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Efficacy of a rubber outsole with a hybrid surface pattern for preventing slips on icy
surfaces
Takeshi Yamaguchi, Jennifer Hsu, Yue Li, Brian E. Maki
Version Post-print/accepted manuscript
Citation (published version)
Yamaguchi, T., Hsu, J., Li, Y., & Maki, B. E. (2015). Efficacy of a rubber outsole with a hybrid surface pattern for preventing slips on icy surfaces. Appl Ergon, 51, 9-17.
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1
Efficacy of a rubber outsole with a hybrid surface pattern for
preventing slips on icy surfaces
Takeshi Yamaguchia, Jennifer Hsu
b, Yue Li
b, and Brian E. Maki
b,c,d
aGraduate School of Engineering, Tohoku University, Sendai, Miyagi, Japan
bToronto Rehabilitation Institute-University Health Network, Toronto, ON, Canada
cSunnybrook Health Sciences Centre, Toronto, ON, Canada
dDepartment of Surgery, Institute of Biomaterials and Biomedical Engineering, and
Institute of Medical Science, University of Toronto, Toronto, ON, Canada
*Corresponding author
6-6-01, Aramaki-Aza-Aoba, Aoba-ku, Sendai, Miyagi 980-8579, Japan
Tel: +81 22 795 6897; Fax: +81 22 795 6897
E-mail: [email protected]
1
ABSTRACT 1
Conventional winter-safety footwear devices, such as crampons, can be effective in 2
preventing slips on icy surfaces but the protruding studs can lead to other problems 3
such as trips. A new hybrid (rough and smooth) rubber outsole was designed to provide 4
high slip resistance without use of protruding studs or asperities. In the present study, 5
we examined the slip resistance of the hybrid rubber outsole on both dry (−10°C) and 6
wet (0°C) icy surfaces, in comparison to three conventional strap-on winter anti-slip 7
devices: 1) metal coils ("Yaktrax Walker"), 2) gritted (sandpaper-like) straps ("Rough 8
Grip"), and 3) crampons ("Altagrips-Lite"). Drag tests were performed to measure 9
static (SCOF) and dynamic (DCOF) coefficients of friction, and gait trials were 10
conducted on both level and sloped ice surfaces (16 participants). The drag-test results 11
showed relatively high SCOF (≧0.37) and DCOF (≧0.31) values for the hybrid 12
rubber sole, at both temperatures. The other three footwear types exhibited lower 13
DCOF values (0.06-0.20) when compared with the hybrid rubber sole at 0°C (p<0.01). 14
Slips were more frequent when wearing the metal coils, in comparison to the other 15
footwear types, when descending a slope at -10°C (6% of trials vs 0%; p<0.05). There 16
were no other significant footwear-related differences in slip frequency, distance or 17
velocity. These results indicate that the slip-resistance of the hybrid rubber sole on icy 18
surfaces was comparable to conventional anti-slip footwear devices. Given the likely 19
advantages of the hybrid rubber sole (less susceptibility to tripping, better slip 20
resistance on non-icy surfaces), this type of sole should contribute to a decrease in fall 21
accidents; however, further research is needed to confirm its effectiveness under a 22
wider range of test conditions. 23
Keywords: friction; crampons; hybrid rubber sole; slip and fall; ice, winter footwear24
2
1. Introduction 25
Slip-and-fall accidents are a major concern because of the severity and costs 26
associated with the injuries incurred. Human factors such as gait biomechanics and 27
extrinsic factors such as footwear influence the occurrence of slips and falls (Gao and 28
Abeysekera, 2004). A large number of slip-and-fall accidents are experienced during the 29
winter on icy or snow-covered surfaces (Gao et al., 2008; Grönqvist and Hirvonen, 1995). 30
Poor grip or low friction between footwear and the underfoot surface on ice and snow is a 31
primary risk factor; hence, winter footwear design is recognized as a key factor that 32
requires more attention (Bentley and Halsam, 1998). 33
The coefficient of friction (COF) between footwear and the underfoot surface is 34
widely used as a measure of slip resistance; however, the choice of either static COF 35
(SCOF) or dynamic COF (DCOF) as a critical frictional parameter for preventing 36
slip-related falls is controversial (Ekkubus and Killey, 1973; Tisserand, 1985; Pilla, 2003; 37
Yamaguchi and Hokkirigawa, 2008). One could argue that both parameters are important, 38
with high SCOF helping to prevent slip initiation and high DCOF helping to stop the slip; 39
therefore, both SCOF and DCOF values should be evaluated (Yamaguchi, et al, 2012). 40
Generally, previous studies have found that DCOF values in the range of 0.2 to 0.4 are 41
required to arrest slips during level walking (Grönqvist et al., 1989, 2003; Redfern and 42
Bidanda,, 1994; Strandberg, 1983). As for the safe limit of the SCOF, Nagata et al. (2009) 43
indicated that SCOF values around 0.4 are required to prevent slips. 44
Friction force is expressed as the sum of an adhesive term and a deformation term 45
(Bowden and Tabor, 1950). Adhesive friction results from contact and subsequent 46
shearing of the contact interface, whereas deformation friction occurs when hard 47
protrusions on the bottom of the footwear (e.g. studs or asperities) "plough" through a 48
3
softer support surface (e.g. ice). The frictional characteristics of icy surfaces are 49
influenced by temperature, and melting (warm) icy surfaces are extremely slippery 50
compared with dry (cold) icy surfaces due to the formation of a water layer (surface melt 51
water due to localized frictional heat, rises in air temperature, etc.) which reduces 52
adhesion friction between the footwear outsole and the icy surface (Oksanen, 1983). 53
Many winter footwear designs and winter anti-slip devices are available and are 54
becoming more popular. Most anti-slip devices, such as crampons (Bruce et al., 1986), 55
are meant to work on icy surfaces under wide temperature conditions by using hard studs 56
or steel coils that penetrate the icy surface, resulting in increased friction due to ploughing 57
of the studs or steel coils through the ice surface. However, these types of anti-slip 58
devices may fail to prevent slips on surfaces that are too hard to allow penetration, or may 59
cause trips to occur when walking on uneven surfaces, e.g. due to "snagging" of the 60
crampons on surface irregularities (Bruce et al., 1986, March and Birkett, 1997). In 61
contrast with the anti-slip devices that rely on ploughing friction, tread rubber footwear 62
can help to prevent slips via increases in contact area, which can increase adhesion 63
friction when the icy surface is dry. However, if the icy surface is wet, tread rubber 64
footwear will produce low adhesion friction if the tread pattern cannot disperse the water 65
film from the interface between the footwear and the ice. In this situation, the frictional 66
force is small and caused mainly by viscous shear in the water layer (Chang, et al., 2001). 67
Yamaguchi et al. (2012a) found that a rubber block with a smooth surface had low 68
SCOF (<0.2) and high DCOF (>1.0) values on a smooth stainless-steel surface covered 69
with a very slippery glycerol water solution, whereas a rubber block with a rough surface 70
had high SCOF (>0.8) and low DCOF (<0.2) values on the same surface. On the basis of 71
these findings, they developed a hybrid (rough and smooth) rubber surface. The hybrid 72
4
pattern provided SCOF and DCOF values of ~0.5 on a glycerol-lubricated surface. 73
Subsequently, Yamaguchi et al. (2014) developed footwear with an outsole that 74
incorporated the hybrid rubber surface pattern, as shown in Fig. 1(a). In this outsole, slip 75
resistance was further enhanced by designing the surface pattern to increase adhesion 76
friction by dispersing the liquid film from the contact interface, as detailed in Fig. 1(b) . 77
Gait trials on a stainless steel floor covered with a glycerol solution demonstrated that this 78
footwear sole has superior slip resistance at both slip initiation and during sliding, and 79
significantly decreases the risk of falling compared with conventional commercial rubber 80
footwear used in food factories and restaurant kitchens (Yamaguchi et al., 2014). 81
The purpose of the present study was to determine whether the hybrid rubber surface 82
pattern sole, which relies on adhesion friction, is effective in preventing slips on dry and 83
wet icy surfaces. Footwear incorporating the hybrid rubber sole was compared to 84
conventional anti-slip devices, which rely on ploughing friction, by means of drag tests 85
and gait trials. We hypothesized that the hybrid rubber sole would provide high (>0.3) 86
static (SCOF) and dynamic (DCOF) coefficient of friction on both dry (−10°C) and wet 87
(0°C) ice due to increased adhesion friction, as measured in the drag tests. We further 88
hypothesized that the frequency and severity of slipping during the gait trials when using 89
the hybrid rubber sole would be equivalent to, or less than, that occurring when using the 90
conventional anti-slip devices, on both dry (−10°C) and wet (0°C) ice. 91
2. Methods 92
A strap-on version of the new hybrid rubber sole was prepared for the purposes of this 93
study, using natural rubber (Shore A-scale hardness of 40 at room temperature and 41 at 94
−10°C). This footwear was compared to three conventional winter strap-on anti-slip 95
devices: 1) metal coils (YAKTRAX Walker Traction Cleats for Snow and Ice, Interex 96
5
Industries, Vancouver, BC), 2) gritted (sandpaper-like) straps (GLIPS-LITE Rough Grips, 97
Winter Walking, Horsham, PA), and 3) crampons (ALTRAGRIPS-LITE, Winter Walking, 98
Horsham, PA), (Fig. 2). Each type of footwear was available in four sizes (S, M, L, and 99
XL). As detailed in the sections that follow, drag tests (King et al., 2013) were performed 100
to measure the relative slipperiness of the hybrid rubber outsole and the conventional 101
anti-slip devices with respect to wet and dry ice surfaces, and a repeated-measures gait 102
protocol was conducted with participants walking on wet and dry ice, on either a level 103
plane or a slope, to characterize the frequency and severity of slips that occurred while 104
wearing each of the four types of footwear. A single sample of each product (size M) was 105
used in the drag tests; four samples of each product were used in the gait trials 106
(size-matched to each subject's footwear). 107
Studies took place in a custom-built environmental chamber (ClimateLab, Toronto 108
Rehabilitation Institute-UHN, Toronto, Canada). The climate-control system includes a 109
refrigeration coil and associated condensing unit, electric heaters, a humidifier, a 110
dehumidifier and a fresh air system. A proportional-integral-derivative feedback 111
controller is used to regulate the environment temperature (measured with a temperature 112
sensor) to within ±0.5°C. For this study, the temperature was maintained at either 0°C or 113
−10°C to create wet or dry ice surfaces, respectively. Relative humidity was 62.1± 14.8% 114
at 0°C and 67.5± 8.7% at −10°C. 115
The interior dimensions of the chamber (4.2m long, 3.7m wide and 2.1m high) 116
allowed for two straight walkways measuring 3.64 m in length and 0.73 m in width. As 117
shown in Fig. 3, each walkway comprised three ice panels, plus a concrete panel at each 118
end, with two of the three ice panels mounted over force plates (BP400600HF, AMTI, 119
Watertown, MA). There was a 1.0-cm gap between panels such that the activation of any 120
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one force plate did not affect the others. For one walkway, the middle ice panel was 121
mounted at a 1:10 rise-to-run slope (5.7°). [A slope between 1:10 and 1:12 is allowed by 122
the standards for Americans with Disabilities (American Department of Justice, 2010).] 123
The thickness of the ice panels was 5 cm. Each ice panel was created by freezing water in 124
a plastic-lined wooden box at −10°C in the chamber. The −10°C trials were performed at 125
least one hour after freezing was complete. For the 0°C trials, the chamber was warmed 126
up to 0°C and maintained at this temperature for one hour before starting the trials. 127
2.1 Drag tests 128
The drag tests were conducted on the second ice panel of the level walkway at either 129
0°C or −10°C. A 6-kg weight was inserted into the shoe. A single type of shoe, with a 130
length of 26 cm (US size 8), was used for all tests. Each of the four anti-slip devices was 131
attached to the shoe, which was then dragged by hand at a particular velocity over the 132
surfaces of interest to measure the slip resistance of the footwear–surface interface, as 133
shown in Fig. 4. The shoe was dragged 0.48 m in 3 s at an approximately constant speed, 134
so that the average sliding velocity (except for the acceleration and deceleration periods) 135
was approximately 0.2 m/s. This procedure was repeated five times for each combination 136
of footwear and surface condition, in random order. During each test, 137
light-emitting-diode markers were attached on the toe and heel of the footwear, and the 138
three-dimensional motion of the markers was tracked with a motion capture system 139
(Phoenix Technologies Inc., Vancouver, British Columbia, Canada). The drag-test 140
motion-capture and force-plate data were sampled at 100 Hz and low-pass filtered 141
digitally at 50 Hz. These data were then used to determine the SCOF (peak pull force 142
required to initiate sliding divided by normal force) and the DCOF (average pull force 143
when dragged at a constant speed divided by normal force). 144
7
The horizontal speed of the midpoint of the toe marker and heel marker was calculated 145
and used as sliding velocity. The resulting horizontal (Fx, Fy) and vertical (Fz) ground 146
reaction forces during dragging were recorded to calculate COF according to the 147
following formula: 148
z
h
F
FCOF (1) 149
22
yxh FFF (2) 150
A representative time variation of COF and sliding velocity is shown in Fig. 5. SCOF 151
was determined as the first peak of COF just before sliding initiation. The mean COF 152
value when the sliding velocity was >0.1 m/s was used as the DCOF, to eliminate data 153
during the acceleration and deceleration periods. 154
Statistical analysis was performed using SPSS ver. 19.0 (SPSS, Inc., Chicago, IL, 155
USA). Two-way analysis of variance (ANOVA) was used to test if the SCOF and DCOF 156
values were affected by the type of footwear or temperature. Post-hoc paired t-testing 157
with a Bonferroni correction was used to determine specific significant differences 158
between footwear conditions. The significance level was set at p = 0.05. 159
2.2 Gait trials 160
Sixteen healthy adults (eight men, eight women; age 27.1 ± 6.3 years, range 20-43; 161
height 1.69 ± 0.095 m, range 1.51-1.83; mass 63.9 ± 10.7 kg, range 45-80) with no 162
neuromusculoskeletal disorders participated in this experiment. Each participant 163
provided written informed consent to comply with ethics approval granted by the 164
institutional review board. 165
Each study participant was tested with each of the four abovementioned types of 166
footwear, which was fit over the participants’ own footwear (participants were instructed 167
8
to bring the shoes or boots that they normally wore in winter weather). For safety, 168
participants were harnessed into a free-standing overhead track at all times while 169
traversing the walkway. The fixed height of the harness was adjusted not to interfere with 170
walking and to suit each participant to prevent injuries to the knees, hip, or head. 171
Participants were asked to wear their own winter coats over clothing that they would 172
normally wear in winter when the outdoor temperature was approximately −10°C. 173
The total number of gait trials was 1152 (16 × 4 × 3 x 2 × 3 = 1152), with 16 174
participants using each of the four types of footwear on each of the three ramp angles (0°, 175
5.7° ascent, 5.7° descent) and each of the two temperature conditions (0 and -10°C), with 176
three repetitions for each test condition (total of 72 trials per subject). The trials at 0°C or 177
−10°C were conducted on different days. The order of footwear type to be tested and the 178
sequence of trials were counterbalanced across participants. 179
Participants were signaled to begin traversing the walkway at a natural pace, with one 180
step on each panel, and to stop at the opposite end. They then turned around and waited 181
for another signal before walking back to the starting point with one step on each panel. 182
After completing three runs on the level walkway and three back-and-forth runs on the 183
sloped walkway (ascent in one direction and descent in the other), participants were asked 184
to step off the walkway, change footwear, and wait outside the chamber until the next 185
condition was set. 186
The frequency of slips and falls was determined using the spatial coordinates of 187
markers attached to the footwear at the heel (at a 2-cm height from the bottom of the shoe) 188
and at the toe. The analyses focused on the second step (middle panel), on both the level 189
and sloped surfaces. The motion-capture and force-plate data from these gait trials were 190
sampled at 100 Hz and then low-pass filtered digitally at 10 Hz. 191
9
A forward slip during the braking phase was considered to have occurred if the heel 192
horizontal (forward) velocity failed to reach 0 m/s (± 20 mm/s) within a 3.0-cm 193
displacement after the foot strike (Maynard, 2002; Fong, et al., 2009). The foot strike was 194
determined to have occurred when the vertical ground reaction force exceeded 5% of the 195
body weight of the participant (Fong, et al., 2009). A backward slip during the propulsive 196
phase [the period after the braking phase in which the antero-posterior ground reaction 197
force is negative and acts to propel the body forward] was defined to have occurred when 198
the backward traveling distance of the toe marker during the propulsion period exceeded 199
3.0 cm (Maynard, 2002; Fong, et al., 2009). Slip velocity and slip distance were also 200
measured, as indicators of slip severity (Brady et al., 2000; Cham and Redfern, 2002a). A 201
fall was defined as a foot slip that did not come to a stop (Cham and Redfern, 2002a). 202
Temporal-spatial gait variables were also measured to identify the effect of the 203
footwear on gait while walking on icy surfaces. Stride time and stride length were 204
determined as the duration and distance between two successive heel-strikes of the same 205
foot on the first and third ice panels. Walking velocity was defined as the stride length 206
divided by stride time. 207
The traction coefficient was calculated for both the braking and propulsion phases, by 208
dividing shear force (horizontal force: Fh) by vertical force (Fz). Two approaches were 209
used to avoid spuriously high RCOF values that can occur when Fz is small: 1) excluding 210
the first 5% of the stance phase (Blanchette, et al. 2011) and 2) excluding Fz values less 211
than 50N (Burnfield and Powers, 2007). Both approaches yielded identical results. The 212
maximum peak values of the traction coefficient defined the required coefficient of 213
friction (RCOF) during the weight acceptance (RCOFh) and toe-off (RCOFt) phases. For 214
each trial, a single step on the middle panel was analyzed. Fig. 6 shows representative 215
10
time changes in traction coefficient for walking on the level walkway. 216
Cochran’s Q test was used to test if the frequency of slips and falls was affected by the 217
type of footwear, for each temperature and surface condition. Two-way 218
repeated-measures ANOVA was used to test if the gait variables and RCOFs were 219
affected by the footwear and temperature conditions. One-way ANOVA was conducted to 220
analyze the difference in slip velocity and slip distance for trials which were identified as 221
slip trials (including forward and backward slips). Post-hoc paired t-testing with a 222
Bonferroni correction was used to determine specific significant differences between 223
footwear conditions. The significance level was set at p = 0.05. 224
3. Results 225
3.1 SCOF and DCOF values during the drag test 226
Table 1 shows the means and standard deviations of the SCOF and DCOF values 227
measured during the drag tests. Fig. 7 shows the relationship between the SCOF and 228
DCOF values. Two-way ANOVA indicated that the mean SCOF and DCOF values were 229
affected significantly by footwear type (p<0.001), temperature condition (p<0.001), and 230
footwear-temperature interaction (p<0.001). Post hoc analysis revealed that SCOF values 231
for all types of footwear at 0°C (wet ice) were lower than those at −10°C (dry ice) 232
(p’s<0.01). The same was true for the DCOF values (p’s<0.001), except that the hybrid 233
rubber sole showed a slightly higher DCOF at 0°C in comparison to −10°C (0.34 vs 0.31; 234
p=0.006). 235
When the temperature was 0°C (wet ice), the hybrid rubber sole showed a SCOF value 236
equivalent to that of the gritted sole and crampons (p>0.05), and higher than that of the 237
metal coils (0.37 vs 0.10; p<0.001). The hybrid rubber sole also exhibited the highest 238
mean DCOF across all footwear types (0.34 vs 0.06-0.20; p<0.001) at this temperature, 239
11
and was the only type of footwear to show both SCOF and DCOF values greater than 0.3. 240
When the temperature was −10°C (dry ice), all of the footwear types demonstrated 241
relatively high frictional coefficients (mean SCOF and DCOF values >0.3). Nonetheless, 242
there were some differences. The hybrid rubber sole was still superior to the metal coils, 243
in terms of higher SCOF values (0.45 vs 0.39; p<0.05); however, the gritted sole showed 244
higher SCOF values than the hybrid rubber sole (0.67 vs 0.45; p<0.001) and the gritted 245
sole and crampons showed higher DCOF values than the hybrid rubber sole (0.37 and 246
0.38 vs 0.31; p<0.01). 247
248
3.2 Frequency of falls and slips during the gait tests 249
No falls were detected during the gait trials on the level walkway or slope at 0°C or 250
−10°C. Slips occurred in 34 of the 1152 trials (3.0% of trials), and all but one subject had 251
one or more slip trials (average number of slip trials per subject = 2.1, range 0 to 6). Table 252
2 shows the means and standard deviations (across subjects) of the frequency of slip trials 253
for each footwear type, under each test condition. Across the test conditions, the mean 254
frequency of slip trials ranged from 0.0% to 6.25%. A Cochran’s Q test revealed that the 255
frequency of forward slip for the metal coils was significantly higher than for the other 256
footwear types when descending the slope at -10°C (6.25% vs 0.0%; p<0.05). There was, 257
however, no significant difference in slip frequency among footwear types when 258
descending the slope at 0°C, or when walking on the level surface or ascending the slope 259
at either temperature (p>0.05). 260
261
3.3 Slip distance and slip velocity during the gait tests 262
Table 3 presents the number of slip trials and the means and standard deviations of the 263
12
slip distance and maximum slip velocity. One-way ANOVA indicated that there was no 264
significant difference in slip distance or slip velocity among footwear types, at either 265
temperature. 266
267
3.4 Peak RCOF values during the gait tests 268
Table 4 shows the means and standard deviations of the peak RCOF values for each 269
footwear type. Two-way repeated measures ANOVA indicated that RCOFh and RCOFt 270
values when walking on the level surface were not affected by temperature condition 271
(p's>0.05), but were significantly affected by footwear type (p<0.05) and 272
temperature-footwear interaction (p<0.01). Post-hoc analysis demonstrated that 273
significant differences between footwear during level walking occurred only at -10°C. At 274
this temperature, the hybrid rubber sole provided higher mean RCOFh values in 275
comparison to the gritted sole (0.14 vs 0.12; p<0.001) and crampons (0.14 vs 0.11; 276
p<0.001), and also provided higher RCOFt values in comparison to the gritted sole (0.22 277
vs 0.20; p<0.01) and crampons (0.22 vs 0.19; p<0.05). 278
For the ascending slope, two-way repeated measures ANOVA indicated that the 279
RCOFt value was affected by temperature-footwear interaction (p<0.05). Post-hoc 280
analysis revealed that the RCOFt value for the hybrid rubber sole was higher than for the 281
gritted sole at -10°C (0.29 vs 0.26; p<0.05). 282
For the descending slope, two-way repeated measures ANOVA indicated that the 283
RCOFt value was significantly affected by footwear type (p<0.05) and 284
temperature-footwear interaction (p<0.05). Post-hoc analysis indicated that RCOFt value 285
for the hybrid rubber sole was higher than for the metal coils at -10°C (0.17 vs 0.14; 286
p<0.001). 287
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288
3.5 Gait variables 289
Table 5 shows the means and standard deviations of the gait variables for each 290
footwear type. Two-way repeated-measures ANOVA indicated footwear type did not 291
significantly affect stride time, stride length, or walking velocity (p > 0.05). 292
4. Discussion 293
The results of this study support our hypotheses that the adhesion friction of the hybrid 294
rubber sole would afford slip protection comparable to, or better than, that provided by 295
commonly-used strap-on winter anti-slip devices which rely on ploughing friction. 296
Although some of these devices yielded somewhat higher coefficients of friction in 297
certain drag-test conditions, the hybrid rubber sole was unique in providing high static 298
(SCOF≧0.37) and dynamic (DCOF≧0.31) coefficients of friction across all test 299
conditions, and was the only footwear type to provide a substantial dynamic coefficient of 300
friction (0.34) on the wet icy surface (0°C). These results compare favorably with respect 301
to the literature which suggests that COF values in the range of 0.2 to 0.4 are required to 302
prevent or arrest slips during level walking (Grönqvist et al., 1989, 2003; Nagata et al. 303
2009; Redfern and Bidanda,, 1994; Strandberg, 1983). In terms of preventing slips during 304
our gait tests, the hybrid rubber soles were found to be just as effective as the crampons 305
and gritted soles, and were actually more effective than the metal coils. 306
As hypothesized, the hybrid rubber sole exhibited high SCOF and DCOF values at 307
both temperatures. Presumably, under dry-ice conditions (-10°C), good adhesion was 308
attained because the low hardness of the hybrid rubber sole at low temperature (shore A 309
scale hardness of 41 at -10°C) contributed to maintaining contact area with the ice surface. 310
The hybrid rubber sole was apparently also able to achieve good adhesion under wet-ice 311
14
conditions (0°C) by dispersing the water film from the shoe–ice surface interface (during 312
static friction tests) or by limiting infiltration of water (during dynamic friction tests); see 313
Fig. 1(b). This mechanisms shown in Fig. 1(b) would be the same as observed in drag 314
tests on a glycerol-lubricated surface (Yamaguchi, et al. 2012a). Somewhat surprisingly, 315
the DCOF was actually higher in the 0°C wet-ice condition (0.34) in comparison to -10°C 316
(0.31). One possible explanation is that the water on the wet icy surface formed bridges 317
(meniscus) at the edge of the smooth part of the hybrid rubber block (Lee, et al., 2012). 318
The other three footwear types were designed to create ploughing friction via 319
penetration of studs, grit (aluminum oxide) particles or steel coils into the ice surface. 320
Such penetration would be most likely to occur when the ice is soft (0°C). Accordingly, 321
the studs and gritted soles did provide high SCOF (≧0.32) at 0°C; however, the low 322
SCOF value (0.10) for the metal coils at 0°C suggests that the coils failed to achieve 323
adequate penetration of the ice. Moreover, the low DCOF values (0.06-0.2) obtained for 324
the studs, grit and coils at 0°C suggest that the ploughing resistance provided by the soft 325
ice during dragging was insufficient to overcome hydroplaning effects due to the water 326
layer. 327
In the gait tests, the hybrid rubber outsoles, studs and gritted soles all tended to show 328
equally low frequency of slipping, and the only statistically significant difference was an 329
elevated frequency of forward slips when using the metal coils to descend the sloped 330
walkway at -10°C. Although the temporal and spatial gait parameters that were analyzed 331
(stride time, length and velocity) were not significantly affected by footwear type, the 332
analyses of the required coefficients of friction (RCOF) suggest that some subtle gait 333
alterations were utilized to reduce risk of slipping. In particular, the RCOF values during 334
weight acceptance (RCOFh) and push-off (RCOFt) at -10°C tended to be lower when 335
15
wearing the crampons, gritted soles or metal coils, in comparison to the hybrid rubber 336
soles, suggesting an adaptation to increase the safety margin between the required and 337
available friction when wearing the ploughing devices on a hard-ice surface. 338
A potential limitation of the present study pertains to the loading that was used during 339
the drag tests. Although the 6-kg load is comparable to loads used in some other studies 340
of footwear coefficients of friction (King et al., 2013; Fong et al., 2005), it has been 341
suggested that such loads may not provide a good simulation of the conditions that lead to 342
slips in daily life (Chang et al., 2001). In particular, it is possible that higher loading 343
would have led to greater penetration of the ice and hence increased coefficients of 344
friction in the devices that rely on ploughing friction. In addition, variation in the speed 345
of drag (due to the fact that the drag was controlled manually) could have possibly had an 346
influence (Chang et al., 2001), although the differences in drag speed were relatively 347
small. An ad hoc comparison (p<0.05) revealed that the mean drag speed was ~10% 348
faster at 0°C in comparison to -10°C, and about 20% faster when testing the gritted sole in 349
comparison to the crampons. It should also be noted that the drag tests fail to emulate 350
dynamic "squeeze-film effects" that can occur on wet surfaces during the landing phase 351
of gait, but it is not clear to what extent this may have affected the findings. The simple 352
drag-test methodology has been used in previous wet-surface footwear studies (Fong, et 353
al., 2005; Fong, et al., 2009; Nagata, et al., 2009; King, et al., 2013) and hence may 354
provide a useful basis for comparison; however, further work is needed to determine how 355
well these test results actually predict the ability to avoid slips during gait. 356
A limitation of the gait trials was that the step/stride length was regulated by the floor 357
panel configuration. The average stride length while walking at a natural pace for an adult 358
male is 1.51 m (Winter, 1999), whereas the average stride length of male participants in 359
16
this study was 1.26 m. Because of the shorter stride length, the walking velocity was also 360
slower than that of the natural gait of young adults. Slip potential, i.e., RCOF, is 361
influenced by step length and walking velocity (Perkins, 1978; Grönqvist et al., 1989; 362
Yamaguchi, et al., 2008); therefore, it will be necessary to investigate the risk of slipping 363
and falling when walking on ice with a longer stride and faster velocity. Another 364
limitation pertains to the possibility of anticipation and adaptation effects. The 365
participants knew prior to the trial that they had to land on the icy surface, and this may 366
have led to a change in their gait to decrease slip potential, as reported by Cham and 367
Redfern (2002b). Further investigations under more unpredictable conditions and during 368
other walking conditions (such as turning, gait initiation and gait termination) should 369
increase slip potential (Yamaguchi et al., 2013) and help to verify that the hybrid rubber 370
sole can be utilized as an effective anti-slip device during winter. Asking subjects to 371
perform a distracting cognitive task during the gait trials may also help to reduce 372
adaptation and increase slipping (Woollacott and Shumway-Cook, 2002). In the present 373
study, slips occurred in only 3% of gait trials. Although these data were sufficient to 374
identify some footwear-related differences, the use of test conditions that increase slip 375
frequency would help to increase the statistical power to detect footwear-related 376
differences. 377
In future work, it would be beneficial to expand the testing to include a wider range of 378
both footwear and anti-slip devices. Although the specific deformation-friction devices 379
that were tested in this initial study were intended to represent the most commonly-used 380
approaches (studs/crampons, metal coils and gritted surfaces), further work will be 381
needed to determine whether similar devices from other manufacturers perform more 382
effectively and to ensure that the specific samples that are tested are representative of the 383
17
product. In addition, it will be important to determine the extent to which the performance 384
is dependent on the type of shoe or boot that is worn in conjunction with the anti-slip 385
device, since footwear features (e.g. outer sole stiffness) could well influence the pattern 386
of contact between the anti-slip device and the icy surface. In the present study, the drag 387
tests used only a single type of shoe, and the gait protocol was restricted to the winter 388
shoes or boots that the subjects normally wear (seven participants wore various models of 389
athletic shoes and nine participants wore various models of winter boots). Finally, there 390
is a need to consider practical issues such as durability and wear resistance, which may 391
have an important influence on cost. In particular, the hybrid rubber sole is very soft (40 392
Shore A hardness) compared with common footwear (>50); hence, further work is needed 393
to evaluate its wear resistance. 394
5. Conclusions 395
Our drag-test and gait-trial results obtained on wet and dry icy surfaces indicated that 396
the hybrid rubber sole showed a slip resistance that was comparable to that of 397
conventional winter anti-slip devices. The hybrid rubber sole exhibited relatively high 398
SCOF (≧0.37) and DCOF (≧0.31) values on the drag test, and no significant differences 399
in the SCOF and DCOF values were observed when the environmental temperature 400
changed. The other three footwear devices had substantially lower DCOF values 401
(0.06-0.20) at 0°C compared with the hybrid rubber sole. Analysis of the frequency of 402
slipping while walking on level and sloped icy surfaces provided no evidence that the 403
hybrid rubber sole was any less effective than the other footwear devices in preventing 404
slips, and in fact showed significantly fewer slips than the metal-coil device. These 405
results suggest that the hybrid rubber sole may be a viable approach to decreasing 406
slip-and-fall accidents on icy surfaces. In addition to providing good slip resistance on icy 407
18
surfaces, the hybrid rubber outsole may have a number of advantages over anti-slip 408
devices that rely on ploughing friction, including reduced susceptibility to tripping 409
(Bruce et al., 1986), better slip resistance on non-icy surfaces (Yamaguchi et al., 2014), 410
and more comfortable walking on non-slippery surfaces. On the other hand, the 411
low-hardness rubber used in the hybrid insole may be more susceptible to wear when 412
walking on concrete pavement or other non-slippery surfaces (Gore and Gates, 1997). 413
Further studies are needed to confirm the effectiveness and wear resistance of the hybrid 414
rubber sole under a wider range of test conditions. 415
416
Conflict of interest statement 417
None of the authors has any conflict of interest, including specific financial interests, 418
relationships, and/or affiliations relevant to the participant matter or materials included in 419
this manuscript. 420
Acknowledgments 421
This study was partially supported by the Excellent Young Researchers Overseas Visit 422
Program from the Japan Society for the Promotion of Science (JSPS). This study was also 423
supported, in part, by the Canadian Institutes of Health Research (grant #MAT-91865). 424
Infrastructure support was provided by the Toronto Rehabilitation Institute (with grants 425
from the Canadian Foundation for Innovation, the Ontario Innovation Trust and the 426
Ministry of Research and Innovation) and by the Sunnybrook Research Institute. 427
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522
24
Captions to Tables 523
Table 1 Means and standard deviations for the static (SCOF) and dynamic coefficients of 524
friction (DCOF) values in the drag test 525
Table 2 Means and standard deviations (across subjects) for the percentage of gait trials in 526
which a slip occurred (n indicates the actual number of slip trials) 527
Table 3 The number of slip trials and means and standard deviations of slip distance and 528
maximum slip velocity in slip trial 529
Table 4 Means and standard deviations of the peak required coefficients of friction 530
(RCOF) values in the gait trials 531
Table 5 Means and standard deviations of the gait variables in the gait trials 532
Captions to Figures 533
Figure 1 (a) Sneaker-type footwear with an outsole having the hybrid rubber surface 534
pattern. 535
Note the smooth and rough rubber blocks, which are designed to provide high SCOF and 536
DCOF under both wet and dry conditions (as detailed in Figure 1(b)), and the channels 537
between adjacent sets of blocks which are intended to aid in dispersion of fluid from the 538
interface with the floor. 539
540
Figure 1(b) Possible mechanisms for the high slip resistance of the hybrid rubber outsole 541
on wet floor surfaces (adapted from the literature (Yamaguchi, et al., 2014)). 542
The figure displays a single set of rough and smooth rubber blocks. As shown in the left 543
side of the figure, the contact pressure is high at points where asperities in the rough 544
rubber block contact the floor. This high pressure disperses the liquid from the points of 545
25
contact and thereby increases adhesion friction between the rough rubber and the floor, 546
resulting in high SCOF. The right side of the figure shows the situation where a slip has 547
occurred. In this situation, the leading edge of each set of rubber blocks may help to 548
reduce infiltration of the liquid (Moldenhauer and Kröger, 2010) and thereby help to 549
secure direct contact between the smooth rubber and the floor during the slip, resulting in 550
high adhesion friction and high DCOF (Yamaguchi et al. 2012a). 551
552
Figure 2 Footwear gear tested 553
A) Hybrid rubber sole: New footwear gear with a rubber outsole having the hybrid 554
surface pattern. The shore A-scale hardness of 40 at room temperature and 41 at −10°C. 555
B) Metal coils consists of steel coiled elastic bands that fit over winter boots or shoes 556
(YAKTRAX Walker Traction Cleats for Snow and Ice, Interex Industries, Vancouver, 557
BC) 558
C) Gritted (sandpaper-like) straps fit over winter shoes and boots and feature bands of 559
sandpaper-like aluminum oxide mounted onto rubber (GLIPS-LITE Rough Grips, Winter 560
Walking, Horsham, PA) 561
D) Crampons (or ice cleats, or studded grips) fit over winter boots or shoes and come with 562
metal studs that are partially embedded in rubber. This example uses 16 tungsten carbide 563
studs (ALTRAGRIPS-LITE, Winter Walking, Horsham, PA). 564
565
Figure 3 566
A) a level walkway and B) a sloped walkway created by mounting the middle flooring 567
block on a slope and lowering the fourth and fifth panels. The first and last blocks are 568
concrete flooring blocks and the middle three flooring blocks are ice blocks. The second 569
26
and third ice flooring blocks are mounted over two forceplates. 570
571
Figure 4 572
A) Schematic diagram of the drag test footwear on the ice surface to measure the static 573
(SCOF) and dynamic coefficients of friction (DCOF) 574
The ice flooring block is fixed atop a support structure that includes the AMTI force 575
plates. 576
577
Figure 5 578
Variation over time of the sliding velocity and the coefficient of friction in the drag test; 579
dashed line means mean DCOF value when the sliding velocity was >0.1 m/s; type of 580
footwear gear is meal coils; temperature is 0 degrees C. 581
582
Figure 6 583
Representative variations over time in the required coefficient of friction (RCOF) while 584
in the stance phase during walking and definition of peak RCOF values during the weight 585
acceptance and toe-off phases while level walking; 0% and 100% of the gait cycle means 586
heel-strike and toe-off, respectively. 587
588
Figure 7 589
Relationship between static (SCOF) and dynamic coefficient of friction (DCOF) values 590
for each footwear gear in the drag tests at (A) 0°C and (B) −10°C 591
The error bars are standard deviations. 592
Measures Temperature, °C Hybrid rubber
sole Metal coils Gritted straps Crampons
SCOF 0 0.37 (0.03) 0.10 (0.04)* 0.32 (0.03) 0.36 (0.10)
−10 0.45 (0.04) 0.39 (0.02)* 0.67 (0.06)* 0.48 (0.04)
DCOF 0 0.34 (0.03) 0.06 (0.01)* 0.08 (0.02)* 0.20 (0.06)*
−10 0.31 (0.03) 0.33 (0.01) 0.37 (0.02)* 0.38 (0.04)*
Table 1 Means and standard deviations for the static (SCOF) and dynamic coefficients of friction (DCOF) values in the drag test
* Significant difference to hybrid pattern
Temperature,
degrees C Trial type Slip direction
Hybrid
rubber sole Metal coils
Gritted
straps Crampons p-value
0
Level walking
Forward 1.04 (4.17)
n = 1
1.04 (4.17)
n = 1
1.04 (4.17)
n = 1
1.04 (4.17)
n = 1 1.00
Backward 0 .00 (0.00)
n = 0
1.04 (4.17)
n = 1
0 .00 (0.00)
n = 0
1.04 (4.17)
n = 1 0.57
Ascending
Forward 2.08 (8.33)
n = 1
0 .00 (0.00)
n = 0
0 .00 (0.00)
n = 0
6.25 (18.13)
n = 3 0.11
Backward 2.08 (8.33)
n = 1
0 .00 (0.00)
n = 0
0 .00 (0.00)
n = 0
0 .00 (0.00)
n = 0 0.39
Descending
Forward 2.08 (8.33)
n = 1
0 .00 (0.00)
n = 0
2.08 (8.33)
n = 0
6.25 (13.44)
n = 3 0.28
Backward 0 .00 (0.00)
n = 0
0 .00 (0.00)
n = 0
0 .00 (0.00)
n = 0
0 .00 (0.00)
n = 0 -
−10
Level walking
Forward 3.13 (6.72)
n = 3
3.13 (6.72)
n = 3
2.08 (5.69)
n = 2
4.17 (12.91)
n = 4 0.87
Backward 0 .00 (0.00)
n = 0
1.04 (4.17)
n = 1
1.04 (4.17)
n = 1
0.00 (0.00)
n = 0 0.57
Ascending
Forward 0 .00 (0.00)
n = 0
2.08 (8.33)
n = 1
0 .00 (0.00)
n = 0
0 .00 (0.00)
n = 0 0.39
Backward 0 .00 (0.00)
n = 0
2.08(8.33)
n = 1
0 .00 (0.00)
n = 0
0 .00 (0.00)
n = 0 0.39
Descending
Forward 0 .00 (0.00)
n = 0
6.25 (18.13)
n =3
0 .00 (0.00)
n = 0
0 .00 (0.00)
n = 0 0.03*
Backward 0 .00 (0.00)
n = 0
0 .00 (0.00)
n = 0
0 .00 (0.00)
n = 0
0 .00 (0.00)
n = 0 -
Table 2 Means and standard deviations (across subjects) for the percentage of gait trials in which a slip occurred
(n indicates the actual number of slip trials)
* Significant difference among footwear gears
Temperature,
degrees C Variables
Hybrid rubber
sole Metal coils Gritted straps Crampons p-value
0
Number of slip trial 3 2 2 8 -
Slip distance, cm 4.9 (1.9) 3.7 (0.2) 4.2 (1.3) 4.5 (2.0) 0.74
Maximum slip
velocity, m/s 0.45 (0.32) 0.39 (0.02) 0.89 (0.66) 0.55 (0.35) 0.94
−10
Number of Slip trial 3 9 3 4 -
Slip distance, cm 3.7 (0.7) 4.3 (1.2) 4.6 (1.6) 4.5 (0.3) 0.88
Maximum slip
velocity, m/s 0.34 (0.21) 0.31 (0.10) 0.36 (0.22) 0.28 (029) 0.51
Table 3 The number of slip trials and means and standard deviations of slip distance and maximum slip velocity in slip trial
Temperature,
degrees C Trial type RCOFs
Hybrid rubber
sole Metal coils Gritted straps Crampons
0
Level walking RCOFh 0.12 (0.03) 0.13 (0.02) 0.13 (0.02) 0.12 (0.02)
RCOFt 0.21 (0.04) 0.21 (0.03) 0.22 (0.03) 0.20 (0.04)
Ascending RCOFh 0.09 (0.03) 0.09 (0.03) 0.08 (0.02) 0.09 (0.03)
RCOFt 0.31 (0.09) 0.31 (0.12) 0.33 (0.11) 0.31 (0.11)
Descending RCOFh 0.19 (0.04) 0.20(0.04) 0.20 (0.03) 0.19 (0.05)
RCOFt 0.16 (0.05) 0.16 (0.04) 0.17 (0.04) 0.16 (0.04)
−10
Level walking RCOFh 0.14 (0.02) 0.13 (0.02) 0.12 (0.02)* 0.11 (0.02)*
RCOFt 0.22 (0.03) 0.21 (0.02) 0.20 (0.03)* 0.19 (0.03)*
Ascending RCOFh 0.08 (0.02) 0.09 (0.03) 0.08 (0.02) 0.09 (0.03)
RCOFt 0.29 (0.04) 0.26 (0.02) 0.26 (0.05)* 0.29 (0.05)
Descending RCOFh 0.20 (0.03) 0.19 (0.03) 0.19 (0.02) 0.19 (0.03)
RCOFt 0.17 (0.05) 0.14 (0.04)* 0.17 (0.03) 0.14 (0.05)
Table 4 Means and standard deviations of the peak required coefficients of friction (RCOF) values in the gait trials
* Significant difference to hybrid pattern
Temperature,
degrees C Trial type Gait parameters
Hybrid rubber
sole Metal coils Gritted straps Crampons
0
Level walking
Stride time, s 1.23 (0.09) 1.25 (0.07) 1.24 (0.10) 1.20 (0.09)
Stride length, m 1.18 (0.16) 1.19 (0.13) 1.16 (0.15) 1.15 (0.16)
Walking velocity, m/s 1.06 (0.18) 1.06 (0.18) 1.10 (0.18) 1.07 (0.17)
Ascending
Stride time, s 1.2 (0.24) 1.19 (0.15) 1.17 (0.31) 1.08 (0.25)
Stride length, m 1.28 (0.22) 1.26 (0.23) 1.24 (0.23) 1.30 (0.32)
Walking velocity, m/s 0.95 (0.20) 0.96 (0.18) 0.99 (0.28) 0.87 (0.22)
Descending
Stride time, s 1.21 (0.13) 1.13 (0.23) 1.25 (0.12) 1.16 (0.22)
Stride length, m 1.34 (0.21) 1.35 (0.25) 1.27 (0.22) 1.21 (0.25)
Walking velocity, m/s 0.99 (0.20) 0.88 (0.26) 1.08 (0.28) 1.07 (0.19)
−10
Level walking
Stride time, s 1.30 (0.12) 1.25 (0.09) 1.25 (0.12) 1.25 (0.06)
Stride length, m 1.19 (0.20) 1.26 (0.23) 1.19 (0.19) 1.21 (0.26)
Walking velocity, m/s 1.14 (0.23) 1.03 (0.18) 1.09 (0.19) 1.09 (0.23)
Ascending
Stride time, s 1.18 (0.17) 1.15 (0.10) 1.25 (0.09) 1.10 (0.12)
Stride length, m 1.28 (0.23) 1.27 (0.21) 1.27 (0.29) 1.35 (0.23)
Walking velocity, m/s 0.94 (0.18) 0.93 (0.15) 0.96 (0.40) 0.84 (0.1)
Descending
Stride time, s 1.25 (0.16) 1.20 (0.24) 1.15 (0.21) 1.25 (0.11)
Stride length, m 1.23 (0.23) 1.28 (0.26) 1.32 (0.28) 1.32 (0.30)
Walking velocity, m/s 1.09 (0.21) 0.96 (0.17) 0.98 (0.24) 1.03 (0.23)
Table 5 Means and standard deviations of the gait variables in the gait trials
Figure 1 (a) Sneaker-type footwear with an outsole having the hybrid rubber surface pattern.
Note the smooth and rough rubber blocks, which are designed to provide high SCOF and DCOF under both wet and dry
conditions (as detailed in Figure 1(b)), and the channels between adjacent sets of blocks which are intended to aid in
dispersion of fluid from the interface with the floor.
Smooth part
Rough part
Channels
Figure 1(b)- Possible mechanisms for the high slip resistance of the hybrid rubber outsole on wet floor surfaces (adapted from the
literature (Yamaguchi et. al., 2014)).
The figure displays a single set of rough and smooth rubber blocks. As shown in the left side of the figure, the contact pressure is high
at points where asperities in the rough rubber block contact the floor. This high pressure disperses the liquid from the points of contact
and thereby increases adhesion friction between the rough rubber and the floor, resulting in high SCOF. The right side of the figure
shows the situation where a slip has occurred. In this situation, the leading edge of each set of rubber blocks may help to reduce
infiltration of the liquid (Moldenhauer and Kröger, 2010) and thereby help to secure direct contact between the smooth rubber and the
floor during the slip, resulting in high adhesion friction and high DCOF (Yamaguchi et al. 2012a)
Figure 2 Footwear gear tested
A) Hybrid rubber sole: new footwear gear with a rubber outsole having the hybrid surface pattern. The shore A hardness was 40
B) Metal coils consists of steel coiled elastic bands that fit over winter boots or shoes (YAKTRAX Walker Traction Cleats for
Snow and Ice, Interex Industries, Vancouver, BC)
C) Gritted (sandpaper-like) straps fit over winter shoes and boots and feature bands of sandpaper-like aluminum oxide mounted
onto rubber (GLIPS-LITE Rough Grips, Winter Walking, Horsham, PA)
D) Crampons (or ice cleats, or studded grips) fit over winter boots or shoes and come with metal studs that are partially embedded
in rubber. This example uses 16 tungsten carbide studs (ALTRAGRIPS-LITE, Winter Walking, Horsham, PA).
A) B)
C) D)
0.84 m 0.76 m 0.76 m 0.84 m 0.44 m
0.73 m
0.84 m 0.76 m 0.76 m 0.84 m 0.48 m
0.01 m
0.73 m 0.048 m
Forceplate
A)
B) Rigid spacer
Concrete flooring block
Ice flooring block
Figure 3 A) a level walkway and B) a sloped walkway created by mounting the middle flooring block on a slope and
lowering the fourth and fifth panels. The first and last blocks are concrete flooring blocks and the middle three flooring
blocks are ice blocks. The second and third ice flooring blocks are mounted over two forceplates.
Sliding distance: 0.48m
Force plate Ice flooring block
LED
marker
Weight
Footwear gear
Figure 4
A) Schematic diagram of the drag test footwear on the ice surface to measure the static (SCOF) and dynamic (DCOF);
coefficients of friction The ice flooring block is fixed atop a support structure that includes the AMTI force plates.
LED marker
SCOF
Sliding initiation
Figure 5 Variation over time of the sliding velocity and the coefficient of friction in the drag test; dashed line means
mean DCOF value when the sliding velocity was >0.1 m/s; the type of footwear gear is metal coils; temperature is 0
degrees C.
Time t, s
Slid
ing
velo
cit
y v
, m
/s
CO
F
Sliding velocity
COF
Mean DCOF (v>0.1 m/s)
Figure 6
Representative variations over time in the required coefficient of friction
(RCOF) while in the stance phase during walking and definition of peak
RCOF values during the weight acceptance and toe-off phases while level
walking; 0% and 100% of the gait cycle means heel-strike and toe-off,
respectively.
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
0 10 20 30 40 50 60 70 80 90 100
RC
OF
Gait cycle, %
RCOFh
RCOFt
Level surface
0
0.1
0.2
0.3
0.4
0.5
0 0.2 0.4 0.6 0.8
SCOF
DC
OF
0
0.1
0.2
0.3
0.4
0.5
0 0.2 0.4 0.6 0.8
SCOF D
CO
F
Hybrid rubber sole
Gritted straps
Metal coils
Crampons
A) 0 degrees C B) −10 degrees C
Figure 7
Relationship between static (SCOF) and dynamic (DCOF) coefficient of friction values for each footwear gear in
the drag tests at (A) 0°C and (B) −10°C
The error bars are standard deviations.
Hybrid rubber sole
Gritted straps
Metal coils
Crampons