The effectiveness of anti-slip devices and studded footwear in preventing slipping on ice

Leskinen, Timo

Accident Prevention Team / Finnish Institute of Occupational Health / Topeliuksenkatu 41a A / FI-00250 Helsinki, Finland+358-40-575 5535 / Timo.Leskinen@ttl.fiVartiainen, MattiFinnish Institute of Occupational Health / Topeliuksenkatu 41a A, / FI- 00250 Helsinki, FinlandPlaketti, PekkaFinnish Institute of Occupational Health / Topeliuksenkatu 41a A, / FI- 00250 Helsinki, FinlandGrönqvist, RaoulFinnish Institute of Occupational Health / Topeliuksenkatu 41a A, / FI- 00250 Helsinki, Finland

ABSTRACT

Slipping is a major risk for serious injuries when walking outside in winter. Shoes with studs and two different anti-slip devices attached under the shoe sole were evaluated and compared to plain winter footwear for the effectiveness in preventing slips and falls on ice. Twenty-four participants were divided into groups for each footwear type. A walking track with an ice generating system on top of a force plate, a motion analysis system, and high speed video cameras were used to analyze foot-floor forces, and foot movements. Half of participants slipped on the ice. None wearing studded shoes slipped. Half of participants with anti-slip devices slipped, but the anti-slip device seemed to assist part of them in recovering from a slip, while those who slipped wearing plain winter footwear just continued the slide after the initial slipping.

Keywords

Slipping, falling, friction, accident prevention

INTRODUCTION

Nearly 200 000 slip and fall accidents causing injury occur every year in Finland. Slipping accidents are common to all age groups, but the consequences of slipping and falling are most severe to elderly people [4]. Many factors affect the safe gait of workers and pedestrians, for instance, the environment indoors and outdoors, slippery wintry weather conditions, temperature and snowfall, awareness of risks, friction between surfaces, and possibility to adapt one's gait proactively in accordance with the conditions available. A decisive factor is the relation between the available friction, i.e. how slippery it is, and the utilized friction at the moment of heel contact or toe push-off [3]. Weather conditions have a big effect on the slipperiness of outdoor walking surfaces, and on specific high risk winter days, when the temperature risesclose to zero, the risk of slipping induced injuries may be many-fold compared to normal winter days [7].

It has been suggested that pedestrians should wear specific anti-slip devices or studded footwear to prevent slipping accident at least on these high risk conditions.

The role of anti-slip devices and studded footwear in preventing slips and falls is particularly important during unexpected or sudden loss of grip. Such situations are usually not recognized quickly enough by the pedestrian to be properly anticipated. The anti-slip devices and studded shoes cannot usually be used when walking insidethe buildings, and they may even reduce the friction on hard stony surfaces. So the usability of those devices depends much on how easy it is to take them on and off [5].

The main goal of the study was to evaluate the effectiveness of two types ofanti-slip devices and one special studded winter footwear in preventing slips and falls when walking on slippery ice.

MATERIAL AND METHODS

The study was undertaken in a laboratory environment to evaluate the advantages and potential disadvantages of anti-slip devices and studded footwear on slippery ice in relation to plain winter working shoes.

Except for the studded shoes the subjects wore their own working shoes for winter conditions, and the anti-slip devices were strapped to the shoe bottom (see Figure 1). One of the anti-slip devices was a heel device (Devisys) with 5 or 4 (smallsize device) studs under the heel, while the other was a forefoot device (Warma 8) with 5 studs under the forefoot. The 16 studs of the special studded sports shoe (Icebug) were located almost uniformly both under the heel and the forefoot and toes. The tested anti-slip devices and the studded shoes were commercially available.

A               B  

C             D  

Figure 1. The footwear and anti-slip devices of the study: A. heel device, B. forefoot device, C. studded sports shoe (Icebug), and D. plain winter working shoe without anti-slip devices.

A special walking and slipping track was constructed to test the footwear on icy as well as dry conditions. A large open box (length 8 m, width 2 m, height 1 m, see Figure 2) was provided with an air cooling system which cooled the air temperature on the floor just below 0 C. The walking track was made of plywood covered with black plastic floor covering inside this box. Two force plates (Kistler 9286A) were installed under the walking track, and on each force plate a water-proof box with a freezing system was installed, so that the upper surface of the ice plate in the box was at the level of the walk track. For video recording from the side the wall panels of the box were provided with glass windows.

Figure 2. Walking and slipping track with air cooling system and water freezing systems on force plates.

For dry surface tests the plates were covered with rectangular pieces of the same plastic floor covering as the rest of the track. For the gait tests in extremely slippery icy conditions the pieces were removed, and moreover, a thin layer of water was melted on top of ice plates by blowing hot air on the icy surface.

The measurements in the experiments on the walking track consisted of foot- floor forces exerted against the two successive force platforms, foot movements,automatic and forced correction movements to loss of footing and balance.

A motion analysis system (Peak Motus) were used to collect and analyze the movement and force data. Two high speed video cameras (Citius Imaging Centurio) were used to record the foot movements through a window in the side wall at 500frames/s, and four normal video cameras (JVC) recorded the whole body movements at 50 frames/s. Reflective markers were used on appropriate sites on the shoes and clothes were used to detect the locations of landmarks from the video recordings.

From the force plate signals friction use was obtained by dividing the horizontalground reaction force with the vertical force. Friction use for each subject on the slippery ice ('available friction') was compared to the same subject on the reference surface ('required friction').

Twenty four voluntary participants 12 women and 12 men took part in the experiments. The participants were workers who are used to walk much outside in their jobs. Each subject gave an informed consent before starting the trials. The subjects were informed of the sequence of walking trials, and that the surface can insome of the trials be slippery, but they were not told when the surface would be slippery. During the trials they wore special spectacles which prevented them from seeing the surface of the gait track. During all walking tests the participants wore asafety harness with a sliding connection to a safety rail installed on top of the walking track.

Each subject started with a few practice walk exercises to learn a steady walking rhythm after accelerating quite fast during just a few steps, which was required for the relatively short gait track. After the exercises two recordings were made while the subject walked on the dry surface, and finally the walk over the icy plates was recorded. The order had to be kept always the same as the first slipping on the track was expected have an effect on the walking technique, i.e. the subject would start walking more cautiously [2, 6]. There were a few minutes breaks between the recordings during which the subject stayed in the room next to the laboratory so that no visual clues were given by the personnel of the preparations for the next trial i.e. if the conditions were to be dry or icy.

In the analysis the results on slippery ice were compared to the results on a reference high grip floor covering. Variables analyzed included friction use, the angle ofthe foot (deviation from horizontal), distance of slide of the heel, and maximumhorizontal velocity of the heel during sliding. The slide of the heel was defined only for the time when the vertical ground reaction force exceeded 40 % of body weight, i.e.the initial slide with less weight on the foot was excluded. The slide exceeding 50 mm was considered slipping, and if the slide exceeded 150 mm, slipping was considered severe, possibly leading to falling.

Paired t-tests were used to compare the statistical differences on the slipperyice and the reference surface. The results of those participant groups who used different anti-slip devices were statistically compared as different groups using t- testing.

RESULTS

On dry surface none of the participants slipped. The slide of the heel varied between 4 mm and 24 mm, the mean was 13 mm. The maximum velocity of the slide varied between 0,25 m/s and 0,82 m/s.

On icy surface twelve participants (50 %) slipped: 4 of 6 wearing plain winter shoes, 6 of 6 using forefoot anti-slip devices, 2 of 6 using heel devices, and nonewearing studded footwear. Two of the forefoot device users succeeded in stopping theslip after 85 and 101 mm slide while all the others who slipped, slipped more severely (>150 mm). The dimensions of the force platform and the ice plate on it(400x600 mm) caused even the severe slips to stop at the edge of the icy area, which helped a slipping participant to recover from the slip, and thus prevented the participants from falling / getting supported by the safety harness.

In Figure 3 the average slide distance with different footwear and anti-slipdevices is shown. In Figure 4 the average peak horizontal slide velocity with different footwear and anti-slip devices is shown.

Slide distance (cm)

30 Forefootdevice, ice25

Winter shoe, ice

Heel device, ice20

15

10 Total average,5 dry

0

Studded shoe, ice

Figure 3. Average horizontal foot slide on ice with different footwear.

Slide velocity (m/s)

3,5

3

2,5

2

1,5

Heel device, ice

Total average,

Forefoot device, ice

Winter shoe, ice

1

0,5

0dry

Studded shoe,ice

Figure 4. Average maximal horizontal foot velocity during slide on ice with different footwear.

Figure 5 illustrates the foot horizontal velocity immediately before and after the heel strike of three different participants, results are shown on the development of a slip. The horizontal foot velocity after the heel strike reduces to almost zero at the heel strike, but in slippery conditions the foot starts to slide when more weight is put on it. The results suggest that if the horizontal velocity does not exceed 1 m/s, the walker can recover from the slip, but if the velocity is higher, the slip is severe.

Horizontal foot velocity on ice

2,5

2,0

1,5

1,0

Severe slip Slipping + recovery No slipping

0,5

0,0

-50            0            50           100          150          200          250

Time (ms)

Figure 5. Horizontal foot velocity on ice of three participants, one slipping severely (plain winter shoes), one slipping with a successful recovery (forefoot devices), and one not slipping (studded shoes). On the time scale, 0 indicates the time when the vertical ground reaction force exceeds 40 % of body weight.

Individual differences in walking technique seemed to have an effect on slipping on ice. Among the heel device users the two who slipped had at the heel strike the steepest foot angle, more than 30°, as well as the highest foot velocity. Among the sub-group wearing plain winter shoes the two who did not slip had at the heel strike the lowest foot angle, as well as the slowest foot velocity.

Figure 6 gives two examples of the difference between available and required friction on slippery conditions. The upper figure shows the results of a participant who requires high friction even in dry conditions, and so he cannot get enough friction onthe icy surface, which leads to slipping, while a participant walking in a more cautious technique requires less friction and survives also on the ice without slipping (lower figure).

Participant not slipping on ice

0,250,2

0,15

0,1

Ice Dry

0,050

0 50 100 150 200

Time (ms)

Figure 6. Friction use after heel strike on dry and icy surface of two participants with plain winter shoes.

DISCUSSION

The results showed that the studded shoes reduced most effectively the number of slip incidents. None of the participants with the studded boots slipped, nor did they subjectively feel that the icy surface condition was unsafe.

A main problem with studded shoes is the fact that studs may harm the indoorfloor materials when walking inside. So the shoes must always be changed to normal shoes when coming in.

Many of the participants wearing anti-slip devices slipped on the slippery ice, whether they wore heel or forefoot devices. Despite slip initiation, the tested anti-slipdevices seemed to assist the participants in recovering from a slip, while plain winter footwear did not really reduce initial heel-slipping in the trials.

The walking technique, as reflected in foot angle and foot velocity at heel strike, seemed to be an important factor both when using plain winter shoes and when usinganti-slip devices. The angle of the foot at heel strike is critical when using anti-slip devices. Even with the heel device two participants slipped, because the smooth back edge of the anti-slip device touched the ice before the studs which cannot be installedquite at the edge of the device. All the participants with forefoot device slipped as there were no studs touching the ground during heel strike, and those who recovered received assistance of the studs under the forefoot only after a short slide. The fact that not all of the participants not using any anti-slip device reflects probably frommore cautious walking technique.

To obtain the wanted benefit of the use of anti-slip devices, it is important to guide the user to use them properly. Foot angle and cadence are important modifiers in adapting to slippery conditions [1, 3, 8], and they should be modified in a properway also when using anti-slip devices. In this study we asked the participants to walk with their normal walking speed, not anticipating slippery conditions. That is why their foot angle was not adapted to proper use of the anti-slip devices, and all forefootdevice users experienced at least an initial slip after heel strike, and even two of the heel device users slipped. Especially when using the forefoot devices the pedestrian should adopt a different walking technique with the foot almost horizontal already at heel strike, i.e. the heel and forefoot should contact the ground almost simultaneouslythus giving sufficient pressure to the forefoot already in the beginning.

CONCLUSIONS

Winter footwear with firmly fixed studs both under the heels and under the ball reduced most effectively slipping when walking unexpectedly over an icy surface. Fixed studs under the shoes can be recommended at least to people who walk long distances outdoors on possibly slippery icy surfaces.

Removable anti-slip devices are more usable when people have to walk both indoors and outdoors. When using removable anti-slip devices it is recommended toadapt the walking technique to get full advantage of the anti-slip properties of the devices.

ACKNOWLEDGMENTS

The study was supported by the Finnish Work Environment Fund.

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