Proc. of the 5th Symp. on Footwear Biomechanics, 2001, Zuerich / Switzerland, (Eds. E. Hennig, A. Stacoff) 64 A RELIABLE MEASURE TO ASSESS FOOTWEAR COMFORT A. Mündermann, B.M. Nigg, D.J. Stefanyshyn, R.N. Humble Human Performance Laboratory, University of Calgary, Canada, AB T2N 1V7 INTRODUCTION Comfort has been proposed as one of the most important factors for footwear in physical activities (Nigg et al., 1999). However, to date a reliable measure to assess comfort has not been developed. In this study, the reliability of comfort measured on a visual analogue scale (VAS) was determined based on data collected for ten sessions per subject. REVIEW AND THEORY Sport shoes, shoe inserts and orthotics should be comfortable. However, little is known about footwear comfort. Comfort has been associated with fit, additional stabilizing muscle work, fatigue, and damping of soft tissue vibrations (Nigg et al., 1999), plantar pressure distribution (Chen et al., 1994) and subject characteristics such as skeletal alignment (Miller et al., 2000). A valid relationship between footwear comfort and biomechanical variables can only be obtained if comfort can be assessed in a reliable way. Visual analogue scales (VAS) have been proposed as a reliable measure for subjective footwear comfort since they have been proven as a reliable measure to assess subjective pain (Gramling & Elliott, 1992). VASs that are 100 to 150 mm in length have been shown to have the greatest sensitivity and were the least vulnerable to distortions or biases in rating (Stevens & Marks, 1980). The ratio scale property of VAS will allow quantification of the difference in comfort between two or more footwear conditions. Therefore, the purpose of this study was to determine the reliability of comfort of shoe inserts using visual analogue scales. PROCEDURES Nine volunteers (4 females, 5 males, age 26.7 ± 4.9 yrs; height 171.2 ± 7.1 cm; weight 71.7 ± 8.5kg) gave informed consent and participated in this study. All subjects were free of lower extremity injury or pain. The four shoe insert conditions tested were (a) a control insert (3 mm EVA, 3 mm spenco), (b) a soft insert (3 mm poron, 3 mm spenco), (c) a hard insert (3 mm EVA, 3 mm korex), and (d) a posted insert (6 mm medial EVA post, 3 mm spenco) with the following shore C values: EVA: 73; spenco: 40; poron: 28; korex: 76. All shoe inserts had no visually noticeable difference and were tested in a standard running shoe (Adidas Super Nova). A 150 mm VAS (left end = ‘not comfortable at all’; right end = ‘most comfortable condition imaginable’) was used to assess footwear comfort. Data for each subject were collected in ten sessions on ten consecutive workdays. In each session, subjects ran two laps on a 450 m indoor running track to warm up. Subjects were instructed that the first, third, fifth and seventh condition were the control condition but were unaware of the order of the test inserts. They then ran one lap using the insert conditions C-Ii-C-Ij-C-Ik-C-R where C corresponded to the control condition, I to an insert condition and R to an arbitrarily chosen repeat insert condition. The sequence of the insert condition was randomly varied. After each lap, the subjects assessed footwear comfort for the tested shoe insert condition. Pearson product moment correlation coefficients (R) for repeated comfort ratings were determined for all subjects and sessions and individually for subjects and sessions. Proc. of the 5th Symp. on Footwear Biomechanics, 2001, Zuerich / Switzerland, (Eds. E. Hennig, A. Stacoff) 65 RESULTS AND DISCUSSION 15 15 2.5 comfort points repeat Comfort points Comfort points 2.0 10 10 1.5 5 1.0 5 0.5 0 1 2 3 4 5 6 7 8 9 10 0 session 0.0 Control Insert Soft Insert 1 1 to 3 4 to 6 7 to 9 0 5 10 15 Hard Insert Medial Wedge comfort points test 1 sessions Figure 1. Comfort ratings for Figure 2. Comfort ratings for subject 1 Figure 3. Average distance from repeated conditions. for all insert conditions and sessions. “true comfort rating”. The Pearson’s correlation Sessions 1 to 3 Sessions 4 to 6 Sessions 7 to 9 coefficient for within session Average Variability 1.89 1.28 1.28 repeatability for all subjects [comfort points] and sessions was R = .799 Table 1. Average variability for all subjects. (Figure 1). After exclusion of subjects with R < .400 (n = 3), the within session repeatability was R = .911. The average difference in the repeat assessments following different preceding conditions was 0.5 for all subjects with an average difference of up to 1.6 comfort points for some subjects. Thus, an insert condition can affect the comfort rating of the following insert condition and, therefore, a control condition should be included for comfort assessment. Comfort ratings of sessions 1 to 3 varied more than comfort ratings of sessions 4 to 10 (Figure 2, Table 1). Therefore, a “true comfort rating” was defined as the average of the ratings of sessions 4 to 10 for each subject and condition. The comfort rating of session 1 differed in the average by more than 2 comfort points from the “true comfort rating” (Figure 3). Thus, the first assessment of footwear comfort does not represent the “true comfort rating”. The average comfort rating for session 4 to 6 differed less than 0.5 comfort points from the “true comfort rating”. The average comfort rating for sessions 7 to 9 did not provide any improvement (Figure 3). Thus, the average comfort rating for session 4 to 6 represents the “true comfort rating” most accurately and is proposed for comfort assessments. The results of this study showed that VASs provide a reliable measure to assess footwear comfort under the conditions that (a) a control condition is included, (b) subjects with low repeatability are excluded, and (c) the average comfort rating of sessions 4 to 6 is used. In this study, subjects were tested on ten days with one session per day. Future research should be conducted to determine the optimal combination of number of conditions per session, running distance per condition, total running distance per day, and number of test days. REFERENCES Chen H. et al. Clin. Biomech., 9, 335-341, 1994. Gramling S.E. & Elliott T.R., Behav. Res. Ther., 30, 71-73, 1992. Miller J.E. et al., Foot & Ankle Int., 21, 759-767, 2000. Nigg B.M. et al. Med. Sci. Sports Exerc., 31, 421-428, 1999. Steven J.C. & Marks L.C., Percept. Psychophys., 12, 417-434, 1980. ACKNOWLEDGMENTS This work was supported by the Canadian Department of Foreign Affairs and International Trade and NSERC (Canada).
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