Elsevier

Gait & Posture

Volume 16, Issue 3, December 2002, Pages 297-303
Gait & Posture

The influence of simulated wear upon the ability of insoles to reduce peak pressures during running when wearing military boots

https://doi.org/10.1016/S0966-6362(02)00021-8Get rights and content

Abstract

Mechanical degradation of three types of shock absorbing insoles equivalent to 100–130 km of running did not reduce their ability to attenuate the peak pressures generated during running when wearing military boots. Pressure measurements at the heel and forefoot were recorded with pressure measuring insoles placed in the boots of nine subjects. Two of the three insoles tested reduced the peak pressures (P<0.05) generated at the heel and forefoot relative to the no-insole (control) condition. The most effective insole reduced the peak pressures at the heel by 37% and at the forefoot by 24%.

Introduction

During initial military training, recruits undergo arduous physical activity including assault course work, long distance marches and runs. The Royal Marines undertake a progressive training course lasting at least 30 weeks during which time they undertake approximately 325 km of structured training wearing military boots, and for much of this training they carry additional loads. It is therefore not surprising that initial military training is associated with a high incidence of lower limb overuse injuries [1], [2]. It has been suggested that shock absorbing insoles, placed within boots, may help to reduce the incidence of overuse injuries, by reducing the magnitude and rate of loading at heel strike, thereby reducing the loads transmitted to the skeletal system [3].

We have previously shown that shock absorbing insoles can reduce the magnitude of peak pressures when running wearing military boots by up to 27% at the heel and 11% at the forefoot [4]. However, studies have shown that the cushioning ability of insole materials is reduced with use [5], [6], [7], [8], [9], suggesting that, although insoles may be effective when issued to military recruits, they may not be for the duration of their training. This may account for the lack of convincing evidence that shock absorbing insoles do reduce the incidence of injuries in military populations [10], [11], [12].

In studies showing reduced cushioning ability with use, the insoles have either been aged using subject wear [5], [6] or using mechanical simulation typical of wear patterns [7], [8], [9]. The studies using subjects to age the insoles did not control the activity performed by subjects during the test period, whereas studies using mechanical loading have not subsequently investigated the effects of insole degradation on the loads occurring during running. The aim of this study was to investigate the influence of controlled degradation of test insoles on the ability of the insoles to reduce peak pressures during running.

It has been demonstrated that the degree of degradation of insole materials following use is dependent on the type of insole material. In particular, foam materials have been found to deteriorate more rapidly than elastomeric materials due to compression set of the material [5], [6], [13]. In the present study, insole materials were selected to include both foam and elastomeric materials. The insoles chosen were a polyurethane foam, a viscoelastic polymer (elastomeric) and an insole constructed from a combination of these materials. It was hypothesised that shock absorbing insoles would reduce peak pressures during running and that following controlled degradation to simulate running impacts, the insoles would exhibit a reduced impact-absorbing ability. In addition, it was hypothesised the insole comprising solely of a viscoelastic polymer would maintain its cushioning ability to a greater extent than the insole constructed of a polyurethane foam.

Section snippets

Subjects

Nine injury free Royal Marine recruits who had completed the first 15 weeks of their 30-week training course volunteered and took part in the study in accordance with the guidelines in the Declaration of Helsinki. The subjects were an average of 19 (S.D. 0.9) years old and their average body mass including the mass of their webbing was 92.2 (S.D. 9.8) kg. The protocol was approved by the Ministry of Defence (Navy) personnel research and ethical committee.

Task

The study was undertaken in a covered

Results

The mean stiffness and peak deceleration of the insoles measured in the laboratory are shown on Fig. 2, Fig. 3. Of the 3 shock absorbing insoles, the stiffness of insoles A and C were similar when new and although their stiffness increased with degradation (with that of insole A increasing the least) they were still less stiff than insole B when new. In terms of shock absorbency (measured as peak deceleration from the drop test and shown on Fig. 3) all 3 shock absorbing insoles reduced the peak

Discussion

The hypothesis that the placement of insoles in military boots reduces peak pressures in running cannot be accepted for all insoles, as only two of the three shock absorbing insoles were effective. The significant reduction in peak pressures for two of the test insoles supports the findings of the previous study in which a range of commercially available insoles were found to all significantly reduce peak pressures [4]. However, the observation that one type of insole does not reduce peak

Conclusion

Although the mechanical properties of shock absorbing insole material may be affected by degradation, the findings show that if an insole is able to reduce the peak pressures during running when new it will continue to do so following a period of degradation that equates to 100–130 km of running.

Acknowledgements

The authors would like to acknowledge the help of Dr A. Collop and Dr T. Singleton and the technical staff in the Department of Civil Engineering and Orthopaedic and Accident Surgery, University of Nottingham. We would also like to thank the Commandant and Medical Staff at Commando Training Centre, Royal Marines and Dr R.J. Pethybridge from the Institute of Naval Medicine for his assistance with the statistical analysis. This work has been carried out with the support of the Ministry of Defence

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