DEVELOPMENT OF A NEW METHOD TO MEASURE THE ...

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male and 5 female) walking on 9 different casual shoes with an instrumented insole ... Ten different combinations of pressure and time were identified for loading ...
Proc. of the 5th Symp. on Footwear Biomechanics, 2001, Zuerich / Switzerland, (Eds. E. Hennig, A. Stacoff)

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DEVELOPMENT OF A NEW METHOD TO MEASURE THE RECOVERY ABILITY OF INSOLE MATERIALS BY SIMULATING PLANTAR PRESSURES E. Alcántara, J. C. Conzález, S. Alemany, A. C. García Instituto de Biomecanica de Valencia. Spain INTRODUCTION A new machine test method simulating plantar pressures was developed to study the recovery ability of materials during walking. Results for a range of insole materials available in the market are shown. REVIEW AND THEORY There is a wide selection of insole materials in the market that help to improve general performance of shoes and that is also used in clinical practice. Different parameters as hardness, loss tangent, stiffness and others may be used to select the most adequate material for each case (Forner et al., 1995; Rome, 1990; Whittle, 1997). Nevertheless, materials’ performance is related to a high extent to their thickness and internal structure. In this sense, under load, a material takes a certain time to recover its original thickness and structure after the load is released, which is called recovery ability. Thus, when walking, if materials’ recovery time is greater than the time between steps it will show a residual compression leading to a thickness and structure and so mechanical properties different from the originally measured ones. However, there is little information in literature about recovery ability of insole materials. In some commercial brochures (Rogers Corp., 1998) it has been measured as rate of recovery, but there is a need for a simple and reliable method of measuring recovery ability of materials. Besides, the materials studied are viscoelastic and non-linear and therefore, machine tests should apply loads similar in magnitude and frequency to those occurring in real conditions. This paper presents the development of a machine test to measure recovery ability of insole materials by simulating plantar pressure. PROCEDURES To define the loading conditions, plantar pressures were registered for ten volunteers (5 male and 5 female) walking on 9 different casual shoes with an instrumented insole [the IBV Biofoot] placed inside the shoe. It consists of a flexible insole with up to 64 piezolectric ceramics distributed according to foot physiology. Two photocell barriers placed at a known distance were used to control walking speed (Figure 1). The foot plant was divided into eleven zones (Figure 2) and the peak of the average of the pressure data for all sensors in each area was computed for analysis. PrD RD MM C5M

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Figure 1. Laboratory set up for plantar pressure distribution testing.

Figure 2. Areas into which the foot plant is divided.

Proc. of the 5th Symp. on Footwear Biomechanics, 2001, Zuerich / Switzerland, (Eds. E. Hennig, A. Stacoff)

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Ten different combinations of pressure and time were identified for loading conditions as follows: Pressure (kPa) 100, 200, 300, 400 and 500 and Time (ms) 400 and 800. For simplicity, the loading conditions are referred to by two numbers, the first one represents the duration (4 or 8) and the second one is the pressure. Thus, the test consists of applying for each loading condition five compression cycles with a time lapse of 1300 ms between two consecutive peaks (walking step time). The test was done using a Universal dynamic INSTRON 8501 testing machine by a cylindrical clamp of diameter 50 mm to compress the material. The force and displacement signals were registered and three different parameters were obtained from them to describe the recovery ability of materials: Integration of displacement on time (Area), Residual compression (Rebote, in m) and Loading work (Wdefor) (integration of force against displacement during loading phase). Fifty-two different materials were tested to analyse the possibility of reducing the number of loading conditions and parameters. A hierarchical clustering analysis was done between variables using the between groups method and the absolute value of Pearson correlation coefficient as distance measurement. The statistical analysis was done using SPSS 7.5.2.s for Windows. The method was used to test some commonly used materials (EVA, PU, etc.). The Coefficient of Variation (COV=sd/mean) over three tests was used to assess the test’s repeatability.

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RESULTS AND DISCUSSION A high statistically significant correlation 0,00035 (r>0.8) was found between the different 0,0003 0,00025 study parameters from each loading set 0,0002 REBOTE44 and also between parameters from REBOTE82 0,00015 0,0001 different sets. Thus finally, the 0,00005 procedure included two tests: (i) rapidly 0 applied high pressure (simulating pressures under the heel and first toe) and (ii) slowly applied low pressure Figure 3. Rebote for a sample of insole materials (simulates pressures under the 5th metatarsal heads). The Coefficient of Variation ranged between 2 and 36% (mean = 13.05% for Rebote44 and of 14.1% for Rebote82), which were considered as acceptable. The values obtained for materials in the market are shown in Figure 3. Polyethylene shows the greater residual displacement for both cases and in general, greater values were registered for Rebote44. This is an easy and reliable method of measuring materials recovery ability that could be used for further studies related to its influence in walking performance and to select insole materials. REFERENCES

Forner, A.; García, A.C.; Alcántara, E.; Ramiro, J.; Hoyos, J.V:, Vera, P.:. Foot Ankle Int., 16 (12): 778-786, 1995 Rome, K.: J. Am. Podiatr. Med. Assoc., 80: 471-478. 1990. Whittle, M. W.: The use of viscoelastic materials in shoes and insoles: a review. Per. Com. 1997. ACKNOWLEDGEMENTS

This work has been carried out with the support of the company Analco S.L.