is systematically used for stepping, the assumption is implicitly made that stepping with the other leg will provide symmetrical data. Such an assumption has strong implications for the clinician because asymmetry in SI is frequently considered to indicate pathology (Hesse et al. 1997). However, this assumption was presumably made with the sole goal of simplifying data collection and analysis. To date, it has not been subjected to experimental verification. There is growing evidence in the literature supporting ‘the concept of side dominance in the postural component of apparently symmetrical motor tasks’ (Gilleard et al. 2008), such as pointing to a target with the preferred or the non-preferred arm (Teyssèdre et al. 2000), bilateral forward pushing with both hands (Mezaour et al. 2009) or rising to stand from a chair (Gilleard et al. 2008). The present study tested the hypothesis that in young healthy subjects, the control of ML postural stability during SI is asymmetrical.