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