Elsevier

Gait & Posture

Volume 9, Issue 1, March 1999, Pages 31-37
Gait & Posture

The shock attenuation characteristics of four different insoles when worn in a military boot during running and marching

https://doi.org/10.1016/S0966-6362(99)00002-8Get rights and content

Abstract

A study was undertaken to determine if placing shock absorbing insoles in the boots of Royal Marine recruits would attenuate the peak pressure at the foot-boot interface, when marching at 4.8 kph carrying a 32 kg (70 lb) Bergen and running at 12.8 kph in loose order plus webbing weighing 10 kg (22 lb). Four types of insoles were assessed: viscoelastic polymetric insole (Cambion®) polymetric foam insole (PPT®) Saran insole (military issue) and Sorbothane®. There was a fifth control condition in which no insoles were used. Pressure measurements during heel strike and forefoot loading were taken using Paratec equipment with pressure measuring insoles placed in the boots. Data were obtained from eleven subjects and indicated that all the insoles significantly (P<0.05) attenuated the peak pressures generated during heel strike and forefoot loading. The performance of the four insoles in terms of peak pressure attenuation ranked in order with the best first were: Sorbothane Cambion PPT Saran. The Sorbothane insole was substantially and significantly (P<0.05) better than the other insoles in terms of attenuating peak pressures during heel strike. During running, mean peak pressure at heel strike was 494 kPa in the control condition, this was reduced to 377 kPa when wearing Sorbothane insoles (a reduction of 27%). When marching the Sorbothane insoles reduced the mean peak pressure at heel strike from 395 kPa (control) to 303 kPa (23% reduction). During forefoot loading the peak pressure attenuation of all four insoles was similar, although on average the Sorbothane insole performed slightly better than the others and was significantly different (P<0.05) to the Cambion insole. Mean peak forefoot loading pressure in the control condition when running was 413 kPa, with the Sorbothane insole it was 367 kPa, during marching the respective mean peak pressures were 397 and 323 kPa. It is concluded that of the four types of insoles assessed the Sorbothane insoles attenuated the greatest amounts of the peak pressure generated at heel strike and during forefoot loading when running and marching wearing military boots.

Introduction

The association between military activities such as marching and the occurrence of stress fractures has long been recognised [1]. A study of British Royal Marines revealed that during their 30 week training programme 16% suffered an injury to their lower limbs of which stress fractures were the most common [2]. Factors that reduce the amount of stress transmitted to the bone could reduce the incidence of these injuries [3] and this might be achieved by placing a shock absorbing insole in the boot.

Longitudinal studies which have been conducted using military recruits have shown some evidence to suggest that shock absorbing insoles do reduce the incidence of overuse injuries; however, this is not conclusive [4], [5], [6]. Although such studies have been conducted, the effectiveness of shock absorbing insoles to attenuate the peak pressures generated at the foot-ground interface when wearing boots has received little attention. It has been shown that viscoelastic insoles did not attenuate the vertical impact peak forces when running at 4 m/s when placed in running shoes [7]. Running shoes however, have inherently greater shock absorbing properties than military boots, hence it should not be assumed that the same result would be found when wearing boots. Indeed, shock absorbing heel inserts placed in street shoes have been shown to reduce the peak accelerations of the lower extremity [8], [9].

The shock absorbing ability of shoes and insoles has been assessed using ‘bench top’ material tests [10], [11]. For such tests, a mass with a specified impact velocity is dropped onto the test material and the peak force during impact measured. The shock absorbency of the material, or cushioning, is determined by the effectiveness of a material to reduce the magnitude of the impact peak [12]. Studies have shown clear differences in the shock absorbing capacities of materials on materials tests, however when the same materials were assessed on human subjects the differences were attenuated [11], [13]. This suggests that to determine the shock absorbency of insoles when worn in military boots, they should be assessed when worn in boots by subjects performing the activities for which the insoles are intended.

In the present study, four types of shock absorbing insole were assessed against a control ‘no insole’ condition during running and marching. A method which assessed local pressures at the foot-boot interface was adopted. It was hypothesised that the insoles would attenuate the peak pressures generated at heel strike and forefoot loading compared to the control ‘no insole’ condition during running and marching when worn in military boots.

Section snippets

Subjects

The study was undertaken in a covered hanger with a stone floor. This surface is representative of that upon which Royal Marine training is conducted. They also train in the field and gymnasium (sprung wooden floor). A stone floor was selected for the trial as it would have the least inherent force attenuation properties and would not be affected by varying weather conditions. Royal Marine recruits in week 18 of their 30 week training course volunteered and took part in the study in accordance

Results

Data were collected from 11 subjects all of whom ran with a heel–toe foot strike. Hence there were 22 sets of results for each condition (11 subjects×2 feet). Table 1 shows the characteristics of the 11 subjects.

The mean pressures (kPa) for the 11 subjects left and right feet (n=22) on sensors 1 and 2 (heel strike) and forefoot pressure for each condition during running and marching are presented in Table 2. The pressures generated during running were greater than those during marching, with

Discussion

The hypothesis that insoles placed in boots would attenuate the peak pressures at heel strike during running and marching compared to the ‘no insole’ condition has been supported. Peak pressures during the forefoot loading of the push-off phase have also been found to be attenuated by the use of insoles. Of the four insoles assessed, the Sorbothane insole has been found to be the most effective in terms of attenuating peak pressures.

The reduction in peak pressures at heel strike demonstrated in

Conclusions

Insoles placed in the combat assault boot during running and marching attenuate the peak pressures generated during heel strike and forefoot loading. Of the four insoles assessed Sorbothane was the most effective.

Acknowledgements

The authors would like to acknowledge the Commandant and Medical Staff at Commando Training Centre, Royal Marines particularly Surgeon Lieutenant Commander S. Murray. We would also like to thank Dr R.J. Pethybridge for his assistance with the statistical analysis and the Technical Services department at the Institute of Naval Medicine. Cambion® and Epoflex® are registered trademarks of the Magister Corporation, Poron® is the registered trademark of the Rogers Corporation. PPT® is the

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