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Testing thermal comfort of trekking boots: An objective and subjective evaluation
Applied Ergonomics 44 (2013) 557e565
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Applied Ergonomics journal homepage: www.elsevier.com/locate/apergo
Testing thermal comfort of trekking boots: An objective and subjective evaluation P.M. Arezes a, *, M.M. Neves b, S.F. Teixeira a, C.P. Leão a, J.L. Cunha b a b
Production and Systems Department, School of Engineering, University of Minho, 4800-058 Guimarães, Portugal Textile Engineering Department, School of Engineering, University of Minho, 4800-058 Guimarães, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 February 2011 Accepted 16 November 2012
The study of the thermal comfort of the feet when using a specific type of shoe is of paramount importance, in particular if the main goal of the study is to attend to the needs of users. The main aim of this study was to propose a test battery for thermal comfort analysis and to apply it to the analysis of trekking boots. Methodologically, the project involves both objective and subjective evaluations. An objective evaluation of the thermal properties of the fabrics used in the boots was developed and applied. In addition, the thermal comfort provided when using the boots was also assessed both subjective and objectively. The evaluation of the thermal comfort during use, which was simulated in a laboratory environment, included the measurement of the temperature and moisture of the feet. The subjective assessment was performed using a questionnaire. From the results obtained, it was possible to define an optimal combination of fabrics to apply to trekking boots by considering the provided thermal insulation, air permeability and wicking. The results also revealed that the subjective perception of thermal comfort appears to be more related to the increase in temperature of the feet than to the moisture retention inside the boot. Although the evaluation of knits used in the boots indicated that a particular combination of fibres was optimal for use in the inner layer, the subjective and objective evaluation of thermal comfort revealed that the evaluation provided by users did not necessarily match the technical assessment data. No correlation was observed between the general comfort and specific thermal comfort assessments. Finally, the identification of thermal discomfort by specific foot areas would be useful in the process of designing and developing boots. Ó 2012 Elsevier Ltd and The Ergonomics Society. All rights reserved.
Keywords: Thermal Comfort Test Subjective Objective Boots
1. Introduction To understand how people develop their perception about comfort is a very complex task. This complexity occurs because subjective perception is based on several variables or parameters. In the particular case of the use of shoes, those variables can also be quite diverse, such as the pressure on the foot (Chen et al., 1994; Chiu et al., 2007), the vertical impact and shock absorption, the specific shape of foot, foot sensibility and the inside shoe climate (Au and Goonetilleke, 2007; Havenith and Heus, 2004). The aim when evaluating the comfort of a specific object or tool, such as a shoe, is to understand how the specific object is ergonomically adapted to its potential users. However, obtaining an evaluation of a single aspect of comfort, such as thermal comfort, can be a very demanding task (González et al., 2001). In addition,
* Corresponding author. E-mail address: [email protected] (P.M. Arezes).
individual perception of comfort can be highly affected by other complementary aspects such as the design of the shoe (Luximon and Luximon, 2009; Rupérez et al., 2010) and its materials (YungHui and Wei-Hsien, 2005) and not particularly by the thermal parameters that are expected to be analysed. In brief, the comfort of shoes is influenced by two main factors, one linked to the mechanical properties of the shoes and the other concerning the thermal factors. The research focused on the study of comfort of shoes is a frequent issue in specialised literature (Llana et al., 2002; Mündermann et al., 2002). However, as reported by González et al. (2001), the subjective evaluation of the thermal behaviour of shoes is not as frequent as the general analysis of their comfort. Bogerd et al. (2012) also reported that only a few studies evaluate the perception parameters, e.g., temperature and comfort, of socks during use. From the literature review, it appears that there is a consensus about the need to study the thermal comfort in different sections of the foot and the water permeability properties of the fabrics used in
0003-6870/$ e see front matter Ó 2012 Elsevier Ltd and The Ergonomics Society. All rights reserved. http://dx.doi.org/10.1016/j.apergo.2012.11.007
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the inner part of shoes (Diebschlag et al., 1976). This type of analysis will enable the acquisition of additional and more diversified information for use in the evaluation and design of shoes. Although the adoption of preliminary tests for characterising the materials used can significantly improve the design of a particular shoe, comfort is mostly dependent on the users’ assessment. Therefore, it is expected that a comfort assessment should combine both mechanical and psychophysical evaluations (Cengiz and BabalIk, 2007; Jordan and Bartlett, 1995; Li, 1997; Li et al., 2007; Witana et al., 2009). The aims of this study were twofold: (i) to propose a test battery for thermal comfort analysis and (ii) to apply this battery to the analysis of trekking boots. Ultimately, the objective was also to characterise and evaluate all the thermal parameters that may play a significant role in the thermal ergonomics of this type of boots to obtain relevant information for use in the design and construction of trekking boots. The adopted evaluation included two different approaches to verify if both evaluation methods presented consistent and coherent results: an objective one, which included the thermal characterisation of the used fabrics, and a subjective evaluation of thermal comfort perceived by the tested subjects. 2. Methodology The current study was developed in the scope of a broader project regarding the development and testing of trekking boots with the aim of improving thermal comfort. Concerning the methodology, the project involved two different stages. The first stage included an objective evaluation of thermal parameters associated with use of the boots. This stage involved both the characterisation and testing of the fabrics developed to be used in the inner layer and the measurement of some objective thermal properties of the feet during a walking test. The second stage consisted of a subjective evaluation of the thermal comfort when using prototypes of the analysed boots. A subjective evaluation assessment was performed using a questionnaire and a comfort evaluation scale, which were combined with a physical task designed to simulate the regular use of the boots in a laboratory environment. 2.1. Selection of the fabrics The improvement of the comfort provided by the use of shoes has been the focus of many studies and, with this purpose, the development of lining materials appears to be of fundamental importance. The selection of a particular material can play a significant role in the wet sensation of the users’ feet. As an example, if the sweat is not transferred from the skin to the surrounding air or to the external shoe layers, it will result in a wet sensation, which is often interpreted as discomfort. In addition, a wet material may also reduce the insulation properties of the shoe (Bergquist and Holmer, 1997). Accordingly, the design of a functional double knit aims to address the problems of moisture transport and of the maintenance of an optimum temperature of the feet. This performance of the knit can be achieved through an appropriate knit structure design in combination with an effective selection of the materials used in its production. Considering these requirements, a specific knit structure, which was developed and tested in a preliminary study and reported elsewhere (Cunha et al., 2008; Neves et al., 2011), was used, and a set of fibres were selected for the development of the inner layer fabrics. The hydrophilic fibres cotton (CO), polylactic acid (PLA),
soybean (SPF) and bamboo (BAM) and the hydrophobic fibres polypropylene (PP) and polyester (PES) were considered. Knits were produced using these fibres such that the hydrophobic fibre was placed on one face and the hydrophilic fibre on the other. Eight combinations of raw materials were applied, leading to a total of 8 samples of tested knits. After their production, these knits were characterised, and laboratory tests were performed to evaluate the heat and moisture transport properties, the air permeability and the wicking properties. A thermal manikin was used for the evaluation of the thermal insulation. With an appropriate knit structure, the air permeability can be substantially improved. In the presence of moisture, this property is not significantly affected because the hydrophobic fibres used in the layer close to the foot skin do not absorb this moisture and therefore do not soak. Thus, this moisture in water vapour form is discarded. The air permeability of the used samples was evaluated using the air permeability tester TEXTEST FX3300 by applying the procedure defined in the standard EN ISO 9237 (ISO, 1995) under a pressure of 100 Pa. The water vapour permeability of the used materials is also an important property for the maintenance of a desirable thermal equilibrium. The tests used to characterise this variable were performed in the Permetest apparatus according to the standard ISO 11902 (ISO, 1993). Thermal insulation is another property that should be considered when thermo-physiological comfort is studied (Al-Ajmi et al., 2008). If the value of this parameter is high, a low heat exchange rate will occur. The total thermal insulation was determined using a thermal manikin according to the procedure defined in ISO 15831 (ISO, 2004) using the serial model. The thermal manikin measures the total thermal insulation based on the analysis of the thermal insulation of body areas. Therefore, to stabilise the thermal insulation along the body, a standard jogging suit was used, leaving the feet area free for testing the materials for the lining, thus preventing heat losses that could compromise the results. For test purposes, socks were produced using the knits under study. All tests were conducted in the steady state. 2.2. Comfort evaluation The laboratory procedure consisted of the simulation of a regular use situation together with the subjective evaluation of the thermal comfort/discomfort using an individual questionnaire and a specific scale. The user’s subjective assessment is quite common when the comfort of a specific product must be known or evaluated. The great majority of the comfort assessment studies are based or focused on the users’ discomfort experience (Kuijt-Evers et al., 2007a,b; Llana et al., 2002). However, subjective evaluations have been referred to in the literature as presenting some disadvantages when compared with other type of assessments (Kuijt-Evers et al., 2007a,b). As an example, it is frequently noted that this type of assessment requires a large number of subjects, is highly time consuming, and is influenced by users’ personal preferences. Moreover, some known sources of uncertainty are present when using these subjective measurements, such as the time error and the context effects (Annett, 2002). Sometimes, factors not directly related to comfort, or discomfort, may influence the results, and thus, objective measurements are used in addition to or simultaneously with subjective ones. Accordingly, this study also intended to present a testing protocol that includes the quantification of some objective parameters, namely the temperature of 2 different foot locations and the moisture retention after use. These characteristics were selected based on the fabric properties, which were previously
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analysed, as well as in the literature review (Au and Goonetilleke, 2007). 2.2.1. Questionnaire development The evaluation of the use of one product is often performed by applying an individual questionnaire about the use itself; concerning the evaluation scales employed, different types of scales might be used (Havenith and Heus, 2004). In most of the studies involving comfort evaluation, researchers often use ordinal scales to assess comfort (Mündermann et al., 2002). However, in such scales, there is no indication of how much, in an absolute sense, any of the objects possesses the attribute being evaluated. Moreover, no information can be obtained concerning how different the conditions are regarding comfort. Some of these disadvantages can be eliminated or mitigated if Visual Analogue Scales (VAS) were used. As an example, Mündermann et al. (2002) used VAS and concluded that these scales provide a reliable measure to assess footwear comfort. Additionally, Yung-Hui and Wei-Hsien (2005) verified that the application of VAS constitutes a reliable indicator of the comfort of shoes. Considering all these aspects, a specific questionnaire was developed and applied to the subjects. A translated version of this questionnaire is included in Appendix I. The entire test protocol was developed considering the need to simultaneously test both type of boots with both types of inner layers. Otherwise, the same person would need to perform the test twice, which may result in an assessment bias. The individual evaluation of thermal discomfort was performed in 2 different phases. The first phase was performed when the subjects began to use the boots and the other one was performed following a walking activity. The discomfort assessment was based on the application of a 100 mm VAS with the left end labelled “very much” (10 discomfort points) and the right end “not at all” (0 discomfort points). In each scale, a different sensation/dimension descriptor was applied. According to the type of sensation, the score could vary, i.e., when the sensation was considered to be “comfortable” (e.g., foot ventilation), the left end was scored 0 points; however, when the sensation was considered to be “uncomfortable” (e.g., humidity sensation), the left end was scored 10 points. The entire procedure consisted of 4 different phases: - In the 1st phase, apart from the completion of the questionnaire header (questionnaire ID, test date, temperature, and humidity monitoring), the researcher also completed section A of the questionnaire (Appendix I), which included the individual characterisation and the objective measurements for the initial phase. This phase also included section B of the questionnaire, referred to as the Initial thermal evaluation, where the (dis)comfort evaluation was performed without any distinction between the left and right foot. The descriptors used were “humidity sensation”, “thermal insulation”, “foot warming”, “general thermal comfort” and “foot ventilation”; - The 2nd phase consisted of a (dis)comfort evaluation after each subject completed the proposed walking task on the treadmill (described in 2.2.2). In this phase, called the Final stage evaluation, subjects were requested to evaluate the thermal (dis) comfort for each foot separately (section C of the questionnaire); - The 3rd phase consisted of the indication of the most uncomfortable foot areas, which was performed using a graphic scheme of both feet divided into several areas (section D of the questionnaire). This evaluation was performed considering the individual perception of the subject about the areas/ zones of higher temperature increase and higher moisture accumulation.
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- In the 4th phase, designated as “General evaluation”, the subjects were requested to provide a general evaluation of the boots tested using scale descriptors not directly associated with thermal comfort, such as “Aesthetics”, “Flexibility”, “Ease of use” and “Overall comfort” (section E of the questionnaire). The main aim of this question was to compare this evaluation with the thermal discomfort evaluation and verify if other aspects not linked to thermal evaluation may or not have biased the latter. The complete filling in of the table in section A was also performed at this stage by measuring the final skin surface temperature at two predefined points of the feet (metatarsal and plant) as well as by measuring the weight of the socks after their removal from the user’s feet. 2.2.2. Evaluation procedure The sample considered in this study was initially composed of 39 subjects. After the first trial of the questionnaire application, 5 subjects were identified as having some sort of musculoskeletal disorders or injuries related to their feet. Of these subjects, 3 of them reported having a type of dermatophytes (also known as ’”athlete’s foot”), 1 reported the existence of some blisters on their feet, and 1 had “flat feet” due to the absence of an arch in the foot. Accordingly, and considering that these injuries/disorders would affect their perception (Grier et al., 2011), these 5 subjects were excluded from the final sample, which was composed of 16 females and 18 males. The sample used had a mean age of 28.9 (8.5) years, an average weight of 64.8 (9.6) kg, an average height of 170.2 (6.5) cm, and a foot size mean of 25.2 (1.3) cm. The dimension of the sample was defined according to the availability of the subjects and their willingness to voluntarily participate in the study. The subjective evaluation of thermal (dis)comfort was performed using prototypes of the developed boots. The subjective evaluation was also performed by applying a laboratory-controlled physical task, which was used to simulate regular use of the boots. In this physical test, all the subjects were requested to walk on a treadmill located in a room with controlled temperature for 10 min with the moving belt positioned in a horizontal position and at a speed of 130 cm/s, which is equivalent to a metabolic rate of 150 W/m2 activity according to ISO 8996 (ISO, 1990). This speed was determined because the task could not be very demanding in terms of the subjects’ feet use while being difficult enough to detect a hypothetical temperature and moisture increase. Moreover, this speed could be maintained at a comfortable level in terms of the plantar pressure and ground reaction force (Murray et al., 1970). The thermal environment of the laboratory was monitored during the tests. The mean air temperature registered was 23.3 C, and the mean air relative humidity was 42%. For the performed tests, some additional equipment was used: - 10 pairs of boots (sizes within the subjects’ foot size range) produced using 2 types of inner layers and several pairs of 100% cotton socks; - A precision scale from Mettler Instruments AG Type AE 200-S, which allowed the measurement of the weight of the socks before and after the walking task and, consequently, the accumulated humidity or moisture retention; - A mechanical treadmill from ProMaster, which permitted the simulation of a regular walking task; - A Bruel & Kjaer Type 1213 Indoor Climate Analyser, which allowed the measurement of the temperature of the foot skin in 2 different locations, the metatarsal/toes area and the foot plantar area (Kuklane et al., 2001).
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As the selection of the specific foot side that each boot would be tested on could result in some bias (Havenith and Heus, 2004), the wearing order of the foot side/inner fabric combination to be tested needed to be balanced over the participants to avoid order effects. A Latin-square design was used for the selection of the type of boot for each foot. Theoretically, the subjects should have tested the boots without socks to more clearly detect the differences between the two different tested boots. However, due to the need to test prototypes more than one time, this procedure would have been inappropriate from a hygiene point of view. Thus, socks were adopted for all the performed tests. Socks composed of 100% cotton were used; they were considered to all have the same thermal properties and therefore were “innocuous”; i.e., they did not interfere with the evaluation. 3. Results 3.1. Inner layer tests As defined in the methodology section, several combinations of fibres were included in various fabrics and were tested for certain parameters. The first step included the air and vapour permeability characteristics evaluation of the fabrics constructed with different combinations of fibres. The results of this evaluation are presented in Fig. 1, in which the Polylactic Acid/Polyester (PLA/PES) combination knit is observed to have the best performance (highest values). The samples produced with the combinations of PLA/PES, Soybean/Polyester (SPF/PES) and Bamboo/Polyester (BAM/PES) presented better (higher) results for the water vapour permeability test. Regarding thermal insulation, Fig. 2 demonstrates that the PLA/ PES knit provided the most thermal insulation, while the BAM/PES knit provided the least. The amount of water absorbed by the different knits (horizontal position) as a function of time was also measured. Fig. 3(a) presents the results of this analysis for different knit combinations. The PLA/ PES knit combination was the optimal in removing water due to its high wicking ability and speed values. The vertical wicking results in the Wales direction are provided in Fig. 3(b). The PLA knits exhibited the best performance for this property. From the combinations of the various materials tested, the PLA/ PES combination appears to be the most suitable for a cold environment due to its high thermal insulation. However, the BAM/PP combination is the most suitable for a warm environment, as it provides low thermal insulation and permeability as well as good
(horizontal and vertical) wicking. Therefore, considering the tested properties and corresponding results, the BAM/PP combination was considered optimal for trekking boot construction. Using the developed knits, 20 pairs of boots were produced (10 pairs with each type of knit lining) in different sizes. These boots were used in the thermal comfort subjective evaluation trials, for which a test protocol was specifically developed. Considering the initial aim of the analysis of fabrics, the BAM/PP and PLA/PES combinations were selected as they provided the best properties for use in the inner layers of the boot. 3.2. Moisture Retention In the current analysis, Moisture Retention (MR) was defined as the difference between the weight of the socks before and after use in the physical task. To assess the difference between the two tested materials, the moisture retention in the boots using BAM/PP and PLA/PES inner layers was measured. The mean values obtained for both combinations are presented in Fig. 4. The statistical analysis used for comparing the difference between the means for the moisture accumulation in the BAM/PP and PLA/PES combinations was the Wilcoxon test, which is a nonparametrical test assuming related samples. The results of the test application (z ¼ 4.280; p < 0.001) had a significant value of p < 0.05, which indicates that the equal means of the difference hypothesis is rejected. Therefore, it can be concluded that the moisture accumulation in boots fabricated using BAM/PP is higher than that in boots fabricated using PLA/PES and that this difference is statistically significant. 3.3. Foot skin surface temperature As described in the methodology section, the analysis of foot temperature was performed by measuring the skin surface temperature of each foot before and after the selected physical task. The temperature was measured in two different zones, the metatarsal/toes and the plantar foot zone. The temperature increase (TI) (in the metatarsal and foot plant areas) was obtained using the value of the difference between the final and initial temperature of the skin surface for each separate foot. The average values for the TI obtained for the two types of inner layers and foot zones are presented in Table 1 and graphically compared in Fig. 5. The statistical analysis used to compare the differences between the means in the previous section was also used in this case.
Fig. 1. (a) Air permeability and (b) water vapour permeability (Permetest) results.
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Fig. 2. Thermal insulation.
As the value obtained for p in the metatarsal zone was 0.401 (z ¼ 0.841), it can be concluded that the difference between the TI in the metatarsal zone for both types of boots is statistically significant. However, the analysis of the results indicates that the p value in the plantar zone is higher than 0.05 (p ¼ 0.779; z ¼ 0.281); thus, it can be concluded that, for this zone, there is no statistically significant difference.
3.4. Thermal discomfort scores Considering the statistical analysis purpose, the following variables were considered: - Questionnaire score related to the initial thermal discomfort (iTD) (section B of the questionnaire e Appendix I), computed from the sum of each answer in the VAS, as previously described; - Questionnaire score related to thermal discomfort (TD), computed from the sum of each answer in the VAS and from the difference between each boot evaluation (section C of the questionnaire e Appendix I) and the initial thermal discomfort scores. For this variable, the higher the score, the higher the discomfort. This evaluation was performed independently for each boot. Because of the subjective evaluation, which was performed through the application of the questionnaire previously mentioned, two different scores were obtained, one regarding the overall initial
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thermal discomfort (iTD) score and the other obtained at the end of the test. The initial score was used as a “baseline” score. Therefore, the difference between the initial and final scores was defined as the thermal discomfort (TD) score for both boot types, BAM/PP and PLA/PES. Fig. 6 presents the results of the TD scores for each boot type. From this analysis, it is possible to verify that concerning the perceived thermal discomfort, the BAM/PP boots have a lower score, i.e., these boots were evaluated as resulting in minor thermal discomfort. Although moisture in clothing has been widely recognised as one of the most important factors contributing to discomfort during wear (Li, 2005), it is possible that the thermal discomfort perceived by the subjects in this study, considering the measurements performed, is much more likely related to the TI than to the MR. 3.5. Thermal discomfort scores validation In the applied questionnaire, the final score in the question related to General Evaluation (GE) was computed using the sum of each VAS answer obtained in the last question (section E of the questionnaire e Appendix I). For this question, all the used scales had the same “score orientation”. Therefore, the higher the score, the better the general evaluation of the boots. This evaluation was applied considering the pair of boots and not particularly each individual tested boot. As the TD scores were obtained from the subjective perception of each subject, it is probable that these scores were closely related to (and influenced by) the overall evaluation of the boots. To verify this possible effect, the scores obtained for thermal discomfort, both initial (iTD) and final (TD) for the two boots, were compared with the scores obtained in the General Evaluation (GE). This analysis was performed by verifying the correlation coefficients between variables, which are presented in Table 2. The results presented in Table 2 demonstrate that there is a statistically significant (and positive) correlation between the TD variables in the PLA/PES and BAM/PP boots (0.509), which indicates that the thermal discomfort scores for both the inner layers are highly correlated. That is, the subjects have experienced thermal discomfort but they experienced the same, or a very similar, sensation for the two types of boots. It appears that GE and the thermal specific comfort evaluation can be independent, as demonstrated by the non-significant correlation between the TD and GE variables (last column in Table 2). Although the correlations are not statistically significant,
Fig. 3. Results of (a) Horizontal and (b) Vertical wicking of the tested knits.
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Table 1 TI (in C) by foot area and tested knit. Metatarsal/Toe
Mean (sd) Median
Plantar area
BAM/PP
PLA/PES
BAM/PP
PLA/PES
2.45 (1.45) 1.90
2.52 (1.41) 2.30
1.73 (1.08) 1.40
1.76 (1.18) 1.80
the correlation between GE and all the other variables related with thermal discomfort are negative, which implies that the subjects tended to inversely score the thermal discomfort and the overall evaluation of the boots. This result suggests that there is a very tenuous line between the overall evaluation, which includes aspects other the thermal experience, and the specific discomfort evaluation when considering the thermal properties. It may also be assumed that those two comfort/discomfort judgments can be distinguished. Alcántara et al. (2005) also observed that people are able to differentiate comfort from other characteristics of the shoes, such as its quality.
Fig. 5. TI (in C) mean in (a) the metatarsal/toes zone and in (b) the plantar zone.
3.6. Foot zones with higher thermal discomfort Certain previous studies, for example Au and Goonetilleke (2007), have reported that a comfortable shoe does not necessarily have the same perceived fit in every region of the shoe. Therefore, it is likely that the same phenomenon may occur in the perceived thermal (dis)comfort. Accordingly, it is important to evaluate (dis)comfort in the different regions or zones of the foot. In a more detailed analysis of the thermal discomfort, subjects were asked to indicate where they felt that the thermal discomfort was more obvious by considering the different zones of the foot and using a visual scheme (section D of the questionnaire e Appendix I). The subjects were able to indicate one or more specific zones of the foot in which they noticed a higher discomfort regarding the two analysed parameters: (1) the perceptible higher temperature increase and (2) the perceptible higher moisture accumulation. The results of this analysis are presented in Fig. 7. Fig. 6. TD mean score (in points) by knit type.
Fig. 7 demonstrates that there is a predominance of indications of discomfort for the PLA/PES boots. This result is visible on both sides of the chart, which suggests that the discomfort is highly perceived in the PLA/PES boots, both in terms of TI and MR. In the specific case of the MR (Fig. 7(b)), this predominance occurs for all the zones considered. Notably, this predominance is not in line with the results obtained in the objective test. In this test, PLA/PES performed better
Table 2 Spearman’s correlation coefficients matrix between scores.
Thermal discomfort in the initial phase (iTD) Thermal discomfort (TD) e BAM/PP Thermal discomfort (TD) e PLA/PES General evaluation (GE) Fig. 4. MR mean value (in gram) in both types of tested boots.
iTD
TD BAM/PP
TD PLA/PES
GE
e
0.343*
0.270
0.010
e
0.509**
0.059
e
0.287 e
* Correlation is significant at the 0.05 level; ** Correlation is significant at the 0.01 level.
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Fig. 7. Number of indications of thermal discomfort regarding (a) temperature increase and (b) moisture retention.
than BAM/PP for the MR; however, this result was not confirmed by the discomfort indications of the subjects. A possible explanation for this result can be the fact that there is a “prevailing” influence of the TI in the discomfort perceived by subjects, i.e., subjects tend to value the TI more than the MR when they evaluate thermal discomfort. Alternatively, other possible explanation may also be related to the characteristics of the requested physical task. As previously detailed, the proposed physical task was not highly challenging from the physical point of view. Therefore, it can be assumed that that MR could have a different impact on the thermal discomfort perception if the proposed physical task required a higher physical activity and, theoretically, a higher moisture retention in the socks used by subjects. Finally, it can also be considered that MR may play a more important role in the users’ (dis)comfort when considering low temperature environments and, consequently, in the discomfort produced by the cold sensation in the foot (Kuklane et al., 2001). From this analysis, two important insights emerge. On one hand, regarding the TI, the subjects perceived a more evident thermal discomfort in the mid-part of the foot. The subjects also seemed to differentiate better between the 2 types of fabrics tested in the mid-foot zone. However, regarding the MR, the most affected area was the upper region of the foot, including the midfoot/metatarsal and the toe area. The subjects did not experience (or at least did not perceive) a significant accumulation of moisture in the heel or in the malleolus/ankle zones. This information may be relevant for the design and development of boots and for the design of socks. 4. Study limitations Regarding the present study, there are two main limitations related to the analysed data that may affect the accuracy of the results. The first limitation concerns the use of a simulated use of the boots in a laboratory environment. This simulation does not seem to fully represent the “real world” use, as both the treadmill walking task and the characteristics of the terrain are not representative. However, the main idea was to compare, in a controlled environment, the properties of the fabrics and the corresponding sensorial perception using a subjective assessment. The use of a real setting (with typical hikers, different types of terrains, climates, etc.) would imply the presence of a high number of uncontrolled variables, which would certainly influence the data obtained. The second limitation is related to the exclusion from the sample of subjects with some type of
disorder in their feet. In reality, some of the people using this type of boots will have that type of problem, such as the case of feet blisters and callosities. However, again, it is the authors’ opinion that the inclusion of these people in the sample would impair the current analysis. 5. Conclusions The definition of a thermally comfortable shoe is one of the most desired targets among the manufacturers of several types of shoes. This goal is even more important when it concerns the design of some specific types of shoes, such as trekking boots. The work described in this manuscript aimed at proposing a battery test for thermal comfort assessment and applying it by identifying and evaluating trekking boots, both objectively, by developing and testing fabrics with particular characteristics that can optimise thermal comfort, and subjectively, by assessing the discomfort perceived and reported by users. Based on the application of the proposed battery tests and on the corresponding results, it was possible to demonstrate that the subjective perception of thermal comfort appears to be related much more to the increased temperature of the foot than to the moisture retention. The use of a combination of objective and subjective assessment techniques revealed that although the objective assessment indicated that a particular combination of fibres appears to be the optimal solution for the inner layer of the boots, the subjective evaluation of the thermal discomfort suggested that users may have a different opinion regarding the used fabrics and, most likely, their opinion is influenced by other parameters. Furthermore, the current study also demonstrates that no correlation was observed between the general comfort evaluation and specific thermal comfort evaluation. This result indicates that it is likely that the applied methodology may have allowed the differentiation of these two aspects in the evaluation of comfort. In the authors’ opinion, this issue could be addressed in future studies, with a larger subject sample, to verify whether the applied scales are adequate to autonomously evaluate the thermal discomfort/ comfort and not include some hypothetical biases from external factors, such as, the consideration of boot aesthetics. Finally, it appears that the identification of thermal discomfort by specific zones of the foot will be useful for the project and construction of the shoes, particularly for previewing the application of fabrics with differentiated thermal behaviour in these areas.
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Appendix I
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References Al-ajmi, F.F., Loveday, D.L., Bedwell, K.H., Havenith, G., 2008. Thermal insulation and clothing area factors of typical Arabian Gulf clothing ensembles for males and females: measurements using thermal manikins. Applied Ergonomics 39 (3), 407e414. Alcántara, E., Artacho, M.A., González, J.C., García, A.C., 2005. Application of product semantics to footwear design. Part I e identification of footwear semantic space applying differential semantics. International Journal of Industrial Ergonomics 35 (8), 713e725. Annett, J., 2002. Subjective rating scales: science or art? Ergonomics 45 (14), 966e 987. Au, E.Y.L., Goonetilleke, R.S., 2007. A qualitative study on the comfort and fit of ladies’ dress shoes. Applied Ergonomics 38 (6), 687e696. Bergquist, K., Holmer, I., 1997. A method for dynamic measurement of the resistance to dry heat exchange by footwear. Applied Ergonomics 28 (5e6), 383e388. Bogerd, C.P., Brühwiler, P.A., Rossi, R.M., 2012. Heat loss and moisture retention variations of boot membranes and sock fabrics: a foot manikin study. International Journal of Industrial Ergonomics 42 (2), 212e218. Cengiz, T.G., BabalIk, F.C., 2007. An on-the-road experiment into the thermal comfort of car seats. Applied Ergonomics 38 (3), 337e347. Chen, H., Nigg, B.M., Koning, J., 1994. Relationship between plantar pressure distribution under the foot and insole comfort. Clinical Biomechanics vol. 9 (6), 335e341. Chiu, M., Mao-Jiun, Wang, J., 2007. Professional footwear evaluation for clinical nurses. Applied Ergonomics 38 (2), 133e141. Cunha, J., Neves, M., Arezes, P., Teixeira, S., 2008. Development of a Lining for the Thermal Comfort of Trekking Boots. Proceedings of the 8th AUTEX Conference e AUTEX2008, Biella: Italy. Diebschlag, W., Mueller-Limmroth, W., Mauderer, V., 1976. Influence of several socks and linings on the microclimate in shoes with upper material of leather or synthetic. Journal of the American Leather Chemists Association 71 (6), 293e 306. González, J.C., Alcántara, E., Bataller, A., García, A.C., 2001. Physiological and subjective evaluation of footwear thermal Response over time. In: Hennig, E., Stacoff, A. (Eds.), Proc. of the 5th Symp.on Footwear Biomechanics, Zuerich/ Switzerland, pp. 40e41. Grier, T., Knapik, J., Alemany, S., Swedler, D., Jones, B., 2011. Footwear in the United States Army Band: Injury incidence and risk factors associated with foot pain. The Foot. 21 (2), 60e65. Havenith, G., Heus, R., 2004. A test battery related to ergonomics of protective clothing. Applied Ergonomics 35 (1), 3e20. ISO, 1990. ISO 8996: Ergonomics e Determination of Metabolic Heat Production. International Organization for Standardization, Geneva, Switzerland. ISO, 1993. ISO 11092: Textiles-determination of Physiological Properties e Measurement of Thermal and Water Vapour Resistance under Steady-state
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Conditions (Sweating Guarded-hotplate Test). International Organization for Standardization, Geneva, Switzerland. ISO, 1995. ISO 9237 Textiles: Determination of the Permeability of Fabrics to Air. International Organization for Standardization, Geneva, Switzerland. ISO, 2004. ISO 15831: Clothing e Physiological Effects e Measurement of Thermal Insulation by Means of a Thermal Manikin. International Organization for Standardization, Geneva, Switzerland. Jordan, C., Bartlett, R., 1995. Pressure distribution and perceived comfort in casual footwear. Gait & Posture 3 (4), 215e220. Kuijt-Evers, L.F.M., Bosch, T., Huysmans, M.A., de Looze, M.P., Vink, P., 2007a. Association between objective and subjective measurements of comfort and discomfort in hand tools. Applied Ergonomics 38 (5), 643e654. Kuijt-Evers, L.F.M., Vink, P., de Looze, M.P., 2007b. Comfort predictors for different kinds of hand tools: differences and similarities. International Journal of Industrial Ergonomics 37 (1), 73e84. Kuklane, K., Gavhed, D., Fredriksson, K., 2001. A field study in dairy farms: thermal condition of feet. International Journal of Industrial Ergonomics 27 (6), 367e373. Li, K.W., Yu, R., Han, X.L., 2007. Physiological and psychophysical responses in handling maximum acceptable weights under different footwear-floor friction conditions. Applied Ergonomics 38 (3), 259e265. Li, Y., 1997. Sensory comfort: fabric transport properties and subjective responses during exercise under cool and hot environmental conditions. Research Journal of Textile and Apparel 1 (1), 84e93. Li, Y., 2005. Perceptions of temperature, moisture and comfort in clothing during environmental transients. Ergonomics 48 (3), 234e248. Llana, S., Brrizuela, G., Durç, J.V., Garcia, A.C., 2002. A study of the discomfort associated with tennis shoes. Journal of Sports Sciences 20, 671e679. Luximon, A., Luximon, Y., 2009. Shoe-last design innovation for better shoe fitting. Computers in Industry 60 (8), 621e628. Mündermann, A., Nigg, B.M., Stefanyshyn, D.J., Humble, R.N., 2002. Development of a reliable method to assess footwear comfort during running. Gait & Posture 16 (1), 38e45. Murray, M.P., Kory, R.C., Sepic, S.B., 1970. Walking patterns of normal women. Archives of Physical Medicine and Rehabilitation 51 (11), 637e650. Neves, M., Arezes, P.M., Alemany, S., Leão, C.P., Teixeira, S., 2011. A lining for the thermal comfort of trekking boots e experimental and numerical studies. Research Journal of Textile and Apparel (RJTA) 15 (3), 50e61. Rupérez, M.J., Monserrat, C., Alemany, S., Juan, M.C., Alcaniz, M., 2010. Contact model, fit process and, foot animation for the virtual simulator of the footwear comfort. Computer-aided Design 42 (5), 425e431. Witana, C.P., Goonetilleke, R.S., Xiong, S., Au, E.Y.L., 2009. Effects of surface characteristics on the plantar shape of feet and subjects’ perceived sensations. Applied Ergonomics 40 (2), 267e279. Yung-Hui, L., Wei-Hsien, H., 2005. Effects of shoe inserts and heel height on foot pressure, impact force, and perceived comfort during walking. Applied Ergonomics 36 (3), 355e362.
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Applied Ergonomics journal homepage: www.elsevier.com/locate/apergo
Testing thermal comfort of trekking boots: An objective and subjective evaluation P.M. Arezes a, *, M.M. Neves b, S.F. Teixeira a, C.P. Leão a, J.L. Cunha b a b
Production and Systems Department, School of Engineering, University of Minho, 4800-058 Guimarães, Portugal Textile Engineering Department, School of Engineering, University of Minho, 4800-058 Guimarães, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 February 2011 Accepted 16 November 2012
The study of the thermal comfort of the feet when using a specific type of shoe is of paramount importance, in particular if the main goal of the study is to attend to the needs of users. The main aim of this study was to propose a test battery for thermal comfort analysis and to apply it to the analysis of trekking boots. Methodologically, the project involves both objective and subjective evaluations. An objective evaluation of the thermal properties of the fabrics used in the boots was developed and applied. In addition, the thermal comfort provided when using the boots was also assessed both subjective and objectively. The evaluation of the thermal comfort during use, which was simulated in a laboratory environment, included the measurement of the temperature and moisture of the feet. The subjective assessment was performed using a questionnaire. From the results obtained, it was possible to define an optimal combination of fabrics to apply to trekking boots by considering the provided thermal insulation, air permeability and wicking. The results also revealed that the subjective perception of thermal comfort appears to be more related to the increase in temperature of the feet than to the moisture retention inside the boot. Although the evaluation of knits used in the boots indicated that a particular combination of fibres was optimal for use in the inner layer, the subjective and objective evaluation of thermal comfort revealed that the evaluation provided by users did not necessarily match the technical assessment data. No correlation was observed between the general comfort and specific thermal comfort assessments. Finally, the identification of thermal discomfort by specific foot areas would be useful in the process of designing and developing boots. Ó 2012 Elsevier Ltd and The Ergonomics Society. All rights reserved.
Keywords: Thermal Comfort Test Subjective Objective Boots
1. Introduction To understand how people develop their perception about comfort is a very complex task. This complexity occurs because subjective perception is based on several variables or parameters. In the particular case of the use of shoes, those variables can also be quite diverse, such as the pressure on the foot (Chen et al., 1994; Chiu et al., 2007), the vertical impact and shock absorption, the specific shape of foot, foot sensibility and the inside shoe climate (Au and Goonetilleke, 2007; Havenith and Heus, 2004). The aim when evaluating the comfort of a specific object or tool, such as a shoe, is to understand how the specific object is ergonomically adapted to its potential users. However, obtaining an evaluation of a single aspect of comfort, such as thermal comfort, can be a very demanding task (González et al., 2001). In addition,
* Corresponding author. E-mail address: [email protected] (P.M. Arezes).
individual perception of comfort can be highly affected by other complementary aspects such as the design of the shoe (Luximon and Luximon, 2009; Rupérez et al., 2010) and its materials (YungHui and Wei-Hsien, 2005) and not particularly by the thermal parameters that are expected to be analysed. In brief, the comfort of shoes is influenced by two main factors, one linked to the mechanical properties of the shoes and the other concerning the thermal factors. The research focused on the study of comfort of shoes is a frequent issue in specialised literature (Llana et al., 2002; Mündermann et al., 2002). However, as reported by González et al. (2001), the subjective evaluation of the thermal behaviour of shoes is not as frequent as the general analysis of their comfort. Bogerd et al. (2012) also reported that only a few studies evaluate the perception parameters, e.g., temperature and comfort, of socks during use. From the literature review, it appears that there is a consensus about the need to study the thermal comfort in different sections of the foot and the water permeability properties of the fabrics used in
0003-6870/$ e see front matter Ó 2012 Elsevier Ltd and The Ergonomics Society. All rights reserved. http://dx.doi.org/10.1016/j.apergo.2012.11.007
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the inner part of shoes (Diebschlag et al., 1976). This type of analysis will enable the acquisition of additional and more diversified information for use in the evaluation and design of shoes. Although the adoption of preliminary tests for characterising the materials used can significantly improve the design of a particular shoe, comfort is mostly dependent on the users’ assessment. Therefore, it is expected that a comfort assessment should combine both mechanical and psychophysical evaluations (Cengiz and BabalIk, 2007; Jordan and Bartlett, 1995; Li, 1997; Li et al., 2007; Witana et al., 2009). The aims of this study were twofold: (i) to propose a test battery for thermal comfort analysis and (ii) to apply this battery to the analysis of trekking boots. Ultimately, the objective was also to characterise and evaluate all the thermal parameters that may play a significant role in the thermal ergonomics of this type of boots to obtain relevant information for use in the design and construction of trekking boots. The adopted evaluation included two different approaches to verify if both evaluation methods presented consistent and coherent results: an objective one, which included the thermal characterisation of the used fabrics, and a subjective evaluation of thermal comfort perceived by the tested subjects. 2. Methodology The current study was developed in the scope of a broader project regarding the development and testing of trekking boots with the aim of improving thermal comfort. Concerning the methodology, the project involved two different stages. The first stage included an objective evaluation of thermal parameters associated with use of the boots. This stage involved both the characterisation and testing of the fabrics developed to be used in the inner layer and the measurement of some objective thermal properties of the feet during a walking test. The second stage consisted of a subjective evaluation of the thermal comfort when using prototypes of the analysed boots. A subjective evaluation assessment was performed using a questionnaire and a comfort evaluation scale, which were combined with a physical task designed to simulate the regular use of the boots in a laboratory environment. 2.1. Selection of the fabrics The improvement of the comfort provided by the use of shoes has been the focus of many studies and, with this purpose, the development of lining materials appears to be of fundamental importance. The selection of a particular material can play a significant role in the wet sensation of the users’ feet. As an example, if the sweat is not transferred from the skin to the surrounding air or to the external shoe layers, it will result in a wet sensation, which is often interpreted as discomfort. In addition, a wet material may also reduce the insulation properties of the shoe (Bergquist and Holmer, 1997). Accordingly, the design of a functional double knit aims to address the problems of moisture transport and of the maintenance of an optimum temperature of the feet. This performance of the knit can be achieved through an appropriate knit structure design in combination with an effective selection of the materials used in its production. Considering these requirements, a specific knit structure, which was developed and tested in a preliminary study and reported elsewhere (Cunha et al., 2008; Neves et al., 2011), was used, and a set of fibres were selected for the development of the inner layer fabrics. The hydrophilic fibres cotton (CO), polylactic acid (PLA),
soybean (SPF) and bamboo (BAM) and the hydrophobic fibres polypropylene (PP) and polyester (PES) were considered. Knits were produced using these fibres such that the hydrophobic fibre was placed on one face and the hydrophilic fibre on the other. Eight combinations of raw materials were applied, leading to a total of 8 samples of tested knits. After their production, these knits were characterised, and laboratory tests were performed to evaluate the heat and moisture transport properties, the air permeability and the wicking properties. A thermal manikin was used for the evaluation of the thermal insulation. With an appropriate knit structure, the air permeability can be substantially improved. In the presence of moisture, this property is not significantly affected because the hydrophobic fibres used in the layer close to the foot skin do not absorb this moisture and therefore do not soak. Thus, this moisture in water vapour form is discarded. The air permeability of the used samples was evaluated using the air permeability tester TEXTEST FX3300 by applying the procedure defined in the standard EN ISO 9237 (ISO, 1995) under a pressure of 100 Pa. The water vapour permeability of the used materials is also an important property for the maintenance of a desirable thermal equilibrium. The tests used to characterise this variable were performed in the Permetest apparatus according to the standard ISO 11902 (ISO, 1993). Thermal insulation is another property that should be considered when thermo-physiological comfort is studied (Al-Ajmi et al., 2008). If the value of this parameter is high, a low heat exchange rate will occur. The total thermal insulation was determined using a thermal manikin according to the procedure defined in ISO 15831 (ISO, 2004) using the serial model. The thermal manikin measures the total thermal insulation based on the analysis of the thermal insulation of body areas. Therefore, to stabilise the thermal insulation along the body, a standard jogging suit was used, leaving the feet area free for testing the materials for the lining, thus preventing heat losses that could compromise the results. For test purposes, socks were produced using the knits under study. All tests were conducted in the steady state. 2.2. Comfort evaluation The laboratory procedure consisted of the simulation of a regular use situation together with the subjective evaluation of the thermal comfort/discomfort using an individual questionnaire and a specific scale. The user’s subjective assessment is quite common when the comfort of a specific product must be known or evaluated. The great majority of the comfort assessment studies are based or focused on the users’ discomfort experience (Kuijt-Evers et al., 2007a,b; Llana et al., 2002). However, subjective evaluations have been referred to in the literature as presenting some disadvantages when compared with other type of assessments (Kuijt-Evers et al., 2007a,b). As an example, it is frequently noted that this type of assessment requires a large number of subjects, is highly time consuming, and is influenced by users’ personal preferences. Moreover, some known sources of uncertainty are present when using these subjective measurements, such as the time error and the context effects (Annett, 2002). Sometimes, factors not directly related to comfort, or discomfort, may influence the results, and thus, objective measurements are used in addition to or simultaneously with subjective ones. Accordingly, this study also intended to present a testing protocol that includes the quantification of some objective parameters, namely the temperature of 2 different foot locations and the moisture retention after use. These characteristics were selected based on the fabric properties, which were previously
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analysed, as well as in the literature review (Au and Goonetilleke, 2007). 2.2.1. Questionnaire development The evaluation of the use of one product is often performed by applying an individual questionnaire about the use itself; concerning the evaluation scales employed, different types of scales might be used (Havenith and Heus, 2004). In most of the studies involving comfort evaluation, researchers often use ordinal scales to assess comfort (Mündermann et al., 2002). However, in such scales, there is no indication of how much, in an absolute sense, any of the objects possesses the attribute being evaluated. Moreover, no information can be obtained concerning how different the conditions are regarding comfort. Some of these disadvantages can be eliminated or mitigated if Visual Analogue Scales (VAS) were used. As an example, Mündermann et al. (2002) used VAS and concluded that these scales provide a reliable measure to assess footwear comfort. Additionally, Yung-Hui and Wei-Hsien (2005) verified that the application of VAS constitutes a reliable indicator of the comfort of shoes. Considering all these aspects, a specific questionnaire was developed and applied to the subjects. A translated version of this questionnaire is included in Appendix I. The entire test protocol was developed considering the need to simultaneously test both type of boots with both types of inner layers. Otherwise, the same person would need to perform the test twice, which may result in an assessment bias. The individual evaluation of thermal discomfort was performed in 2 different phases. The first phase was performed when the subjects began to use the boots and the other one was performed following a walking activity. The discomfort assessment was based on the application of a 100 mm VAS with the left end labelled “very much” (10 discomfort points) and the right end “not at all” (0 discomfort points). In each scale, a different sensation/dimension descriptor was applied. According to the type of sensation, the score could vary, i.e., when the sensation was considered to be “comfortable” (e.g., foot ventilation), the left end was scored 0 points; however, when the sensation was considered to be “uncomfortable” (e.g., humidity sensation), the left end was scored 10 points. The entire procedure consisted of 4 different phases: - In the 1st phase, apart from the completion of the questionnaire header (questionnaire ID, test date, temperature, and humidity monitoring), the researcher also completed section A of the questionnaire (Appendix I), which included the individual characterisation and the objective measurements for the initial phase. This phase also included section B of the questionnaire, referred to as the Initial thermal evaluation, where the (dis)comfort evaluation was performed without any distinction between the left and right foot. The descriptors used were “humidity sensation”, “thermal insulation”, “foot warming”, “general thermal comfort” and “foot ventilation”; - The 2nd phase consisted of a (dis)comfort evaluation after each subject completed the proposed walking task on the treadmill (described in 2.2.2). In this phase, called the Final stage evaluation, subjects were requested to evaluate the thermal (dis) comfort for each foot separately (section C of the questionnaire); - The 3rd phase consisted of the indication of the most uncomfortable foot areas, which was performed using a graphic scheme of both feet divided into several areas (section D of the questionnaire). This evaluation was performed considering the individual perception of the subject about the areas/ zones of higher temperature increase and higher moisture accumulation.
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- In the 4th phase, designated as “General evaluation”, the subjects were requested to provide a general evaluation of the boots tested using scale descriptors not directly associated with thermal comfort, such as “Aesthetics”, “Flexibility”, “Ease of use” and “Overall comfort” (section E of the questionnaire). The main aim of this question was to compare this evaluation with the thermal discomfort evaluation and verify if other aspects not linked to thermal evaluation may or not have biased the latter. The complete filling in of the table in section A was also performed at this stage by measuring the final skin surface temperature at two predefined points of the feet (metatarsal and plant) as well as by measuring the weight of the socks after their removal from the user’s feet. 2.2.2. Evaluation procedure The sample considered in this study was initially composed of 39 subjects. After the first trial of the questionnaire application, 5 subjects were identified as having some sort of musculoskeletal disorders or injuries related to their feet. Of these subjects, 3 of them reported having a type of dermatophytes (also known as ’”athlete’s foot”), 1 reported the existence of some blisters on their feet, and 1 had “flat feet” due to the absence of an arch in the foot. Accordingly, and considering that these injuries/disorders would affect their perception (Grier et al., 2011), these 5 subjects were excluded from the final sample, which was composed of 16 females and 18 males. The sample used had a mean age of 28.9 (8.5) years, an average weight of 64.8 (9.6) kg, an average height of 170.2 (6.5) cm, and a foot size mean of 25.2 (1.3) cm. The dimension of the sample was defined according to the availability of the subjects and their willingness to voluntarily participate in the study. The subjective evaluation of thermal (dis)comfort was performed using prototypes of the developed boots. The subjective evaluation was also performed by applying a laboratory-controlled physical task, which was used to simulate regular use of the boots. In this physical test, all the subjects were requested to walk on a treadmill located in a room with controlled temperature for 10 min with the moving belt positioned in a horizontal position and at a speed of 130 cm/s, which is equivalent to a metabolic rate of 150 W/m2 activity according to ISO 8996 (ISO, 1990). This speed was determined because the task could not be very demanding in terms of the subjects’ feet use while being difficult enough to detect a hypothetical temperature and moisture increase. Moreover, this speed could be maintained at a comfortable level in terms of the plantar pressure and ground reaction force (Murray et al., 1970). The thermal environment of the laboratory was monitored during the tests. The mean air temperature registered was 23.3 C, and the mean air relative humidity was 42%. For the performed tests, some additional equipment was used: - 10 pairs of boots (sizes within the subjects’ foot size range) produced using 2 types of inner layers and several pairs of 100% cotton socks; - A precision scale from Mettler Instruments AG Type AE 200-S, which allowed the measurement of the weight of the socks before and after the walking task and, consequently, the accumulated humidity or moisture retention; - A mechanical treadmill from ProMaster, which permitted the simulation of a regular walking task; - A Bruel & Kjaer Type 1213 Indoor Climate Analyser, which allowed the measurement of the temperature of the foot skin in 2 different locations, the metatarsal/toes area and the foot plantar area (Kuklane et al., 2001).
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As the selection of the specific foot side that each boot would be tested on could result in some bias (Havenith and Heus, 2004), the wearing order of the foot side/inner fabric combination to be tested needed to be balanced over the participants to avoid order effects. A Latin-square design was used for the selection of the type of boot for each foot. Theoretically, the subjects should have tested the boots without socks to more clearly detect the differences between the two different tested boots. However, due to the need to test prototypes more than one time, this procedure would have been inappropriate from a hygiene point of view. Thus, socks were adopted for all the performed tests. Socks composed of 100% cotton were used; they were considered to all have the same thermal properties and therefore were “innocuous”; i.e., they did not interfere with the evaluation. 3. Results 3.1. Inner layer tests As defined in the methodology section, several combinations of fibres were included in various fabrics and were tested for certain parameters. The first step included the air and vapour permeability characteristics evaluation of the fabrics constructed with different combinations of fibres. The results of this evaluation are presented in Fig. 1, in which the Polylactic Acid/Polyester (PLA/PES) combination knit is observed to have the best performance (highest values). The samples produced with the combinations of PLA/PES, Soybean/Polyester (SPF/PES) and Bamboo/Polyester (BAM/PES) presented better (higher) results for the water vapour permeability test. Regarding thermal insulation, Fig. 2 demonstrates that the PLA/ PES knit provided the most thermal insulation, while the BAM/PES knit provided the least. The amount of water absorbed by the different knits (horizontal position) as a function of time was also measured. Fig. 3(a) presents the results of this analysis for different knit combinations. The PLA/ PES knit combination was the optimal in removing water due to its high wicking ability and speed values. The vertical wicking results in the Wales direction are provided in Fig. 3(b). The PLA knits exhibited the best performance for this property. From the combinations of the various materials tested, the PLA/ PES combination appears to be the most suitable for a cold environment due to its high thermal insulation. However, the BAM/PP combination is the most suitable for a warm environment, as it provides low thermal insulation and permeability as well as good
(horizontal and vertical) wicking. Therefore, considering the tested properties and corresponding results, the BAM/PP combination was considered optimal for trekking boot construction. Using the developed knits, 20 pairs of boots were produced (10 pairs with each type of knit lining) in different sizes. These boots were used in the thermal comfort subjective evaluation trials, for which a test protocol was specifically developed. Considering the initial aim of the analysis of fabrics, the BAM/PP and PLA/PES combinations were selected as they provided the best properties for use in the inner layers of the boot. 3.2. Moisture Retention In the current analysis, Moisture Retention (MR) was defined as the difference between the weight of the socks before and after use in the physical task. To assess the difference between the two tested materials, the moisture retention in the boots using BAM/PP and PLA/PES inner layers was measured. The mean values obtained for both combinations are presented in Fig. 4. The statistical analysis used for comparing the difference between the means for the moisture accumulation in the BAM/PP and PLA/PES combinations was the Wilcoxon test, which is a nonparametrical test assuming related samples. The results of the test application (z ¼ 4.280; p < 0.001) had a significant value of p < 0.05, which indicates that the equal means of the difference hypothesis is rejected. Therefore, it can be concluded that the moisture accumulation in boots fabricated using BAM/PP is higher than that in boots fabricated using PLA/PES and that this difference is statistically significant. 3.3. Foot skin surface temperature As described in the methodology section, the analysis of foot temperature was performed by measuring the skin surface temperature of each foot before and after the selected physical task. The temperature was measured in two different zones, the metatarsal/toes and the plantar foot zone. The temperature increase (TI) (in the metatarsal and foot plant areas) was obtained using the value of the difference between the final and initial temperature of the skin surface for each separate foot. The average values for the TI obtained for the two types of inner layers and foot zones are presented in Table 1 and graphically compared in Fig. 5. The statistical analysis used to compare the differences between the means in the previous section was also used in this case.
Fig. 1. (a) Air permeability and (b) water vapour permeability (Permetest) results.
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Fig. 2. Thermal insulation.
As the value obtained for p in the metatarsal zone was 0.401 (z ¼ 0.841), it can be concluded that the difference between the TI in the metatarsal zone for both types of boots is statistically significant. However, the analysis of the results indicates that the p value in the plantar zone is higher than 0.05 (p ¼ 0.779; z ¼ 0.281); thus, it can be concluded that, for this zone, there is no statistically significant difference.
3.4. Thermal discomfort scores Considering the statistical analysis purpose, the following variables were considered: - Questionnaire score related to the initial thermal discomfort (iTD) (section B of the questionnaire e Appendix I), computed from the sum of each answer in the VAS, as previously described; - Questionnaire score related to thermal discomfort (TD), computed from the sum of each answer in the VAS and from the difference between each boot evaluation (section C of the questionnaire e Appendix I) and the initial thermal discomfort scores. For this variable, the higher the score, the higher the discomfort. This evaluation was performed independently for each boot. Because of the subjective evaluation, which was performed through the application of the questionnaire previously mentioned, two different scores were obtained, one regarding the overall initial
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thermal discomfort (iTD) score and the other obtained at the end of the test. The initial score was used as a “baseline” score. Therefore, the difference between the initial and final scores was defined as the thermal discomfort (TD) score for both boot types, BAM/PP and PLA/PES. Fig. 6 presents the results of the TD scores for each boot type. From this analysis, it is possible to verify that concerning the perceived thermal discomfort, the BAM/PP boots have a lower score, i.e., these boots were evaluated as resulting in minor thermal discomfort. Although moisture in clothing has been widely recognised as one of the most important factors contributing to discomfort during wear (Li, 2005), it is possible that the thermal discomfort perceived by the subjects in this study, considering the measurements performed, is much more likely related to the TI than to the MR. 3.5. Thermal discomfort scores validation In the applied questionnaire, the final score in the question related to General Evaluation (GE) was computed using the sum of each VAS answer obtained in the last question (section E of the questionnaire e Appendix I). For this question, all the used scales had the same “score orientation”. Therefore, the higher the score, the better the general evaluation of the boots. This evaluation was applied considering the pair of boots and not particularly each individual tested boot. As the TD scores were obtained from the subjective perception of each subject, it is probable that these scores were closely related to (and influenced by) the overall evaluation of the boots. To verify this possible effect, the scores obtained for thermal discomfort, both initial (iTD) and final (TD) for the two boots, were compared with the scores obtained in the General Evaluation (GE). This analysis was performed by verifying the correlation coefficients between variables, which are presented in Table 2. The results presented in Table 2 demonstrate that there is a statistically significant (and positive) correlation between the TD variables in the PLA/PES and BAM/PP boots (0.509), which indicates that the thermal discomfort scores for both the inner layers are highly correlated. That is, the subjects have experienced thermal discomfort but they experienced the same, or a very similar, sensation for the two types of boots. It appears that GE and the thermal specific comfort evaluation can be independent, as demonstrated by the non-significant correlation between the TD and GE variables (last column in Table 2). Although the correlations are not statistically significant,
Fig. 3. Results of (a) Horizontal and (b) Vertical wicking of the tested knits.
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Table 1 TI (in C) by foot area and tested knit. Metatarsal/Toe
Mean (sd) Median
Plantar area
BAM/PP
PLA/PES
BAM/PP
PLA/PES
2.45 (1.45) 1.90
2.52 (1.41) 2.30
1.73 (1.08) 1.40
1.76 (1.18) 1.80
the correlation between GE and all the other variables related with thermal discomfort are negative, which implies that the subjects tended to inversely score the thermal discomfort and the overall evaluation of the boots. This result suggests that there is a very tenuous line between the overall evaluation, which includes aspects other the thermal experience, and the specific discomfort evaluation when considering the thermal properties. It may also be assumed that those two comfort/discomfort judgments can be distinguished. Alcántara et al. (2005) also observed that people are able to differentiate comfort from other characteristics of the shoes, such as its quality.
Fig. 5. TI (in C) mean in (a) the metatarsal/toes zone and in (b) the plantar zone.
3.6. Foot zones with higher thermal discomfort Certain previous studies, for example Au and Goonetilleke (2007), have reported that a comfortable shoe does not necessarily have the same perceived fit in every region of the shoe. Therefore, it is likely that the same phenomenon may occur in the perceived thermal (dis)comfort. Accordingly, it is important to evaluate (dis)comfort in the different regions or zones of the foot. In a more detailed analysis of the thermal discomfort, subjects were asked to indicate where they felt that the thermal discomfort was more obvious by considering the different zones of the foot and using a visual scheme (section D of the questionnaire e Appendix I). The subjects were able to indicate one or more specific zones of the foot in which they noticed a higher discomfort regarding the two analysed parameters: (1) the perceptible higher temperature increase and (2) the perceptible higher moisture accumulation. The results of this analysis are presented in Fig. 7. Fig. 6. TD mean score (in points) by knit type.
Fig. 7 demonstrates that there is a predominance of indications of discomfort for the PLA/PES boots. This result is visible on both sides of the chart, which suggests that the discomfort is highly perceived in the PLA/PES boots, both in terms of TI and MR. In the specific case of the MR (Fig. 7(b)), this predominance occurs for all the zones considered. Notably, this predominance is not in line with the results obtained in the objective test. In this test, PLA/PES performed better
Table 2 Spearman’s correlation coefficients matrix between scores.
Thermal discomfort in the initial phase (iTD) Thermal discomfort (TD) e BAM/PP Thermal discomfort (TD) e PLA/PES General evaluation (GE) Fig. 4. MR mean value (in gram) in both types of tested boots.
iTD
TD BAM/PP
TD PLA/PES
GE
e
0.343*
0.270
0.010
e
0.509**
0.059
e
0.287 e
* Correlation is significant at the 0.05 level; ** Correlation is significant at the 0.01 level.
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Fig. 7. Number of indications of thermal discomfort regarding (a) temperature increase and (b) moisture retention.
than BAM/PP for the MR; however, this result was not confirmed by the discomfort indications of the subjects. A possible explanation for this result can be the fact that there is a “prevailing” influence of the TI in the discomfort perceived by subjects, i.e., subjects tend to value the TI more than the MR when they evaluate thermal discomfort. Alternatively, other possible explanation may also be related to the characteristics of the requested physical task. As previously detailed, the proposed physical task was not highly challenging from the physical point of view. Therefore, it can be assumed that that MR could have a different impact on the thermal discomfort perception if the proposed physical task required a higher physical activity and, theoretically, a higher moisture retention in the socks used by subjects. Finally, it can also be considered that MR may play a more important role in the users’ (dis)comfort when considering low temperature environments and, consequently, in the discomfort produced by the cold sensation in the foot (Kuklane et al., 2001). From this analysis, two important insights emerge. On one hand, regarding the TI, the subjects perceived a more evident thermal discomfort in the mid-part of the foot. The subjects also seemed to differentiate better between the 2 types of fabrics tested in the mid-foot zone. However, regarding the MR, the most affected area was the upper region of the foot, including the midfoot/metatarsal and the toe area. The subjects did not experience (or at least did not perceive) a significant accumulation of moisture in the heel or in the malleolus/ankle zones. This information may be relevant for the design and development of boots and for the design of socks. 4. Study limitations Regarding the present study, there are two main limitations related to the analysed data that may affect the accuracy of the results. The first limitation concerns the use of a simulated use of the boots in a laboratory environment. This simulation does not seem to fully represent the “real world” use, as both the treadmill walking task and the characteristics of the terrain are not representative. However, the main idea was to compare, in a controlled environment, the properties of the fabrics and the corresponding sensorial perception using a subjective assessment. The use of a real setting (with typical hikers, different types of terrains, climates, etc.) would imply the presence of a high number of uncontrolled variables, which would certainly influence the data obtained. The second limitation is related to the exclusion from the sample of subjects with some type of
disorder in their feet. In reality, some of the people using this type of boots will have that type of problem, such as the case of feet blisters and callosities. However, again, it is the authors’ opinion that the inclusion of these people in the sample would impair the current analysis. 5. Conclusions The definition of a thermally comfortable shoe is one of the most desired targets among the manufacturers of several types of shoes. This goal is even more important when it concerns the design of some specific types of shoes, such as trekking boots. The work described in this manuscript aimed at proposing a battery test for thermal comfort assessment and applying it by identifying and evaluating trekking boots, both objectively, by developing and testing fabrics with particular characteristics that can optimise thermal comfort, and subjectively, by assessing the discomfort perceived and reported by users. Based on the application of the proposed battery tests and on the corresponding results, it was possible to demonstrate that the subjective perception of thermal comfort appears to be related much more to the increased temperature of the foot than to the moisture retention. The use of a combination of objective and subjective assessment techniques revealed that although the objective assessment indicated that a particular combination of fibres appears to be the optimal solution for the inner layer of the boots, the subjective evaluation of the thermal discomfort suggested that users may have a different opinion regarding the used fabrics and, most likely, their opinion is influenced by other parameters. Furthermore, the current study also demonstrates that no correlation was observed between the general comfort evaluation and specific thermal comfort evaluation. This result indicates that it is likely that the applied methodology may have allowed the differentiation of these two aspects in the evaluation of comfort. In the authors’ opinion, this issue could be addressed in future studies, with a larger subject sample, to verify whether the applied scales are adequate to autonomously evaluate the thermal discomfort/ comfort and not include some hypothetical biases from external factors, such as, the consideration of boot aesthetics. Finally, it appears that the identification of thermal discomfort by specific zones of the foot will be useful for the project and construction of the shoes, particularly for previewing the application of fabrics with differentiated thermal behaviour in these areas.
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Appendix I
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