An investigation into plantar pressure measurement protocols for footwear research
Introduction
The magnitude and distribution of pressure on the plantar surface of the foot has been associated with foot pain [1], [2], comfort [1] and devastating clinical problems such as foot ulcers [3]. Footwear research has therefore sought to investigate specific footwear designs that might reduce plantar pressures (PP) [4], [5], [6]. A wide range of footwear styles have been investigated, including running shoes, high-heeled shoes, boots, rollover footwear and instability footwear [4], [5], [7], [8], [9], [10], [11].
Many aspects of gait (PP [4], lower limb kinematics [12], kinetics [13] and muscle activity [14]) are sensitive to footwear design choices. Therefore, footwear can be said to represent a potential perturbation in the mechanical conditions under which gait is performed. Like any perturbation in the conditions of walking, some period of acclimatisation to the footwear is likely necessary and this is widely acknowledged by researchers in their protocols. Footwear that is similar to that worn by a participant on a daily basis might represent a small or indeed no perturbation at all, and requires little acclimatisation. Investigation of some footwear styles, however, (e.g. stiletto, rollover footwear) might require more acclimatisation if participants are unfamiliar with their use.
Protocols used in prior research to acclimatise to footwear vary considerably and often only scant details are offered. Some studies require the participant to familiarise themselves with the study protocol rather than the footwear being tested [1], [15], [16], [17] and others had participants acclimatise to footwear walking on a treadmill but collected data during overground walking [18]. Other investigators allow a period of time between conditions but do not explain the purpose of this period or what each participant did [19]. There are studies which allow the participant to acclimatise to each footwear condition [20], [21] and 5 min is the most common period of time provided. The variation in methodologies highlights the lack of understanding of the acclimatisation effect. Furthermore, no previous studies have reported quantitative criteria to define when a participant has acclimatised to a specific shoe design.
A further issue regarding PP protocol design is how many steps are required to produce a valid representation of gait. Since no two steps during gait are the same [22] numerous steps must be measured and thereafter averaged to represent the individual participant and experimental condition. One study has suggested that 200 steps are required when walking on a treadmill [23] although this was based on ground reaction rather than PP data. In PP studies, it is typical to use fewer than 200 steps [17], [19], [21], [24].
The only comparable study based on PP data focussed on people with diabetes wearing rocker profile footwear [25]. Based on data from 20 steps, just 12 were required for valid data. However, people with diabetes exhibit different gait patterns and rocker footwear has features that are distinct from many footwear styles (very stiff sole, distinct angular sole profile). Thus, the generalizability of these data is low.
The two aims to this study are:
- 1)
To identify the minimum number of steps required to produce an average which accurately represents normal gait.
- 2)
To identify the minimum number of steps required for participants to acclimatise to a range of footwear types.
Our purpose in meeting these aims was to offer recommendations for future studies concerned with footwear effects on PP.
Section snippets
Participants
Twenty young adults (10 female, mean (SD) age = 28 (7.1) years, height = 1.692 m (0. 074); mass = 69.9 kg (14.9)) were each tested in one session. All were pain, injury and pathology free. The study was approved by the university ethics committee and all participants provided informed consent.
Footwear
Five footwear designs for females and three for males were selected to provide a range of perturbations and therefore possible acclimatisation periods and number of steps for valid data. For men: a Duna (Italy)
Results
In females, the highest minimum number of steps was 39 (MT5, soft upper heeled shoe) (Table 1), lowest was 14 (hallux, Starlit and Rocker). In males, the highest minimum number of steps required to provide representative peak PP data was 34 (midfoot, Starlit shoe), the lowest was 2 (hallux, Kalenji shoe). When considering both genders, the foot area requiring the highest minimum number of steps was MT1 (27.9 steps, based both on shoes common to male and females) and the area requiring the least
Discussion
We implemented a new approach to determine the minimum number of steps over which PP data should be recorded in order to provide valid peak PP data. Furthermore, we implemented a novel approach to establish the number of steps required to acclimatise to unfamiliar footwear. The values recommended for future research studies (30 steps for valid data and 166 steps to acclimatise to footwear) are the upper boundaries of what is required. This is because we used the mean value +2 standard
Funding
Declaration regarding funding and role of RB:
Acknowledgement
The work in this paper was supported in part by funding by Scholl Footcare Brand (Reckitt Benkiser). The company supported PhD studentship of J Melvin investigating the relationship between foot pressure and footwear designs. The work in this paper was carried out to support that work. No financial benefit was received by the authors for this work. Company representatives played no part in the work in this paper.
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