MAXIMUM ACHIEVABLE INCLINES FOR FOOTWEAR WHILE
WALKING ON ICY SLOPES AND CROSS-SLOPES
Jennifer Hsu1,2 and Geoff Fernie1,21Toronto Rehabilitation Institute – University Health Network, 550 University Avenue, Toronto,
ON M5G 2A2 Canada
2
University of Toronto, Department of Mechanical and Industrial Engineering, 5 King’s
College Road, Toronto, ON M5S 3G8 Canada

More informative tests of winter footwear performance are required in order toprevent injurious slips and falls on icy conditions. In this study, 8 participants
tested 4 styles of winter boots on longitudinal and cross-sloped surfaces consistingof smooth, wet ice as it was progressively tilted until participants could no longer
continue standing or walking. The maximum achievable slopes provided consistentmeasures of slip resistance, demonstrated over a wide range of observable
performance. One footwear soling material and tread combination outperformed
the others on the wet ice surface allowing participants to successfully walk on
longitudinal slopes of 17.5° ± 1.9° (mean ± SD) while others failed beyond 3.3° ±
0.5°, 5.1° ± 0.8°, and 7.5° ± 0.9°. By further exploiting the methodology to include
additional surfaces and contaminants, such tests can be used to optimize tread
designs and materials that are ideal for reducing the risk of slips and falls.

Introduction
Slips and falls are one of the most prevalent and injurious types of accidents on-the-job and in
public spaces. Winter conditions increase the likelihood of pedestrian slips and falls as
precipitation and cold temperatures create hazards and obstacles (Gard & Lundborg, 2000). In
winter-experiencing environments, foot slippage is estimated to account for 43% of all falls and
16% of all accidents in Nordic countries (Lund, 1984). Medical care costs of slipping injuries
are estimated to be the same as that of all traffic injuries in the same period (Björnstig et al.,1997).
Slip-resistant footwear can reduce the risks of slips and falls (Abeysekera & Gao, 2001). In
order for footwear designers and manufacturers to optimize the slip resistant properties of
winter footwear and to appropriately inform consumers of footwear performance, it isimperative that methods of footwear assessment be relevant and reliable. However, themeasurement of footwear slip resistance is not straightforward (Chang et al., 2001) and while
there are standard mechanical methods for measuring footwear coefficient of friction (COF), theoutput validity is dependent on their ability to simulate human gait (Chang et al., 2001b). To
improve validity and because users sometimes prefer direct walking tests, small-scale ramp tests
have been developed to assess footwear slip resistance (James, 1999). In this study, a novel
ramp test allowing sustained, natural gait was used to evaluate the slip resistance of winterfootwear.

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Methods
Footwear

A total of 8 young, healthy males took part in this study to test the performance of 4 men’s winterboots with different outsole designs. Three of the styles consisted of identical uppers. Of these, 2styles used identical tread patterns composed of different materials: NCI rubber compound (BootN) and Green Diamond technology which consists of aluminium oxide and silicon carbide
granules embedded in rubber (Boot G). The outsole of the third boot style had no tread andconsisted of a proprietary JStep rubber compound (Boot J). The final boot style tested was alighter-weight boot, whose outsole includes a proprietary (IcePaw) design embedded in rubber.
The test boots are depicted in Figure 1. An active motion tracking system (Phoenix Technologies
Inc., British Columbia, Canada) was used to identify the positions of markers attached to the heels
and toes of each pair of boots during participant testing.

Figure 1. Test footwear; from left to right: Boot B, Boot G, Boot J, and Boot N

Test Conditions

The boots were tested on a 5.5m long walkway of smooth, wet ice. Ice was created by freezingwater and maintaining the ice temperature at -0.5°C ± 0.5°C via glycol tubes below the ice surface.
The ambient temperature of the room was held at 10.6°C ± 0.6°C and all test boots wereconditioned at this temperature for 1 hour prior to testing. The warmer ambient temperature in
combination with the near freezing ice temperature maintained a thin layer of melting water overthe ice surface. The COF of the ice floor was measured according to ASTM F1679 using a Neolite
pad on the English XL Variable Incidence Tribometer. The ice conditions were measured before
and after each test session and the average COF across all readings was 0.08 ± 0.05. The entire
laboratory was mounted over a motion base which could tilt the entire floor surface in any
direction to create sloped conditions. The laboratory could be tilted up to maximum slopes of 20°.

Test Protocol

Participants wore typical winter garments including winter coats throughout this study under afull-body safety harness. The harness was attached to a fall-arrest system connected to a robotic
overhead gantry that automatically followed the position of the participant. A controlled-descentline, balanced at 2kg, prevented injury from falls by allowing a maximum downward velocity of
0.7m/s. This study was approved by the Research Ethics Board of the Toronto Rehabilitation
Institute–UHN.
During the experiment, participants were asked to complete two sets of tests while wearing
each style of footwear: standing tests followed by walking tests. For the standing tests, participants
were asked to stand with their feet shoulder-width apart. They were instructed that they could
adjust their posture as required to maintain balance while the laboratory was tilted. The floor wastilted from level at a rate of 0.25° per second, pausing for 5 seconds at each full degree, until bothof the participants’ feet began to slide simultaneously. This test was conducted in 4 slope
directions, in random order, with the participant facing downhill, uphill, with their left side higher,
and their right side higher, see Figure 2. The final angle at which the participant was able toprevent both feet from sliding simultaneously was recorded as the maximum achievable angle foreach piece of footwear in each slope direction.

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Downhill
UphillLeft Side Up
Right Side Up
Longitudinal Slopes
Cross-Slopes
Figure 2. Two slope types (longitudinal and cross-slopes) and 4 slope directions (downhill,
uphill, left side higher, and right side higher) tested while standing and walking
The walking tests were conducted on longitudinal slopes and cross-slopes, with the tilting axisof the floor perpendicular to the direction of travel and parallel to the direction of travel,
respectively. For each slope type, participants walked in 2 slope directions; downhill and uphill on
the longitudinal slope and right side higher and left side higher on the cross-slopes, see Figure 2. Inthese tests, participants were asked to walk at a natural pace and in a controlled manner from one
end of the laboratory to the other and back again. Between laps, the slope angle of the walkway
was progressively increased. Each style of footwear was first tested on the level at 0° andsubsequent tests started from 5° less than the critical angles determined during standing tests,
changing in 1° increments until failure. For each participant, critical angles for each type offootwear were computed as the lowest maximum slope achieved in any slope direction during thestanding tests. If the critical angle was less than 5°, walking tests after the level began at 1° and for
critical angles greater than 16° participants included a 10° trial. A test angle for each slope typewas deemed a failure if both feet slipped simultaneously (not including controlled slides toterminate gait), or the participant could not initiate gait, in either slope direction. The order of slope
type and starting directions were randomized.

Results
Participants

The 8 males who participated in this study were 31.6 ± 8.1 years of age (mean ± SD), 179cm ±7cm in height, and weighed 80.2kg ± 15.4kg. All participants had lived in a winter-experiencing
climate for at least one winter season, averaging 18.5 ± 11.8 years spent living in winterexperiencing regions. All participants indicated via questionnaire that they spent at least 3-5days per week outdoors in winter and on average 1-2 hours outdoors during those days. 62% of
the participants indicated that they engage in winter sports such as skiing, skating or
snowboarding. Half of the participants had fallen in the previous 2 years but only one
participant had fallen outdoors and indicated that the fall was on ice. None of the participants
had experience using the test footwear prior to the experiment and all participants in this study
were right-foot dominant.

Standing Tests

A two-way repeated measures analysis of variance (ANOVA) was used to determine the effects
of footwear and slope direction on the maximum achievable slope angle while standing. The
results show that footwear significantly affected the maximum achievable angle,
F(3,21)=1476.22, p<.001. The main effect of slope direction and the interaction effect between
slope direction and footwear were not significant. All pairwise comparisons were correctedusing Bonferroni adjustments. Comparisons showed that there were significant differences
between all types of footwear (all p<.001). Boot J significantly outperformed all other footwear
with an average maximum achievable slope angle of 19.3° across all slope directions. Boot G
allowed participants to climb on average 9.4° slopes and performed significantly better than Boot
N and Boot B. Boot N averaged 6.0° slopes, which was significantly steeper than the 3.4° slopes
achieved by Boot B. Figure 3 shows the average maximum achievable inclines while standing in- 225 -

each boot for each slope direction along with the equivalent static coefficient of friction (staticCOF). The equivalent static COF is calculated as the tan of the maximum achievable angles while
standing. In increasing order, mean static COF for Boots B, N, G, and J were therefore 0.06, 0.11,
0.17, and 0.35, respectively, on the smooth, wet ice surface.

Figure 3. Average maximum slope angles (and equivalent static COF) at which
participants could maintain standing posture without sliding plotted with standard
deviation error bars

Walking Tests

A two-way repeated measures ANOVA was also used to determine the effects of footwear andslope type on the maximum slopes achieved during the walking task. The main effects offootwear (F(1.7,11.9)=397.94, p<.001) and slope type (F(1,7)=36.76, p<.05) were significant
and there was a significant interaction effect between footwear and slope type (F(3,21)=13.86,
p<.001). Pairwise comparisons indicated that each type of footwear performed significantly
differently from one another (all p<.05) during the walking tests, but ranked in the same orderas that of the standing tests. The results also showed that the maximum achievable angles weresignificantly lower while walking on cross-slopes than on longitudinal slopes. This phenomenon
is most evident with Boot J indicating that at steeper angles, differences between maximum
achievable angles on the longitudinal slope compared to cross-slopes are more pronounced,
which also gives rise to the significant interaction effect. Maximum achievable angles while
walking on ice are shown in Figure 4 along with the equivalent dynamic COF, calculated as tanof the maximum achievable angles while walking.

Figure 4. Average maximum achievable slope angles (and equivalent dynamic COF) while
walking plotted with standard deviation error bars

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When test type was included for analysis, a three-way ANOVA indicated that participants
were able to achieve significantly steeper slopes while standing compared with walking (p<.05).
As expected, static COF was thus significantly greater than dynamic COF.

Gait speed

Gait speed was calculated across the middle 2.5m section of the walkway for successful
walking trials using the position of the heel at the first heel strike in the 2m section and theposition of the same heel at its final heel strike in the section. A one-way ANOVA indicated thatgait speed on level ice was siginificantly affected by footwear (F(3,21)=6.47, p<.05) andparticipants walked significantly slower in Boot B (0.72m/s ± 0.16m/s) than in Boot J (0.99m/s± 0.24m/s) at 0° incline, p<.05. Gait speed decreased linearly with increasing slope angles in all
slope directions for each type of footwear with R2 values of .77 ± .22, see Figure 5. The ratio ofchange in gait speed per degree change in slope angle was analyzed using a two-way repeatedmeasures ANOVA to determine the effects of footwear and slope direction. Only the main
effect of footwear was significant, F(3,21)=5.62, p<.05). Results showed that participants
walked significantly slower per degree of incline in Boot B than in Boot G. See Table 1 formean reductions in gait speed per degree change in incline angle, for each type of footwear.

Figure 5. Representative data from one participant showing reduction in gait speed atincreasingly steep slope angles
Table 1. Mean and standard deviation of change in gait speed per degree of incline
Footwear
Mean ± SD (cm/s/°)
Boot B-10.0 ± 5.7
Boot N
-7.19 ± 5.4
Boot G-4.91 ± 3.8
Boot J-4.94 ± 4.1

Discussion
The smooth, JStep sole significantly outperformed all of the other boots in both the standing
tests and walking tests on the smooth, wet ice surface, allowing participants to walk on
longitudinal slopes of 17.5° ± 1.9° and cross-slopes of 15.1° ± 1.6°. The second best performing
footwear utilized the Green Diamond sole followed by the NCI rubber footwear, and finally the
IcePaw sole. Because the material and tread of the JStep footwear differed from those of theother test boots, further testing is required to determine the contribution of tread material andtread pattern independently, in the performance observed in this study. Furthermore, while thesmooth, wet ice represents one challenging winter surface, tests involving snow and othertypical winter conditions are necessary to develop complete performance ratings.
This study showed that steeper inclines could be achieved while walking on longitudinal- 227 -

slopes than cross-slopes. Postural changes on longitudinal inclines moved the body’s centre ofmass along the longer axis of the base of support while on cross-slopes the body’s centre ofmass was more likely to move outside the shorter axis of the base of support.
It should be noted that the two poorer performing styles of footwear failed beyond 3.3° ±
0.5° and 5.1° ± 0.8°, respectively, on longitudinal slopes. The maximum allowable slope of curb
ramps is 5.7° as specified in the Americans with Disability Act Standards for Accessible Design(Kirschbaum et al., 2001). Results of this study indicate that while wearing Boot B or Boot N,
users would be unable to maintain stable balance on standard curb ramps covered with wet ice.
Gait speed decreased the most per degree of incline while using the worst performing boots
in which gait speed decreased at a rate of 10cm/s per degree of incline while speed in the otherboots styles reduced at rates of 5cm/s/° to 7cm/s/°. In this experiment, participants were asked toself-select gait speed in order to complete the walking task. The young, healthy participants
were able to dramatically reduce their gait speed when necessary. However, for older users orusers with mobiltiy limitations, failure may have occurred at less steep angles. Alternative
definitions of footwear failure, such as speed thresholds may be explored for determining
failure.
The results also indicated that neither measuring gait speed on slippery level surfaces nor
determining the reduction in gait speed at steeper inclines could provide ratings of performanceto the resolution accomplished in this study. The performance outcomes from this study can beused to inform consumers making decisions regarding winter footwear. Current standard
assessments of footwear slip resistance measure coefficient of friction. However, there are nostandards for thresholds of COF, the results are difficult for consumers to interpret, and themeasurement mechanisms do not replicate all aspects of human gait. User-based determinationof maximum achievable angle on slippery slopes is a more useful representation of footwear
performance, which could also be interpreted using equivalent static and dynamic COF values,
if desired (as shown in Figure 3 and Figure 4).
This experiment showed that a wide spread of footwear performance can be assessed bywalking on increasingly steep slopes under controlled surface conditions, and that the results are
consistent and require a relatively small sample size. This study also demonstrates the feasibilityof conducting similar tests for other types of protective footwear, such as those for indoor
workers where floor surfaces and contaminant conditions are easier to predict and recreate.

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