Postural stability when walking and exposed to lateral oscillations

Abstract

The role of accelerations encountered in daily life in causing postural instability or falls is not well understood, especially when walking. Lateral oscillations disturb walking stability during train journeys but it is desirable that passengers feel comfortable and they do not fall due to loss of balance. This research was designed to improve understanding of the mechanisms of walking stability and to construct a model for predicting the probability of losing balance in walking railway passengers. Postural stability was assessed using both a subjective measure (the reported probability of losing balance) and objective measures of centre of pressure (COP).

The first of four experiments investigated how postural stability when walking depends on the frequency (0.5 to 2.0 Hz) and the magnitude (0.1 to 2.0 ms-2 r.m.s.) of transient lateral oscillation. The probability of losing balance reported by 20 subjects was used to obtain stability thresholds for the lateral accelerations experienced in trains. It was shown that postural stability cannot be predicted solely from either the peak or the r.m.s. value of lateral acceleration but can be predicted from the peak or the r.m.s. velocity of sinusoidal lateral oscillation.

The second experiment with 20 subjects investigated the extent to which a hand support (rigid vertical bar) modifies postural stability when walking during lateral oscillation. The hand support improved postural stability at all frequencies (0.5 to 2 Hz) and at all velocities (0.05 to 0.16 ms-1 r.m.s.). The improvement in postural stability from holding the support and the forces applied to the hand support were independent of support height and were greater during perturbed walking (30-50% when the support was held throughout the oscillation, 20-30% when the support was held if required ) than during normal walking (15%). When it was required, subjects preferred to hold the hand support at a height of 126 cm above the surface supporting the feet.

The third experiment investigated how the postural stability of walking people is influenced by the waveform of lateral oscillations. Twenty subjects were exposed to a range of 1 Hz and 2 Hz lateral oscillations having the same r.m.s. magnitude but different waveforms. The reported probability of losing balance and the lateral COP velocity was found to be sensitive to the peak magnitude of the oscillations especially at 1 Hz. It was concluded that the r.m.s. value is not an optimum method for predicting the postural stability of walking subjects exposed to low frequency lateral oscillations and that peaks in the motion should also be considered.

The influence of subject characteristics (age, gender, weight, stature, shoe width, fitness) on postural stability was investigated in a fourth experiment with 100 subjects. Age had the greatest influence on postural stability, with an increase in COP measures with increasing age. There was no significant effect of any subject characteristic on self-reported probability of losing balance. The stability thresholds of young males (determined in the first experiment) can therefore be applied to a wider age range (18 to 70 years) of fit and healthy people, including females.

The subjective experimental findings have been used to develop an empirical model for predicting the probability of losing balance in walking people exposed to lateral oscillation. Analysis of the objective measure of COP revealed that the ‘stepping strategy’ is the principal means of maintaining postural stability when walking is perturbed by lateral oscillation. The developed model can be used to predict the perceived risk of fall when walking and exposed to lateral oscillations from the peak and r.m.s. velocity of oscillations. The model predicts the perceived probability of losing balance during exposure to various waveforms of oscillations and is applicable to males and females with variety of ages (18 to 70 years).

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Identifiers

ORCID for M.J. Griffin: orcid.org/0000-0003-0743-9502

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