Academic journal article Human Factors

The Retention of Balance: An Exploratory Study into the Limits of Acceleration the Human Body Can Withstand without Losing Equilibrium

Academic journal article Human Factors

The Retention of Balance: An Exploratory Study into the Limits of Acceleration the Human Body Can Withstand without Losing Equilibrium

Article excerpt

INTRODUCTION

Everyone who regularly travels by public transportation has seen someone lose his or her equilibrium because of acceleration of the vehicle. In the Netherlands, passengers have been swung against the door and have fallen out of the bus in a sharp turn, resulting in serious injuries or even death. A recent poll (Consumentengids, 1995, pp. 128-131) shows that complaints regarding excessive braking and acceleration are common, especially in buses and trams. The records of the Consumer Safety Institute show that each year 2300 passengers need first aid treatment in a hospital because of accidents occurring in the Dutch public transportation systems (Mulder, 1993). In accidents in which people lost their balance (claimed in 1227 cases), the direct cause is not unequivocally clear. However, it seems reasonable to look for a clue in the nature of the accelerations or decelerations to which the victims were exposed.

Worldwide, some research has been done on the effect of acceleration on postural balance, but this research is of a fragmentary nature and has frequently focused on compensatory responses to very small disturbances (Allum, 1983; Dietz, 1986; Nashner, Woollacott, & Tuma, 1979). Guedry (1974) reported on measurements of the (lower) perceptual threshold for accelerations, and human reactions to extreme stresses are known from military aviation research. However, there is a paucity of data in the international scientific literature on experiences with accelerations in everyday life and their role in causing people to lose their postural balance completely.

In the 1940s, however, Jongkees and Groen (1942) investigated a series of sudden accelerations causing people to lose their balance. They used a small vehicle to accelerate 50 standing participants in a forward, backward, and side-ward direction. They found that healthy individuals who stand upright with closed eyes and feet together are able to endure an acceleration of up to 76 cm/[s.sup.2] in a backward direction, a forward acceleration of up to 48 cm/[s.sup.2], and a sideward acceleration of up to 33 cm/[s.sup.2]. (An acceleration in a backward direction is produced by a sudden backward movement of the floor surface - in their case a small vehicle - relative to the body.)

The stimulus Jongkees and Groen used in their experiments was a sudden, constant acceleration. It is arguable that higher limiting values might be obtainable if the level of acceleration were increased gradually rather than in steps: This would give the participant's balance reflexes some time to adapt to the disturbance.

In practice, different figures from those Jongkees and Groen (1942) reported are found. The standards for acceleration and deceleration in European road traffic define acceleration levels of 100 to 150 cm/[s.sup.2] in a longitudinal direction as manageable and decelerations of 150 cm/[s.sup.2] as comfortable. The most "sensitive" road users (i.e., standing bus passengers; Westerduin, 1974) were taken as the criterion in choosing these values. If we evaluate these data in light of Jongkees and Groen's findings, we can see that these bus passengers are at minimum assumed to be capable of holding on tightly. Accelerations in a transverse or vertical direction should not exceed a value of 50 or 25 cm/[s.sup.2], respectively (Westerduin, 1974). No justification is provided for the European road traffic standards figures, and we do not know of any fundamental research on this topic.

The chief aim of the present study is to explore the most extreme human limiting values for linear accelerations - that is, the values obtained under the most favorable sensory conditions. The scope of the study was restricted to stationary people (either standing still or moving steadily in relation to the surface of the earth, such as in the bus) exposed to a sudden (constant or gradual) acceleration or deceleration. The data obtained in the laboratory were compared with situations occurring during travel on public transport. …

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