Elsevier

Safety Science

Volume 47, Issue 10, December 2009, Pages 1429-1433
Safety Science

Human walks carefully when the ground dynamic coefficient of friction drops below 0.41

https://doi.org/10.1016/j.ssci.2009.04.005Get rights and content

Abstract

This study investigated the available and utilized friction during non-slip gait in level walking, and determined the limit which human starts to walk carefully to adapt to slippery surface. Sixteen floor–footwear-contaminant conditions with different slipperiness (dynamic coefficient of friction, DCOF, from 0.11 to 1.06) were employed. Fifteen harnessed Chinese male performed ten self-paced walking trials in each condition without slips. The utilized friction (COFu) was obtained from the maximum value of shear to normal ground reaction force ratio during the first 25% stance. ANOVA and Tukey tests showed three subsets with similar COFu, and confirmed the hypothesis that the utilized friction drops gradually when the available friction drops below a certain critical limit. Non-linear regression models were applied to the data to determine the COFu to be 0.20 and the limit of available ground friction which human starts to walk carefully to adapt to slippery surface (DCOFlimit) to be 0.41.

Introduction

Slips and falls cause fracture, disability, medical expenditure, and deaths (Courtney and Webster, 1999, Cumming and Klineberg, 1994, Runge, 1993). Slips are caused by multiple, interacting environmental and human factors (Redfern et al., 2001). When the extrinsic environmental factors introduced a potential slippery surface which could be anticipated, i.e. an icy and snowy surface (Gao and Abeysekera, 2004), human could evoke changes in intrinsic factors, i.e. gait patterns (Cham and Redfern, 2002), in order to reduce the slip probability. The most influential environmental factors are the surface contaminant and shoe type (Redfern and Bidanda, 1994). The most important outcome parameter that attracted researchers’ interest was the ground reaction force during foot contact, as reflected by numerous kinetics studies on slip event.

Theoretically, a slip does not occur when the available friction is greater than the required friction (Redfern et al., 2001). When the available friction becomes smaller than the requirement, a slip might occur. Cham and Redfern (2002) reported that successful non-slip gait could be achieved on slippery walking surfaces with less available friction by walking carefully to reduce the required friction by 16–33%. Therefore, the value of available friction quantifies the slip-resistance or slipperiness of a flooring condition. Despite that the measurement methods and devices varies (Chang et al., 2001), most researchers measure the dynamic coefficient of friction (DCOF), which is the ratio of shear to normal ground reaction force during a machine-induced sliding motion (Redfern and Bidanda, 1994). The safe limit of DCOF for level walking was suggested to be 0.20–0.40 by various research groups (Gronqvist et al., 1989, Gronqvist et al., 2003, Redfern and Bidanda, 1994, Strandberg, 1983). The required friction to walk on dry level surface without slip was represented by the required coefficient (RCOF) of friction or utilized coefficient of friction (COFu), which was reported to be 0.17–0.23 by various studies (Buczek and Banks, 1996, Burnfield et al., 2005, Redfern et al., 2001, Strandberg, 1983). Durance stance, peak normal and shear forces occur at 25% and 19% stance, respectively (Redfern et al., 2001). As the peak normal force is gained later than the peak shear force, the COFu is very large before 19% stance, which often happens within 0.1 s after heel strike (Strandberg and Lanshammar, 1981).

The suggested safe limits of available friction, as represented by DCOF value, were mostly obtained from measured data by slip-simulating meters (Gronqvist et al., 1989, Gronqvist et al., 2003, Redfern and Bidanda, 1994). However, human does walk carefully as demonstrated by significant gait changes when anticipating a slippery condition (Cham and Redfern, 2002). This study investigated the relationship between the available friction and the utilized friction during non-slip gait in level walking. The hypothesis is that the utilized friction would drop gradually when the available friction drops below a certain critical limit. Finally, non-linear regression models were applied to fit the curve to determine the limit which human starts to walk carefully to adapt to the slippery surface.

Section snippets

Methods

Two walking surfaces (wood, cement), two kinds of footwear (safety shoe, cloth shoe) and four surface contaminants (dry, sand, water, and oil) were used in order to provide a wide range of available ground friction for this study. A pair of anti-skid safety shoe and a pair of cloth sport shoe of size 42 (length = 265 mm) were used (Fig. 1). The anti-skid safety shoe conformed to the main regulations provided by the EEC/89/686 European Directive with harmlessness, comfort, solidity, and protection

Results

The 16 conditions had the DCOF value ranging from 0.11 to 1.06 (Table 1). The measurements were highly repeatable as shown by the small standard deviation values ranging from 0.01 to 0.06, which were only 2–9% of the mean values.

A total of 2400 trials were collected during the human walking test. Three and 18 trials were discarded from the wood/oil/safety shoe condition and the wood/oil/cloth shoe condition, respectively, due to slip occurrence. All trials from the other fourteen conditions

Discussion

The DCOF value of the sixteen conditions in this study ranged from 0.11 to 1.06 and provided a wide range of available ground friction to investigate its effect on the utilized friction for non-slip gait. The range was comparable to 0.11–0.57 in Myung and Smith’s study (1997), 0.10–0.75 in Dura et al.’s study (2005) and 0.16–1.43 in Hanson et al.’s study (1999). From Fig. 2a, the DCOF values distributed well along the whole X-axis. The only two gaps were from 0.30 to 0.55 and from 0.80 to 1.05.

Conclusions

This study showed that the utilized friction for normal non-slip gait on non-slippery level surfaces represented by the utilized coefficient of friction (normal COFu) was determined to be 0.20. Such utilized friction would drop gradually when the available friction, as represented by the dynamics coefficient of friction (DCOF), drops below a certain limit. Such limit of available friction for human to walk carefully to adapt to the slippery surface (DCOFlimit) was determined to be 0.41.

Acknowledgement

This study was financially supported by the Hong Kong Occupational Safety and Health Council.

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