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| Journal of the New Zealand Medical Association,
03-July-2009, Vol 122 No 1298
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Preventing winter falls: a randomised controlled
trial of a novel intervention
Lianne Parkin, Sheila M Williams, Patricia Priest
There are anecdotal reports that pedestrians who wear socks
over top of their footwear are less likely to slip and fall in icy conditions.
Advocates of this practice include our local council (in Dunedin) which advises
residents who prefer to walk (rather than drive) in icy conditions to “put
a pair of old socks over your shoes to increase
grip”.1
Methods to enhance footwear traction have particular
relevance for our population. While the university, hospital, and business areas
of Dunedin are located on relatively flat land, most residential areas are
clustered on the surrounding hills. In winter, damp weather followed by freezing
conditions can transform a quick journey to work into a lengthy and perilous
expedition.2
Searches of Medline and the Cochrane Library (using the
terms “ice”, “falls”, “prevention”, and
“socks”) failed to locate any evaluations of unorthodox sock
wearing. To remedy this surprising gap in falls prevention research, we decided
to undertake a randomised controlled trial to investigate the hypothesis that
wearing socks over shoes improves traction on icy footpaths.
MethodsParticipants and settings—We
initially considered recruiting volunteers to walk down a short suburban street
(Baldwin Street) which, according to the Guinness Book of Records, is the
steepest street in the world. However this proved impractical for two reasons.
First, requiring volunteers to traverse a 1 in 2.86 gradient in icy conditions
seemed ethically and legally unwise. Second, in order to travel downhill in this
cul-de-sac, the researchers and volunteers would need to scale the incline. This
was not an attractive prospect.
We therefore decided to adopt a pragmatic approach and
intercept passing pedestrians at two other sites (Figure 1). These particular
sites were chosen because many university employees, students, and members of
the public used these routes each morning. Moreover, painful experience meant
that all of us were acquainted with their slippery nature in icy
conditions.
Figure 1. Study sites
To be eligible for inclusion in the trial, passing
pedestrians simply needed to be travelling in a downhill direction. It was
decided a priori that persons already wearing socks over their shoes
would not be eligible.
At both sites, the researchers were divided into two
groups: recruiters and outcome assessors. These groups were stationed at the
uphill and downhill ends, respectively, of the study slopes. Recruiters asked
pedestrians whether they were willing to take part in a study to assess the
anti-slip performance of different types of footwear and different types of
socks worn over the top of footwear.
Participants gave verbal consent and completed a
questionnaire which collected demographic data, as well as information about
experience with icy conditions, previous falls on ice, injuries, familiarity
with the route, and type of footwear (also photographed). Once this information
had been recorded, recruiters opened a sealed envelope to ascertain the group to
which the participant was allocated.
Intervention—Participants in the
intervention group were provided with a pair of socks to put on over their
footwear (Figure 2). The acrylic-blend work socks (size 11–14) were
purchased in bulk from a budget department store using independent research
funds.
Individuals in the intervention (socks) and control (no
socks) groups were directed to walk downhill as normally as possible (given the
conditions). In light of the observed behaviour of pedestrians (often young men)
at these sites on previous mornings, participants were asked to refrain from
deliberately skidding or sliding.
Figure 2. Correctly fitted socks
 Outcomes—On reaching the outcome
assessors, participants were asked to complete an assessment form. Self-rated
slipperiness (the primary outcome) was measured using a validated slipperiness
scale.3 Participants were asked to indicate on
the 5-point scale how slippery they found their descent: “not
slippery”, “somewhat slippery”, “slippery”,
“very slippery”, or “extremely slippery”. Previous
research has shown a strong, statistically significant correlation between
subjective reports of slipperiness and objective measures of friction
(r=0.90).4 Participants were also asked to
report any falls and to make any other comments they wished.
To validate self-reported slipperiness, outcome
assessors independently recorded (using the 5-point scale) how slippery
participants appeared to have found the footpath. Assessors were also asked to
document any falls and to comment on the demeanour of the participants during
their descent (for example, “walked confidently”, “clung to
fences or parked cars”, “crawled”). Finally, to detect any
risk compensation in the intervention group, the assessors used stop-watches
(standard issue obtained from one electronics shop) to time the descent of each
participant. Landmarks such as water valve covers and traffic signs were used as
starting and stopping points.
Sample size—Sample size
calculations were undertaken using PS
software.5 To detect a difference of 1.5 in
mean self-rated slipperiness, with a 1:1 ratio of intervention to control
participants, using an alpha of 0.05, 90% power, and a within group standard
deviation of 1.1,3 we calculated that 12 people
in each arm of the trial would be required.
Randomisation—Microsoft Excel
software was used to generate the random allocation sequence which was
stratified by site. Sheets of paper noting the allocation status (socks / no
socks) were placed in numbered opaque envelopes which were then sealed.
Recruiters were instructed to use these envelopes in numerical sequence after
they had administered the baseline questionnaire.
Blinding—It was not possible to
blind the participants and outcome assessors to treatment allocation. However,
certain measures were employed to conceal the exact nature of the study
hypothesis, and hence minimise biased assessment of outcome.
First, to avoid any implication that socks were
superior, all recruiters and outcome assessors were instructed to wear
unmodified footwear. Second, participants and assessors were simply told that we
were interested in assessing the performance of different types of footwear and
different types of socks worn over the top. Third, participants’ footwear
was photographed for later reference and this might have encouraged participants
and assessors to think that the characteristics of footwear were important.
Fourth, because we had heard anecdotal accounts about the supremacy of certain
types of socks, we deliberately allocated socks of three different colours to
confuse any avid sock supporters about what, exactly, was being tested. To avoid
any disclosure of the true state of affairs (that the socks were of identical
composition), the labels were removed by the principal investigator. This action
was also necessary because the socks were labelled “alpha
♂™” and we did not wish to encourage any rash behaviour in the
intervention group.
Statistical methods—Observer
comments about the demeanour of participants were summarised into three
categories: “confident”, “cautious, but did not hold onto
supports” (fences, railings, or parked cars), and “held onto
supports”. The data were analysed according to intention to treat. The
groups were compared using a t-test for the continuous or ordinal variables and
a Fisher’s exact test for the categorical variables. Because men were
known to be more intrepid than women, the sample size was increased to allow
adjustment for sex.
Ethical approval—Ethical
approval was granted by the University of Otago Ethics Committee at departmental
level.
ResultsParticipants—The trial was conducted
on 15 August 2008. A total of 30 pedestrians underwent randomisation (Figure 3).
One young woman after agreeing to participate, and appearing to understand the
instructions, inexplicably turned to walk back uphill and disappeared. The most
common reason given for not participating in the trial was “running late
for lectures”.
No-one was already wearing socks over their shoes. Only one
participant did not fully comply with the study protocol: a segment of redundant
sock at the toes (resulting from improper application) created a hazard. In
accordance with intention to treat principles, her data were analysed as
randomised.
The baseline characteristics of the participants are shown
in Table 1. High proportions of both groups had previously fallen on ice. All
participants were wearing sensible footwear.
Table 1. Baseline characteristics of study
participants
Figure 3. Flow of participants through the
trial
 Outcomes—Wearing socks over footwear
significantly improved traction (Table 2). The mean self-reported slipperiness
scores in the intervention and control groups were 1.6 (SD 1.14) and 2.9 (SD
1.32) respectively (difference in means 1.3, 95%CI: 0.4–2.3). This
difference increased to 1.4 (95%CI: 0.4–2.3) after adjusting for sex.
There was a high level of agreement between self-rated and observer-rated
slipperiness (r=0.70).
A higher proportion of the intervention group (71%, 10/14)
than the control group (53%, 8/15) appeared confident while descending the study
slopes, although the difference was not statistically significant (p=0.45).
There was no evidence of risk compensation in the intervention group (difference
in mean descent times 1.9 seconds, 95%CI: -6.1–10.0).
Table 2. Pre-specified primary and secondary
outcomes
*Observer-rated
slipperiness score was missing for one control.
Two members of the control group and one in the intervention
group (who tripped on improperly applied socks) slipped, but only one fell (a
control). Although participants in the intervention group were told that they
could keep their socks, many (who appeared to have image issues) opted to return
them to the outcome assessors —including one young man who promptly fell
on leaving the assessment area. Falls were also observed, incidentally, in
non-sock-wearing pedestrians negotiating intersecting streets. No obvious
injuries were sustained in the vicinity of the study sites.
Feedback from the intervention group about the use of socks
was informative: “socks are key!!”, “that was sweet as”,
“recommend socks for hungover people”, “socks helped with
slipperiness but wouldn’t wear them to uni[versity]!”
Adverse events—The only adverse
events were short periods of embarrassment for the image-conscious in the
intervention group.
DiscussionWearing socks over footwear significantly reduced the
self-reported slipperiness of icy footpaths and a higher proportion of
sock-wearers displayed confidence in descending the study slopes. The only falls
occurred in people who were not wearing (external) socks.
The trial had other unanticipated benefits. For example, a
retired couple who lived beside one of the study sites provided a compelling
oral history covering several decades of ice-related mishaps on their street.
It was not possible to blind participants or outcome
assessors. However, some obfuscation of the exact hypothesis reduced the
potential for biased outcome assessment. Moreover, it was reassuring to learn
that many of the participants had previously been unaware of this novel use of
socks. Apart from sex, no adjustment was made for imbalances in the baseline
characteristics of the groups as these were not specified before beginning the
study. It is possible that the control group, having a larger proportion of
participants who had previously fallen on ice, were more inclined to report
slipperiness and to be less confident. However, the difference in proportions
was related to the sex imbalance between the two groups and the fact that men in
the control group were more likely to have fallen than the women. We did not
enquire about the circumstances of previous falls, but if the excess among the
men resulted from deliberate attempts to slide this would make it less likely
that our results are an artefact of a higher level of trepidation within the
control group.
The research presented several unique challenges.
Unfortunately, freezing conditions, unless accompanied by a certain degree of
moisture, do not guarantee a slippery footpath. Thus we could not set a specific
date for data collection. Although our Head of Department’s suggestion to
furtively spray the study slopes with water had some practical merit, we were
obliged to reject his idea and wait for suitable conditions. Inevitably, when
they did occur, it was difficult to reach the study sites.
It has been suggested that new arrivals to cold climates
should be warned about the dangers of falling on ice and, moreover, should be
given special training on how to walk in such
conditions.6 As part of this preventive
approach, perhaps municipal authorities in colder regions of the country could
consider issuing a large pair of socks (in local colours) to each new resident.
Research questions for the future include “does wool
perform better than synthetic?” and “which socks perform best in a
cost-effectiveness analysis?” Other suggestions for future investigations
are of a practical nature: provide a thick rug for participants to sit on while
putting on socks, supply socks of varying sizes, and pack a thermos flask.
ConclusionDespite some residual scientific uncertainty, because of the
high frequency of ice-related falls in our population, the cheap and simple
nature of the socks-over-shoes intervention, and the absence of physical harm
(if correctly fitted), we feel inspired to join an eminent professor, herself a
long-time proponent of socks, in adopting this practice this winter.
Competing interests: None known. In
particular, none of the authors has financial links with sock manufacturers and
none of us own sheep.
Author information: Lianne Parkin, Senior
Lecturer in Epidemiology, Sheila M Williams, Research Associate Professor,
Patricia Priest, Senior Lecturer in Epidemiology; Department of Preventive and
Social Medicine, University of Otago, Dunedin
Acknowledgements: We thank the participants
for their time and good humour. Emma Wyeth, Sue McAllister, Dan Jahnke, G. Peter
Herbison, and Kimberly Cousins all assisted with the data collection. Professor
Charlotte Paul provided the inspiration for the study.
Correspondence: Lianne Parkin, Department
of Preventive and Social Medicine, University of Otago, PO Box 913, Dunedin, New
Zealand. Fax: (03) 4797298; email: lianne.parkin@stonebow.otago.ac.nz
References:
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