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The identification of factors in the systematic evaluation of slip prevention on icy surfaces
International Journal of Industrial Ergonomics 28 (2001) 303–313
The identification of factors in the systematic evaluation of slip prevention on icy surfaces John Abeysekera*, Chuansi Gao Division of Industrial Ergonomics, Department of Human Work Sciences, Lulea( University of Technology, S-971 87 Lulea(, Sweden Received 24 July 2000; received in revised form 30 November 2000; accepted 26 April 2001
Abstract Slips and falls on icy roads often result in fractures or sprains and is a major problem in Nordic countries. Walking trials by 25 subjects wearing four types of winter shoes on five different icy walking surfaces provided subjective and objective measures of tendency to slip and number of slips, respectively. Since friction is a major determinant of a slip, the influence of material spread on icy surfaces, the surface temperatures and the shoe soling characteristics versus the Coefficient of Friction (COF) of the shoes were measured. Sand and gravel on icy roads had positive effects on improving COF. The study revealed that the aetiology of slips and falls is multi-faceted and attempts to solve the problem must adopt a systems approach. Perception of risk, aging, training, experience and postural balance are other factors to be considered in preventing slips and falls. Future research should concentrate on the degree of impact of each factor to the aetiology of slips and falls, which can help to decide priority action in preventing slips and falls. Relevance to industry The personal protective devices used by outdoor workers during winter season have to provide two types of protection, namely, protection from an occupational hazard and protection from the cold climate. Safety shoes used on snow or ice covered surfaces add a third type of protection, namely, an anti-slip quality of the shoe. The 3rd protection can be achieved from among other things, by improving the friction of the shoe soles. r 2001 Elsevier Science B.V. All rights reserved. Keywords: Icy roads; Anti-slip material; Footwear; Coefficient of friction
1. Introduction and Literature Slips and falls can result in minor or major injuries (e.g., sprains and fractures) or permanent disabilities to the victims. Since the rehabilitation *Corresponding author. Tel.: +46-920-910-00; fax: +46-920910-30. E-mail address: [email protected] (J. Abeysekera).
and the healing process of these types of injuries usually take a long time, they can be costly with higher insurance payments and compensation claims both to society and to the enterprises (Gro. nqvist, 1995a). In Nordic regions, slips and falls on icy roads are a major problem during winter. Foot slippage caused 43% of all falls and 16% of all accidents in Nordic countries of which the majority had occurred on surfaces covered with ice and snow (Lund, 1984a,b).
0169-8141/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 8 1 4 1 ( 0 1 ) 0 0 0 2 7 - 0
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It is known that the elderly are more vulnerable to slips and falls. In the north of Sweden the snowrelated injuries to the elderly over 60 yr have accounted for 37% of the total injuries to pedestrians (Lund, 1984a,b). Bj.ornstig et al. (1997) have reported that the ‘‘cost’’ of medical care for slipping injuries had been almost the same as the ‘‘cost’’ of all traffic injuries during the same period. The literature also reveals that trying to escape from falls can damage the back. It is reported that the muscle forces exerted in trying to prevent a fall which can cause harmful loading of the spine (Lavender et al., 1988) and injuries to the vertebral bodies can result in permanent injuries to the back (Wickstrom et al., 1985). Crowded orthopaedic clinics during winter in the north of Sweden are a common sight. Although this can be assumed to be due to victims of fractures from slips and falls on snow, no systematic investigation seemed to have been done to ascertain the causes of overcrowded orthopaedic clinics and no genuine attempt seems to have been taken to prevent fracture injuries during winter. Risk factors for slipping can be divided into (a) primary and (b) secondary (Gro. nqvist, 1995a). A major risk factor for slips is defined as poor grip and low friction between footwear and the underfoot surface (Smith and Falk, 1987). When the Coefficient of Friction (COF) between the shoe and walking surface does not provide sufficient resistance to counteract the forward resulting force at the point of contact, a slip can occur. There are critical gait phases in the slip, namely, heel strike (forward slip) and toe off (rearward slip) of which the forward slip is said to be more dangerous and more frequent (Leamon and Son, 1989). Postural control influenced by sensory modes helps to adapt the gait and when conditions are stable it is easier to adapt the gait (Gro. nqvist, 1995b). It is believed also that the risk of slipping is influenced to a great extent by the subjective awareness to the potential slipperiness of the shoe-floor coupling. Secondary risk factors include (a) extrinsic or environmental factors, e.g., inadequate lighting, slippery roads, etc., (b) intrinsic or human factors, e.g. old age, absence of perception of risk, etc., and (c) integrated or systems factors, e.g., using solings
of low friction properties, elderly walking on poorly lit snowy roads, etc. (Waller, 1978; Pyykko. et al., 1990). Hardness of rubber refers to its elasticity and is based on a measurement of the indentation of a rigid ball into the material (Gro. nqvist, 1995a). Hardness is one of the more widely used mechanical properties to describe footwear sole characteristics. It was reported that there was negative correlation between sole hardness and COF on lubricant/floor and on dry ice (91C to 101C), (Leclercq et al., 1994; Bruce et al., 1986). However there was no clear correlation on wet ice (01C). Surface roughness is defined by parameters such as Ra and Rz and Sm (ISO 468, 1982). Ra is the arithmetic average roughness or mean deviation of the profile. Rz is the height of the profile irregularities in ten points. Rtm is peak-to-peak roughness (Gro. nqvist, 1995a). It is reported that shoe sole roughness had a positive influence on COF. Successive abrasion treatment of soling made it rougher and recorded a higher COF (Manning et al., 1990). The second classic law of friction states that ‘‘the COF is independent of the apparent contact area’’. The apparent contact area does not reflect the true contact area which depends on normal force and liquid or solid contaminants between the interacting surfaces. However it is still under debate whether increased contact area should be recommended (Leclercq et al., 1994; Stevenson et al., 1989; Stevenson, 1997; Strandberg, 1985; Gro. nqvist, 1995a). Research in the past on slips and falls has concentrated more on oily and wet shop floor surfaces, and improving COF of shoes. Regarding improving COF, literature reveals some studies on anti-slip soling material (Bruce et al., 1986; Gro. nqvist, 1995a,b) and anti slip devices (Bruce et al., 1986; Gard and Lundborg, 1994; Noguchi and Saito, 1996). Research is lacking on combined effects of all factors that influence slips and falls on icy surfaces. Unless and until the effects of these integrated factors are researched and studied, it becomes extremely difficult to prevent slips and falls on icy surfaces. This investigation is one of the very first
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attempts of planning to carry out research on the systems approach strategy to prevent slip and fall accidents. This study, therefore, is a preliminary investigation aimed to ascertain the aetiology of slips and falls on icy surfaces and to develop a hierarchical model containing important factors with their degrees of impact and influence to the design of anti-slip shoes and anti-slip walking surfaces. This preliminary study in this long research process, consisted of subjective and objective testing of four different types of winter shoes. The results of this study can become a stepping stone for systematic research for an integrated or systems approach to prevent slip and fall accidents on icy roads.
2. Materials and methods In any ergonomic analysis users’ viewpoints are key issues. The subjective impressions of tendency to slip of a shoe or a walking surface can complement the objective data such as COF. Both subjective and objective data were collected while subjects participated in a walking trial wearing four types of shoes. Objective measurements of the footwear, namely, COF for each type of footwear as well as for each type of icy surface, centre of gravity (COG), hardness and roughness of soling and tread area were measured on each type of footwear.
observer could easily perceive as a slip. ‘Tendency to slip’ was defined as the feeling of the subject of the possibility to slip. The trials were carried out with the subjects being aware of the potential slipperiness of the walking surfaces. By direct observation the number of slips of each subject was counted for each pair of shoes. All walking trials were filmed on videotape. The direct observations made on the number of slips was checked and clarified by watching the video film. Footwear. Four different types (1–4) of new footwear (Figs. 1–4) were selected from Swedish manufacturers as well as from the market, considering different footwear properties, i.e., material, weight, center of gravity, sole and heel design, hardness and roughness, etc. The specifications of the four types of footwear are shown in Table 1. Subjects. Twenty-five subjects, 15 males and 10 females, 22–62 yr old with average age of 31.2 yr (S.D. 8.65), took part in the outdoor experiment. All subjects had more than 5 months experience of walking on icy roads and 7 months living experience in Nordic region. Icy Surfaces. Gravel, sand and salt are common materials spread on ice and snow for preventing slips and falls in Sweden. In order to reveal the interaction if any, and anti-slip effects between the materials and footwear, five different outdoor icy walking surfaces, i.e. pure ice, ice covered with
2.1. Outdoor walking trials Outdoor walking trials were carried out with 25 subjects who wore four different types of footwear and walked on five different icy surfaces (i.e., pure ice and ice covered with gravel, sand, snow and salt). One main aim was to obtain the subjective impressions. Gro. nqvist (1995a) defined ‘slipping’ as a sudden loss of grip, resulting in sliding of the foot on a surface due to lower COF than that required for the momentary activity. For the purpose of this study and walking trials, a ‘slip’ was interpreted as a temporary loss of foot balance or movement of the foot away from its intended path, which the
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Fig. 1.
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Fig. 2. Fig. 4.
gravel (F 4–8, 150 g/m2), sand (180 g/m2), snow (3–5 mm) and salt (9 g/m2) were used, each of which was 10 m long and 0.95 m wide. The walking surfaces were prepared in a shaded area to avoid the direct sunlight. During the experiment, the environmental dry air temperature was 0–21C. In order to control bias of the assessment, the type of footwear worn and the walking surface for each trial were selected at random. After each trial, each subject was asked to rate the tendency to slip by using 5-point scale, that is, 1-very high, 2-high, 3- moderate, 4-low, 5-very low. 2.2. Objective measurements Fig. 3.
Coefficient of friction. According to the first classic law of friction, the friction force is
Table 1 The specifications of the four types of footwear Type
Model/Code
Manufacturer
Out sole/heel tread design
Out sole and heel material
Upper part material
Weight (g) (size)
1 2
Gepard 1100 Stilex PU-boot, Occup. footwear 520 St(alex WS (steel toe safety boot) Art. Falcon, BN (farmer’s boot)
From market Swedish
Rough Smooth flat cleat, no ridges, no asperity Rough
Synthetic material ‘‘BioForm’’ Polyurethane
Synthetic material Leather
387 (41), 431 (43) 452 (40), 484 (42)
Nitril rubber, heat & oil resistant
Leather, Thinsulate fibre-fur lining
812 (40), 913 (42)
Lots of small ridges
Natural rubber
Natural rubber
824 (40), 898 (42)
3
4
Swedish
Swedish
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proportional to the vertical load (normal force) which exists between the actual surfaces Fm ¼ mFN ; where Fm is the friction force, FN is the normal force, and m is the COF. The kinetic COF was assessed with a stationary step simulator, using a method developed at the Finnish Institute of Occupational Health (Gro. nqvist, 1989). The apparatus is equipped with a cooling system. It can simulate the movements of a human foot and the forces applied to the underfoot surface during an actual slip. The ice is formed on a force platform, the surface temperature can be adjusted (Gro. nqvist, 1995a). The left shoe of each footwear was fixed on the last (artificial foot) of the simulator rig (Fig. 5). For each footwear and icy surface a total of five measurements was taken and the averages were recorded. Sole hardness and roughness. As revealed by other researchers (Manning et al., 1991; Tisserand, 1985; Gro. nqvist and Hirvonen, 1994, 1995), hardness and roughness had significant effects on slipperiness. In this study, the out sole hardness and roughness were measured by using a Hardness Tester (made in Japan) and Sultronic 10Ra Roughness Tester, made in UK, respectively.
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These tests were also carried out at the Finnish Institute of Occupational Health. Centre of gravity of footwear. It is assumed that a wearer can better control one’s balance while walking or during a slip, if the COG of the shoe is situated at or closer to the fulcrum of the wearer’s foot (Abeysekera and Khan, 1997). The location of the COG of the 4 types of shoes was used to test this assumption. Due to the different sizes of the same type of shoes having different locations of COG, two parameters ‘‘d’’ and ‘‘d=L’’ were introduced in order to describe where the COGs were located. The d value is influenced by the sizes of the footwear, whereas d=L is independent of footwear sizes if footwear are of the same type. Therefore, the COG location was interpreted as a distance ratio d=L (see Fig. 6). Tread areas of footwear. The out sole contact areas on surfaces were measured by spreading ink on the bottom of the sole, standing static on a piece of white paper to get the imprint of the static position. To get the dynamic imprint of the shoe the body weight (75 kg) was moved forward and backward (at toe and heel), so that the whole out sole was in contact with the paper. The areas of the imprints (static and dynamic) were measured with a Planimeter (Model KP 27, made in Japan). 3. Results 3.1. Subjective ratings of tendency to slip Myung et al. (1993) argued, a large sample size (n ¼ 100) helped satisfy the central limit theorem,
Fig. 5. The stationary step simulator rig (Gro¨nqvist, 1995a).
Fig. 6. The location of COG of footwear (described as the distance ratio).
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Fig. 7. Subjective assessment of tendency to slip.
which allows the use of a parametric test for subjective ratings. In this study, there were 500 trials. Therefore, ANOVA was used to analyse the interaction between footwear and icy surfaces. Two-way within subject ANOVA (4 5) showed that there was no interaction effects between footwear and icy surfaces on tendency to slip (F ¼ 1:025; P ¼ 0:425 > 0:05; two-tailed) as shown in Fig. 7. Friedman test showed that there was significantly different effect of different footwear in the tendency to slip (K2r ¼ 10:746; N ¼ 23; df=3, P ¼ 0:013o0:05). The rank of tendency to slip (from the lowest to the highest) was footwear type 4, 1, 3, and 2. Type 2 was most slippery, while the type 4 was the least slippery one. Friedman test showed that there was highly significant effect of different icy surfaces on the tendency to slip (K2¼ r 52:324; N ¼ 25; df=4, P ¼ 0:000o0:01). The rank of tendency to slip (from the lowest to the highest) is ice covered with sand, gravel, salt, snow, and pure ice (Fig. 7). The ice covered with sand was the least slippery surface, while the pure ice was the most slippery one of the five different icy surfaces.
3.2. Direct observations of slip The number of slips (defined earlier) observed for each type of shoes on each walking surface during the walking trials of the subjects are given in Table 2. Chi-square test of the directly observed number of slip showed that there were highly significantly different effects on the slip among different types of footwear (K2 ¼ 32:462; df=3, P ¼ 0:000o0:01), and among different types of icy surfaces (K2¼ 514:609; df=4, P ¼ 0:000o0:01), with the least observed number of slips on ice covered with sand, while with the most on pure ice. The slip resistance ranks are as follows: ice covered with sand>gravel>salt>snow>pure ice; footwear type 3>4>1>2 (see Table 2). The results of direct observation for walking surfaces were consistent with that of subjective ratings.
3.3. Objective measurements of COF The results of the objective measurement of COF of surfaces and subjective ratings are shown in Table 3 using footwear type 2 as a reference
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Shoe 1 Shoe 2 Shoe 3 Shoe 4 Total Rank of surfaces (least to most)
Gravel
Sand
Snow
Ice
Salt
Total
Rank of footwear (least to most)
17 21 15 9 62 2
2 2 1 1 6 1
31 36 21 24 112 4
94 108 53 75 330 5
18 33 19 18 88 3
162 200 109 127
3 4 1 2
Table 3 Subjective ratings and COF on icy surfaces
Rating COF (S.D.)
Pure ice (01C)
Ice covered with snow (3–5 mm)
Ice covered with sand (180 g/m2)
Ice covered with gravel (150 g/m2)
Ice covered with salt (9 g/m2)
1.16 0.065 0.003
3.18 0.054 0.007
4.24 0.280 0.011
3.22 0.266 0.023
3.41 0.130 0.006
Table 4 Kinetic COF of four types of footwear on pure ice (01C) Footwear
Type 1
Type 2
Type 3
Type 4
Kinetic COF S.D.
0.080 0.001
0.065 0.003
0.077 0.002
0.078 0.005
shoe. The results of kinetic COF of four types of footwear on pure ice (01C) are shown in Table 4. 3.4. Correlation between subjective and objective measurements The correlation between subjective ratings and COF shows that there is significant correlation between kinetic COF and ratings (r ¼ 0:900; p ¼ 0:037o0:05) (see also Table 2). The results of sole hardness, roughness, and COG are shown in Table 5. The hardness of the four types of shoe soles are of the same magnitude. Statistical analyses did not show any significant correlation between subjective ratings of footwear slipperiness and COG, hardness, and roughness. There is no significant correlation between COF and hardness ( p ¼ 0:559 > 0:05), roughness ( p ¼ 0:140 > 0:05) on pure wet ice (01C).
The results of the out sole contact areas are shown in Table 6. There is no significant correlation between subjective rating and dynamic area of contact ( p ¼ 0:256 > 0:05), nor static area of contact, ( p ¼ 0:167 > 0:05). Since the results of subjective ratings were consistent with direct observations of the tendency to slip, subjective ratings were used to study the correlation effects with the other characteristics of the shoes, e.g., COF, COG, hardness, roughness and tread areas. The summary of correlation results is shown in Table 7.
4. Discussion Although the four types of shoes were selected considering different slipperiness properties, no systematic procedure could be observed in terms of a good experimental set up as the four shoes varied in material, weight, size and sole properties. The five different walking surfaces or roads with pure ice, ice covered with gravel, sand, snow and salt were constructed on a flat surface situated outdoors. A safety harness was not used as this will negate the reality in walking on slippery surfaces. In real life pedestrians change their gait
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Table 5 The sole hardness, roughness, and COG of footwear Type
Sole hardnessa (Shore A) (S.D.)
Sole roughnessa (mm) (S.D.)
The distance from COG to heelFd (cm)
COGb d/L
1
60 (1.66)
17.7 (4.29)
2
58 (2.26)
4.3 (1.12)
3
57 (1.06)
24.1 (6.74)
4
60 (2.01)
16.5 (2.93)
13.2(s-41) 14.2(s-43) 11.6(s-40) 12.2(s-42) 12.7(s-40) 13.7(s-42) 10.3(s-40) 10.6(s-42)
0.47 0.47 0.42 0.42 0.44 0.45 0.37 0.37
a ANOVA showed that there were significant differences in the hardness (F ¼ 4:624; p ¼ 0:008o0:01) and roughness (F ¼ 20:917; p ¼ 0:000o0:01) of the four types of footwear. b The location of COG is expressed as the distance ratio (d=L; see Fig. 6).
Table 6 The out sole contact areasa Footwear (size: 42) 2
Static contact area (cm ) Dynamic contact areab (cm2)
1
2
3
4
42.9 61.0
26.3 56.7
34.6 68.9
48.1 78.2
a Only one measurement was made on each shoe (static and dynamic). b The dynamic contact area was measured when moving the body weight forward and backward (at toe and heel). Significant correlation between subjective rating and dynamic area of contact ( p ¼ 0:256 > 0:05), nor static area of contact, ( p ¼ 0:167 > 0:05).
based on their perception of risk. On the other hand if a safety harness was used the subjects would not have walked the way they do in real life. Since this study was not restricted to testing of material (in which case a safety harness can be used) and the chances of potential injury was considered remote, a safety harness was not used.
Many studies in the past have shown that COF is a good indicator of the slipperiness of shoes and/ or floors. The coefficient of kinetic friction can be influenced by many factors, including properties of the walking surfaces, e.g., material spread on icy surfaces, surface temperatures of ice (Gro. nqvist and Hirvonen, 1994), properties of the shoe soling material, e.g., rubber, synthetic, polyurethane, etc., (Gro. nqvist and Hirvonen, 1994), properties of anti-slip devices attached to shoe soles, e.g., chains, crampons, metal studs, etc., (Bruce et al., 1986; Gard and Lundborg, 1994) and shoe soling material characteristics, e.g., roughness (Manning et al., 1990), hardness, etc. The risk of slipping as mentioned before is also influenced by environmental factors, e.g., uneven surfaces, inadequate lighting, etc., and human factors, e.g., perception of risk, aging, postural control, etc. It is visualized that most slipping accidents occur unexpectedly when the victim is unaware of the hazard. Therefore the perception of the hazard is important to consider in preventing slip and fall accidents. In
Table 7 Correlation between subjective ratings of tendency to slip and other properties of footwear
Subjective ratings
COF
COG
Hardness
Roughness
Sole contact area (static)
Sole contact area (dynamic)
r ¼ 0:900; p ¼ 0:037
NSb
NS
NS
NS
NS
a a b
There is significant correlation. NS: Not significant.
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the present study the subjects were aware of the slipping or non-slipping conditions of the walking surfaces. The tendency to slip in the walking trials would therefore be influenced very much by the characteristics of the shoe soling and/or road conditions. The present study investigated the correlation between COF and soling characteristics and antislip materials spread on icy roads. Further, the influence of intrinsic and extrinsic risk factors namely, loss of postural control by the wrong placement of the shoe COG (e.g. top heavy) and wet ice (e.g. melting environment temperature), respectively, were studied. Thus the approach investigated in preventing slips on ice in this study was the integrated or systems approach. Improving COF with anti-slip devicesFalthough it seems to be an effective method used by the elderly to prevent slipping (Gard and Lundberg, 1994)Fwas not included in this study as this would interfere with the characteristics of the soling of the shoes studied. It is also believed that the devices can be a hindrance to wearability of the shoes. All four types of shoes had low COF on pure ice at 01C and the differences between the COF were not significant. Therefore on pure ice the characteristics of the soling seems to have hardly any effect in preventing slipping. Strategies to prevent slipping under those conditions should be different, such as changing gait patterns, improving the balancing properties of the shoe, e.g., the COG of the shoe placed closer to the fulcrum of the foot, etc. using anti-slip devices, etc. The materials spread on the road had a significant effect on COF (Table 3). Further, the subjective ratings for the different materials significantly correlated with COF. Although there were no interactive effects of different types of shoes and road characteristics, when they were tested separately they were significantly correlated with COF values. The correlation results (Table 7) indicate that although the subjects could perceive the effects of COF on the shoes, the effects of COG, hardness, roughness, and sole contact areas cannot be perceived by the subjects. Further the effects if any by these characteristics (except COF) to
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prevent slipping would have been also negligible in the shoes tested. Significant differences were seen in the hardness (F ¼ 4:624; p ¼ 0:008o0:01) and roughness (F ¼ 20:917; p ¼ 0:000o0:01) of the four types of footwear (Table 5). It was also observed that there was no significant correlation between COF and hardness and roughness (Table 7). This may be due to the fact that COF values were measured only with temperature at 01C. In this study the characteristics of the shoes were analyzed separately with subjective ratings or COF measurements but the combined effect of hardness, roughness, COG and tread areas on COF of the shoe has to be further tested. Results of the combined effects may provide useful information in the design of anti-slip shoes and surfaces. Based on the multiplicity of factors that influence a slip on an icy surface, an area of research that should be emphasized for the future is the design of soling with optimum COF which is durable and have optimum wearability. Preliminary trials carried out by Noguchi and Saito (1996) of Japan with permanent fixtures of anti-slip material on relevant parts of the shoe soling are considered an area of research to be pursued.
5. Recommendations and future research 1. This study is limited in terms on number and ages of subjects, the effect of surface temperature, the variations of hardness, roughness and sizes of shoes used. From the results of this study and the information from literature, it is possible to provide some guidelines and trends to prevent slipping under the following headings, viz. preventing primary and secondary risk factors, vulnerability to slip, and strategies of slip prevention, (Fig. 8). 2. Slips and falls on icy surfaces is a multifaceted problem as revealed in this study. Only through an integral approach an optimum solution could be found to prevent slip accidents, towards which future research must be directed. 3. The influence from COG, tread areas, roughness and hardness, and sole material on COF
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Fig. 8. Prevention of risk factors, vulnerability to slip and strategies of slip prevention.
may be considerable. The combined effect of the shoe characteristics will shed more light to the new design of anti-slip shoes. 4. Permanent anti-slip devices on shoe soling with optimum COF without hindrance to wearability, deserves further investigation. 5. Perception of risk, training and experience are important. The influence of these factors to the prevention of slips and falls has to be investigated. 6. Subjective impressions are as important as objective measurements in testing personal protective wear such as shoes, gloves, etc.
Acknowledgements We acknowledge with thanks the financial support received from the Swedish Council for Work Life Research (RALF and now VINNOVA) and COLDTECH , Lule(a, Sweden to carry out this research. We also express our gratitude to Dr. Raoul Gro. nqvist of the Liberty Mutual Research Center for Safety and Health, Hopkinton, MA, USA and Mikko Hirvonen of the Finnish Institute of Occupational Health and Safety, Helsinki and Glenn Lundborg of Lule(a University of Technol-
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ogy for their help in carrying out COF tests and walking trials, respectively.
References Abeysekera, J., Khan, Z., 1997. Slipping and falling accidents on icy surfaces: a case study from northern Sweden. Proceedings of International Symposium on Problems with Cold Work, Stockholm, Sweden. Bjo. rnstig, U., Bjo. rnstig, J., Dahlgren, A., 1997. Slipping on ice and snowFelderly women and young men are typical victims. Accident Analysis and Prevention 29 (2), 211–215. Bruce, M., Jones, C., Manning, D., 1986. Slip resistance on icy surfaces of shoes, crampons and chainsFa new machine. Journal of Occupational Accidents 7, 273–283. Gard, G., Lundborg, G., 1994. Pedestrians transportation problems on different slippery surfaces during winter and the functionality of different anti-skid devices. Proceedings of International Conference on Developing and Commercial Utilisation of Technologies in Polar Regions (PolarTech), Sweden. Gro. nqvist, R., 1989. Development of method for assessing the risk of slipping during walking. Liukedumisuaaran arviointimenctelman Kehittaminen Lisensiaattibyo. Tampereen tecknillinen korkeakoulu, Tampere (in Finnish with English summary). Gro. nqvist, R., 1995a. A dynamic method for assessing pedestrian slip resistance. People at Work, Research Report, Finnish Institute of Occupational health, Hakapaino, Helsinki, 1995. Gro. nqvist, R., 1995b. Mechanisms of friction and assessment of slip resistance of new and used footwear soles on contaminated floors. Ergonomics 38 (2), 224–241. Gro. nqvist, R., Hirvonen, M., 1994. Pedestrian safety on icy surfaces: antislip properties of footwear. In: Aghazadeh, F. (Ed.), Advances in Industrial Ergonomics and Safety VI. Taylor & Francis, pp. 315–322. Gro. nqvist, R., Hirvonen, M., 1995. Slipperiness of foot-wear and mechanisms of walking friction on icy surfaces. International Journal of Industrial Ergonomics 16, 191–200. ISO 468, 1982. (E) International Standard. Surface roughnessParameters, their values and general rules for specifying requirements. First edition, 1982-08-01. Lavender, S.A., Sommerich, C.M., Sudhaker, L.R., Marras, W.S., 1988.Trunk muscle loading in non-sagitaly segmental problems as a result of sudden unexpected loading conditions. Proceedings of the Human Factors Society 32nd Annual Meeting. Anaheim, CA, October 24–28, 1988,
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Vol. 1. The Human Factors Society, Santa Monica, CA , pp. 665–669. Leamon, T., Son, D., 1989. The natural history of a microslip. In: Mital, A. (Ed.), Advances in Industrial Ergonomics and Safety I. Taylor & Francis, London, pp. 633–638. Leclercq, S., Tisserant, M., Saulnier, H., 1994. Slip resistant footwear: a means for the prevention of slipping. In: Aghazadeh, F. (Ed.), Advances in Industrial Ergonomics and Safety VI. Taylor & Francis, London, pp. 329–337. Lund, J., 1984a. Falls and their causes. Journal of Gerontological Nursing 9, 142–149. Lund, J., 1984b. Accidental falls at work in the home and during leisure avtivities. Journal of Occupational Accidents 6, 181–193. Manning, D., Jones, C., Bruce, M., 1991. A method of ranking the grip of industrial footwear on water wet, oily and icy surfaces. Safety Science 14, 1–12. Manning, D.P., Jones, C., Bruce, M., 1990. Proof of shoe slipresistance by a walking traction test. Journal of Occupational Accidents 12, 255–270. Myung, R., Smith, J., Leamon, T., 1993. Subjective assessment of floor slipperiness. International Journal of Industrial Ergonomics 11, 313–319. Noguchi, T., Saito, J., 1996. Proper arrangement system fastened anti-slip device on winter shoes for the disabledFbasic ways of thinking and trial application. Tomakomai National college of Technology, Japan. Pyykko. , I., J.antti, P., Aalto, H., 1990. Postural control in elderly subjects. Age and Aging 19, 215–221. Smith, G.S., Falk, H., 1987. Unintentional injuries. American Journal of Preventive Medicine 3 (5, Suppl.), 143–163. Stevenson, M., 1997. Evaluation of the slip resistance of six types of women’s safety shoe using a newly developed testing machine. Journal of Occupational Health SafetyFAustralia and New Zealand 13 (2), 175–182. Stevenson, M., Hoang, K., Bunterngchit, Y., Lloyd, D., 1989. Measurement of slip resistance of shoes on floor surfaces, Part 1: methods. Journal of Occupational Health SafetyFAustralia and New Zealand 5 (2), 115–120. Strandberg, L., 1985. The effect of conditions underfoot on falling and overexertion accidents. Ergonomics 28 (1), 131–147. Tisserand, M., 1985. Progress in the prevention of falls caused by slipping. Ergonomics 28 (7), 1027–1042. Waller, J.A., 1978. Falls among the elderly-human and environmental factors. Accident Analysis and Prevention 10, 21–33. Wickstrom, G., Niskanen, T., Riihimaki, H., 1985. Strain in the back in concrete reinforcement work. British Journal of Industrial Medicine 42, 233–239.
The identification of factors in the systematic evaluation of slip prevention on icy surfaces John Abeysekera*, Chuansi Gao Division of Industrial Ergonomics, Department of Human Work Sciences, Lulea( University of Technology, S-971 87 Lulea(, Sweden Received 24 July 2000; received in revised form 30 November 2000; accepted 26 April 2001
Abstract Slips and falls on icy roads often result in fractures or sprains and is a major problem in Nordic countries. Walking trials by 25 subjects wearing four types of winter shoes on five different icy walking surfaces provided subjective and objective measures of tendency to slip and number of slips, respectively. Since friction is a major determinant of a slip, the influence of material spread on icy surfaces, the surface temperatures and the shoe soling characteristics versus the Coefficient of Friction (COF) of the shoes were measured. Sand and gravel on icy roads had positive effects on improving COF. The study revealed that the aetiology of slips and falls is multi-faceted and attempts to solve the problem must adopt a systems approach. Perception of risk, aging, training, experience and postural balance are other factors to be considered in preventing slips and falls. Future research should concentrate on the degree of impact of each factor to the aetiology of slips and falls, which can help to decide priority action in preventing slips and falls. Relevance to industry The personal protective devices used by outdoor workers during winter season have to provide two types of protection, namely, protection from an occupational hazard and protection from the cold climate. Safety shoes used on snow or ice covered surfaces add a third type of protection, namely, an anti-slip quality of the shoe. The 3rd protection can be achieved from among other things, by improving the friction of the shoe soles. r 2001 Elsevier Science B.V. All rights reserved. Keywords: Icy roads; Anti-slip material; Footwear; Coefficient of friction
1. Introduction and Literature Slips and falls can result in minor or major injuries (e.g., sprains and fractures) or permanent disabilities to the victims. Since the rehabilitation *Corresponding author. Tel.: +46-920-910-00; fax: +46-920910-30. E-mail address: [email protected] (J. Abeysekera).
and the healing process of these types of injuries usually take a long time, they can be costly with higher insurance payments and compensation claims both to society and to the enterprises (Gro. nqvist, 1995a). In Nordic regions, slips and falls on icy roads are a major problem during winter. Foot slippage caused 43% of all falls and 16% of all accidents in Nordic countries of which the majority had occurred on surfaces covered with ice and snow (Lund, 1984a,b).
0169-8141/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 8 1 4 1 ( 0 1 ) 0 0 0 2 7 - 0
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It is known that the elderly are more vulnerable to slips and falls. In the north of Sweden the snowrelated injuries to the elderly over 60 yr have accounted for 37% of the total injuries to pedestrians (Lund, 1984a,b). Bj.ornstig et al. (1997) have reported that the ‘‘cost’’ of medical care for slipping injuries had been almost the same as the ‘‘cost’’ of all traffic injuries during the same period. The literature also reveals that trying to escape from falls can damage the back. It is reported that the muscle forces exerted in trying to prevent a fall which can cause harmful loading of the spine (Lavender et al., 1988) and injuries to the vertebral bodies can result in permanent injuries to the back (Wickstrom et al., 1985). Crowded orthopaedic clinics during winter in the north of Sweden are a common sight. Although this can be assumed to be due to victims of fractures from slips and falls on snow, no systematic investigation seemed to have been done to ascertain the causes of overcrowded orthopaedic clinics and no genuine attempt seems to have been taken to prevent fracture injuries during winter. Risk factors for slipping can be divided into (a) primary and (b) secondary (Gro. nqvist, 1995a). A major risk factor for slips is defined as poor grip and low friction between footwear and the underfoot surface (Smith and Falk, 1987). When the Coefficient of Friction (COF) between the shoe and walking surface does not provide sufficient resistance to counteract the forward resulting force at the point of contact, a slip can occur. There are critical gait phases in the slip, namely, heel strike (forward slip) and toe off (rearward slip) of which the forward slip is said to be more dangerous and more frequent (Leamon and Son, 1989). Postural control influenced by sensory modes helps to adapt the gait and when conditions are stable it is easier to adapt the gait (Gro. nqvist, 1995b). It is believed also that the risk of slipping is influenced to a great extent by the subjective awareness to the potential slipperiness of the shoe-floor coupling. Secondary risk factors include (a) extrinsic or environmental factors, e.g., inadequate lighting, slippery roads, etc., (b) intrinsic or human factors, e.g. old age, absence of perception of risk, etc., and (c) integrated or systems factors, e.g., using solings
of low friction properties, elderly walking on poorly lit snowy roads, etc. (Waller, 1978; Pyykko. et al., 1990). Hardness of rubber refers to its elasticity and is based on a measurement of the indentation of a rigid ball into the material (Gro. nqvist, 1995a). Hardness is one of the more widely used mechanical properties to describe footwear sole characteristics. It was reported that there was negative correlation between sole hardness and COF on lubricant/floor and on dry ice (91C to 101C), (Leclercq et al., 1994; Bruce et al., 1986). However there was no clear correlation on wet ice (01C). Surface roughness is defined by parameters such as Ra and Rz and Sm (ISO 468, 1982). Ra is the arithmetic average roughness or mean deviation of the profile. Rz is the height of the profile irregularities in ten points. Rtm is peak-to-peak roughness (Gro. nqvist, 1995a). It is reported that shoe sole roughness had a positive influence on COF. Successive abrasion treatment of soling made it rougher and recorded a higher COF (Manning et al., 1990). The second classic law of friction states that ‘‘the COF is independent of the apparent contact area’’. The apparent contact area does not reflect the true contact area which depends on normal force and liquid or solid contaminants between the interacting surfaces. However it is still under debate whether increased contact area should be recommended (Leclercq et al., 1994; Stevenson et al., 1989; Stevenson, 1997; Strandberg, 1985; Gro. nqvist, 1995a). Research in the past on slips and falls has concentrated more on oily and wet shop floor surfaces, and improving COF of shoes. Regarding improving COF, literature reveals some studies on anti-slip soling material (Bruce et al., 1986; Gro. nqvist, 1995a,b) and anti slip devices (Bruce et al., 1986; Gard and Lundborg, 1994; Noguchi and Saito, 1996). Research is lacking on combined effects of all factors that influence slips and falls on icy surfaces. Unless and until the effects of these integrated factors are researched and studied, it becomes extremely difficult to prevent slips and falls on icy surfaces. This investigation is one of the very first
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attempts of planning to carry out research on the systems approach strategy to prevent slip and fall accidents. This study, therefore, is a preliminary investigation aimed to ascertain the aetiology of slips and falls on icy surfaces and to develop a hierarchical model containing important factors with their degrees of impact and influence to the design of anti-slip shoes and anti-slip walking surfaces. This preliminary study in this long research process, consisted of subjective and objective testing of four different types of winter shoes. The results of this study can become a stepping stone for systematic research for an integrated or systems approach to prevent slip and fall accidents on icy roads.
2. Materials and methods In any ergonomic analysis users’ viewpoints are key issues. The subjective impressions of tendency to slip of a shoe or a walking surface can complement the objective data such as COF. Both subjective and objective data were collected while subjects participated in a walking trial wearing four types of shoes. Objective measurements of the footwear, namely, COF for each type of footwear as well as for each type of icy surface, centre of gravity (COG), hardness and roughness of soling and tread area were measured on each type of footwear.
observer could easily perceive as a slip. ‘Tendency to slip’ was defined as the feeling of the subject of the possibility to slip. The trials were carried out with the subjects being aware of the potential slipperiness of the walking surfaces. By direct observation the number of slips of each subject was counted for each pair of shoes. All walking trials were filmed on videotape. The direct observations made on the number of slips was checked and clarified by watching the video film. Footwear. Four different types (1–4) of new footwear (Figs. 1–4) were selected from Swedish manufacturers as well as from the market, considering different footwear properties, i.e., material, weight, center of gravity, sole and heel design, hardness and roughness, etc. The specifications of the four types of footwear are shown in Table 1. Subjects. Twenty-five subjects, 15 males and 10 females, 22–62 yr old with average age of 31.2 yr (S.D. 8.65), took part in the outdoor experiment. All subjects had more than 5 months experience of walking on icy roads and 7 months living experience in Nordic region. Icy Surfaces. Gravel, sand and salt are common materials spread on ice and snow for preventing slips and falls in Sweden. In order to reveal the interaction if any, and anti-slip effects between the materials and footwear, five different outdoor icy walking surfaces, i.e. pure ice, ice covered with
2.1. Outdoor walking trials Outdoor walking trials were carried out with 25 subjects who wore four different types of footwear and walked on five different icy surfaces (i.e., pure ice and ice covered with gravel, sand, snow and salt). One main aim was to obtain the subjective impressions. Gro. nqvist (1995a) defined ‘slipping’ as a sudden loss of grip, resulting in sliding of the foot on a surface due to lower COF than that required for the momentary activity. For the purpose of this study and walking trials, a ‘slip’ was interpreted as a temporary loss of foot balance or movement of the foot away from its intended path, which the
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Fig. 1.
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Fig. 2. Fig. 4.
gravel (F 4–8, 150 g/m2), sand (180 g/m2), snow (3–5 mm) and salt (9 g/m2) were used, each of which was 10 m long and 0.95 m wide. The walking surfaces were prepared in a shaded area to avoid the direct sunlight. During the experiment, the environmental dry air temperature was 0–21C. In order to control bias of the assessment, the type of footwear worn and the walking surface for each trial were selected at random. After each trial, each subject was asked to rate the tendency to slip by using 5-point scale, that is, 1-very high, 2-high, 3- moderate, 4-low, 5-very low. 2.2. Objective measurements Fig. 3.
Coefficient of friction. According to the first classic law of friction, the friction force is
Table 1 The specifications of the four types of footwear Type
Model/Code
Manufacturer
Out sole/heel tread design
Out sole and heel material
Upper part material
Weight (g) (size)
1 2
Gepard 1100 Stilex PU-boot, Occup. footwear 520 St(alex WS (steel toe safety boot) Art. Falcon, BN (farmer’s boot)
From market Swedish
Rough Smooth flat cleat, no ridges, no asperity Rough
Synthetic material ‘‘BioForm’’ Polyurethane
Synthetic material Leather
387 (41), 431 (43) 452 (40), 484 (42)
Nitril rubber, heat & oil resistant
Leather, Thinsulate fibre-fur lining
812 (40), 913 (42)
Lots of small ridges
Natural rubber
Natural rubber
824 (40), 898 (42)
3
4
Swedish
Swedish
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proportional to the vertical load (normal force) which exists between the actual surfaces Fm ¼ mFN ; where Fm is the friction force, FN is the normal force, and m is the COF. The kinetic COF was assessed with a stationary step simulator, using a method developed at the Finnish Institute of Occupational Health (Gro. nqvist, 1989). The apparatus is equipped with a cooling system. It can simulate the movements of a human foot and the forces applied to the underfoot surface during an actual slip. The ice is formed on a force platform, the surface temperature can be adjusted (Gro. nqvist, 1995a). The left shoe of each footwear was fixed on the last (artificial foot) of the simulator rig (Fig. 5). For each footwear and icy surface a total of five measurements was taken and the averages were recorded. Sole hardness and roughness. As revealed by other researchers (Manning et al., 1991; Tisserand, 1985; Gro. nqvist and Hirvonen, 1994, 1995), hardness and roughness had significant effects on slipperiness. In this study, the out sole hardness and roughness were measured by using a Hardness Tester (made in Japan) and Sultronic 10Ra Roughness Tester, made in UK, respectively.
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These tests were also carried out at the Finnish Institute of Occupational Health. Centre of gravity of footwear. It is assumed that a wearer can better control one’s balance while walking or during a slip, if the COG of the shoe is situated at or closer to the fulcrum of the wearer’s foot (Abeysekera and Khan, 1997). The location of the COG of the 4 types of shoes was used to test this assumption. Due to the different sizes of the same type of shoes having different locations of COG, two parameters ‘‘d’’ and ‘‘d=L’’ were introduced in order to describe where the COGs were located. The d value is influenced by the sizes of the footwear, whereas d=L is independent of footwear sizes if footwear are of the same type. Therefore, the COG location was interpreted as a distance ratio d=L (see Fig. 6). Tread areas of footwear. The out sole contact areas on surfaces were measured by spreading ink on the bottom of the sole, standing static on a piece of white paper to get the imprint of the static position. To get the dynamic imprint of the shoe the body weight (75 kg) was moved forward and backward (at toe and heel), so that the whole out sole was in contact with the paper. The areas of the imprints (static and dynamic) were measured with a Planimeter (Model KP 27, made in Japan). 3. Results 3.1. Subjective ratings of tendency to slip Myung et al. (1993) argued, a large sample size (n ¼ 100) helped satisfy the central limit theorem,
Fig. 5. The stationary step simulator rig (Gro¨nqvist, 1995a).
Fig. 6. The location of COG of footwear (described as the distance ratio).
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Fig. 7. Subjective assessment of tendency to slip.
which allows the use of a parametric test for subjective ratings. In this study, there were 500 trials. Therefore, ANOVA was used to analyse the interaction between footwear and icy surfaces. Two-way within subject ANOVA (4 5) showed that there was no interaction effects between footwear and icy surfaces on tendency to slip (F ¼ 1:025; P ¼ 0:425 > 0:05; two-tailed) as shown in Fig. 7. Friedman test showed that there was significantly different effect of different footwear in the tendency to slip (K2r ¼ 10:746; N ¼ 23; df=3, P ¼ 0:013o0:05). The rank of tendency to slip (from the lowest to the highest) was footwear type 4, 1, 3, and 2. Type 2 was most slippery, while the type 4 was the least slippery one. Friedman test showed that there was highly significant effect of different icy surfaces on the tendency to slip (K2¼ r 52:324; N ¼ 25; df=4, P ¼ 0:000o0:01). The rank of tendency to slip (from the lowest to the highest) is ice covered with sand, gravel, salt, snow, and pure ice (Fig. 7). The ice covered with sand was the least slippery surface, while the pure ice was the most slippery one of the five different icy surfaces.
3.2. Direct observations of slip The number of slips (defined earlier) observed for each type of shoes on each walking surface during the walking trials of the subjects are given in Table 2. Chi-square test of the directly observed number of slip showed that there were highly significantly different effects on the slip among different types of footwear (K2 ¼ 32:462; df=3, P ¼ 0:000o0:01), and among different types of icy surfaces (K2¼ 514:609; df=4, P ¼ 0:000o0:01), with the least observed number of slips on ice covered with sand, while with the most on pure ice. The slip resistance ranks are as follows: ice covered with sand>gravel>salt>snow>pure ice; footwear type 3>4>1>2 (see Table 2). The results of direct observation for walking surfaces were consistent with that of subjective ratings.
3.3. Objective measurements of COF The results of the objective measurement of COF of surfaces and subjective ratings are shown in Table 3 using footwear type 2 as a reference
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Shoe 1 Shoe 2 Shoe 3 Shoe 4 Total Rank of surfaces (least to most)
Gravel
Sand
Snow
Ice
Salt
Total
Rank of footwear (least to most)
17 21 15 9 62 2
2 2 1 1 6 1
31 36 21 24 112 4
94 108 53 75 330 5
18 33 19 18 88 3
162 200 109 127
3 4 1 2
Table 3 Subjective ratings and COF on icy surfaces
Rating COF (S.D.)
Pure ice (01C)
Ice covered with snow (3–5 mm)
Ice covered with sand (180 g/m2)
Ice covered with gravel (150 g/m2)
Ice covered with salt (9 g/m2)
1.16 0.065 0.003
3.18 0.054 0.007
4.24 0.280 0.011
3.22 0.266 0.023
3.41 0.130 0.006
Table 4 Kinetic COF of four types of footwear on pure ice (01C) Footwear
Type 1
Type 2
Type 3
Type 4
Kinetic COF S.D.
0.080 0.001
0.065 0.003
0.077 0.002
0.078 0.005
shoe. The results of kinetic COF of four types of footwear on pure ice (01C) are shown in Table 4. 3.4. Correlation between subjective and objective measurements The correlation between subjective ratings and COF shows that there is significant correlation between kinetic COF and ratings (r ¼ 0:900; p ¼ 0:037o0:05) (see also Table 2). The results of sole hardness, roughness, and COG are shown in Table 5. The hardness of the four types of shoe soles are of the same magnitude. Statistical analyses did not show any significant correlation between subjective ratings of footwear slipperiness and COG, hardness, and roughness. There is no significant correlation between COF and hardness ( p ¼ 0:559 > 0:05), roughness ( p ¼ 0:140 > 0:05) on pure wet ice (01C).
The results of the out sole contact areas are shown in Table 6. There is no significant correlation between subjective rating and dynamic area of contact ( p ¼ 0:256 > 0:05), nor static area of contact, ( p ¼ 0:167 > 0:05). Since the results of subjective ratings were consistent with direct observations of the tendency to slip, subjective ratings were used to study the correlation effects with the other characteristics of the shoes, e.g., COF, COG, hardness, roughness and tread areas. The summary of correlation results is shown in Table 7.
4. Discussion Although the four types of shoes were selected considering different slipperiness properties, no systematic procedure could be observed in terms of a good experimental set up as the four shoes varied in material, weight, size and sole properties. The five different walking surfaces or roads with pure ice, ice covered with gravel, sand, snow and salt were constructed on a flat surface situated outdoors. A safety harness was not used as this will negate the reality in walking on slippery surfaces. In real life pedestrians change their gait
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Table 5 The sole hardness, roughness, and COG of footwear Type
Sole hardnessa (Shore A) (S.D.)
Sole roughnessa (mm) (S.D.)
The distance from COG to heelFd (cm)
COGb d/L
1
60 (1.66)
17.7 (4.29)
2
58 (2.26)
4.3 (1.12)
3
57 (1.06)
24.1 (6.74)
4
60 (2.01)
16.5 (2.93)
13.2(s-41) 14.2(s-43) 11.6(s-40) 12.2(s-42) 12.7(s-40) 13.7(s-42) 10.3(s-40) 10.6(s-42)
0.47 0.47 0.42 0.42 0.44 0.45 0.37 0.37
a ANOVA showed that there were significant differences in the hardness (F ¼ 4:624; p ¼ 0:008o0:01) and roughness (F ¼ 20:917; p ¼ 0:000o0:01) of the four types of footwear. b The location of COG is expressed as the distance ratio (d=L; see Fig. 6).
Table 6 The out sole contact areasa Footwear (size: 42) 2
Static contact area (cm ) Dynamic contact areab (cm2)
1
2
3
4
42.9 61.0
26.3 56.7
34.6 68.9
48.1 78.2
a Only one measurement was made on each shoe (static and dynamic). b The dynamic contact area was measured when moving the body weight forward and backward (at toe and heel). Significant correlation between subjective rating and dynamic area of contact ( p ¼ 0:256 > 0:05), nor static area of contact, ( p ¼ 0:167 > 0:05).
based on their perception of risk. On the other hand if a safety harness was used the subjects would not have walked the way they do in real life. Since this study was not restricted to testing of material (in which case a safety harness can be used) and the chances of potential injury was considered remote, a safety harness was not used.
Many studies in the past have shown that COF is a good indicator of the slipperiness of shoes and/ or floors. The coefficient of kinetic friction can be influenced by many factors, including properties of the walking surfaces, e.g., material spread on icy surfaces, surface temperatures of ice (Gro. nqvist and Hirvonen, 1994), properties of the shoe soling material, e.g., rubber, synthetic, polyurethane, etc., (Gro. nqvist and Hirvonen, 1994), properties of anti-slip devices attached to shoe soles, e.g., chains, crampons, metal studs, etc., (Bruce et al., 1986; Gard and Lundborg, 1994) and shoe soling material characteristics, e.g., roughness (Manning et al., 1990), hardness, etc. The risk of slipping as mentioned before is also influenced by environmental factors, e.g., uneven surfaces, inadequate lighting, etc., and human factors, e.g., perception of risk, aging, postural control, etc. It is visualized that most slipping accidents occur unexpectedly when the victim is unaware of the hazard. Therefore the perception of the hazard is important to consider in preventing slip and fall accidents. In
Table 7 Correlation between subjective ratings of tendency to slip and other properties of footwear
Subjective ratings
COF
COG
Hardness
Roughness
Sole contact area (static)
Sole contact area (dynamic)
r ¼ 0:900; p ¼ 0:037
NSb
NS
NS
NS
NS
a a b
There is significant correlation. NS: Not significant.
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the present study the subjects were aware of the slipping or non-slipping conditions of the walking surfaces. The tendency to slip in the walking trials would therefore be influenced very much by the characteristics of the shoe soling and/or road conditions. The present study investigated the correlation between COF and soling characteristics and antislip materials spread on icy roads. Further, the influence of intrinsic and extrinsic risk factors namely, loss of postural control by the wrong placement of the shoe COG (e.g. top heavy) and wet ice (e.g. melting environment temperature), respectively, were studied. Thus the approach investigated in preventing slips on ice in this study was the integrated or systems approach. Improving COF with anti-slip devicesFalthough it seems to be an effective method used by the elderly to prevent slipping (Gard and Lundberg, 1994)Fwas not included in this study as this would interfere with the characteristics of the soling of the shoes studied. It is also believed that the devices can be a hindrance to wearability of the shoes. All four types of shoes had low COF on pure ice at 01C and the differences between the COF were not significant. Therefore on pure ice the characteristics of the soling seems to have hardly any effect in preventing slipping. Strategies to prevent slipping under those conditions should be different, such as changing gait patterns, improving the balancing properties of the shoe, e.g., the COG of the shoe placed closer to the fulcrum of the foot, etc. using anti-slip devices, etc. The materials spread on the road had a significant effect on COF (Table 3). Further, the subjective ratings for the different materials significantly correlated with COF. Although there were no interactive effects of different types of shoes and road characteristics, when they were tested separately they were significantly correlated with COF values. The correlation results (Table 7) indicate that although the subjects could perceive the effects of COF on the shoes, the effects of COG, hardness, roughness, and sole contact areas cannot be perceived by the subjects. Further the effects if any by these characteristics (except COF) to
311
prevent slipping would have been also negligible in the shoes tested. Significant differences were seen in the hardness (F ¼ 4:624; p ¼ 0:008o0:01) and roughness (F ¼ 20:917; p ¼ 0:000o0:01) of the four types of footwear (Table 5). It was also observed that there was no significant correlation between COF and hardness and roughness (Table 7). This may be due to the fact that COF values were measured only with temperature at 01C. In this study the characteristics of the shoes were analyzed separately with subjective ratings or COF measurements but the combined effect of hardness, roughness, COG and tread areas on COF of the shoe has to be further tested. Results of the combined effects may provide useful information in the design of anti-slip shoes and surfaces. Based on the multiplicity of factors that influence a slip on an icy surface, an area of research that should be emphasized for the future is the design of soling with optimum COF which is durable and have optimum wearability. Preliminary trials carried out by Noguchi and Saito (1996) of Japan with permanent fixtures of anti-slip material on relevant parts of the shoe soling are considered an area of research to be pursued.
5. Recommendations and future research 1. This study is limited in terms on number and ages of subjects, the effect of surface temperature, the variations of hardness, roughness and sizes of shoes used. From the results of this study and the information from literature, it is possible to provide some guidelines and trends to prevent slipping under the following headings, viz. preventing primary and secondary risk factors, vulnerability to slip, and strategies of slip prevention, (Fig. 8). 2. Slips and falls on icy surfaces is a multifaceted problem as revealed in this study. Only through an integral approach an optimum solution could be found to prevent slip accidents, towards which future research must be directed. 3. The influence from COG, tread areas, roughness and hardness, and sole material on COF
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Fig. 8. Prevention of risk factors, vulnerability to slip and strategies of slip prevention.
may be considerable. The combined effect of the shoe characteristics will shed more light to the new design of anti-slip shoes. 4. Permanent anti-slip devices on shoe soling with optimum COF without hindrance to wearability, deserves further investigation. 5. Perception of risk, training and experience are important. The influence of these factors to the prevention of slips and falls has to be investigated. 6. Subjective impressions are as important as objective measurements in testing personal protective wear such as shoes, gloves, etc.
Acknowledgements We acknowledge with thanks the financial support received from the Swedish Council for Work Life Research (RALF and now VINNOVA) and COLDTECH , Lule(a, Sweden to carry out this research. We also express our gratitude to Dr. Raoul Gro. nqvist of the Liberty Mutual Research Center for Safety and Health, Hopkinton, MA, USA and Mikko Hirvonen of the Finnish Institute of Occupational Health and Safety, Helsinki and Glenn Lundborg of Lule(a University of Technol-
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ogy for their help in carrying out COF tests and walking trials, respectively.
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