User Trial and Insulation Tests to Determine Whether Shock-Absorbing Insoles Are Suitable for Use by Military Recruits during Training

Military Medicine, Sep 2004 by House, Carol M, Dixon, Sharon J, Allsopp, Adrian J

A user trial was undertaken to determine whether a shock-absorbing insole is suitable for military use. Two thicknesses of insole (3 mm and 6 mm) were studied and were issued to 38 Royal Marine recruits to wear in their military boots for weeks 12 to 30 of training. Biomechanical measurements showed that both thicknesses of insole significantly (p

Introduction

Overuse injuries of the lower limbs are a frequent occurrence during military training1-4 and are extremely costly in terms of medical treatment, rehabilitation, and morale. It has been suggested that shock-absorbing insoles protect against such injuries by reducing the magnitude and rate of loading of the peak forces generated at foot strike during running and walking; this redistributes the ground reaction forces across the foot and reduces the loads transmitted to the skeleton.5 In addition, it has been proposed that inadequate cushioning can result in unnecessary and possibly excessive joint movements of the lower extremities, which might be associated with the occurrence of lower limb injuries.6,7

Previous work demonstrated that, of four different types of commercially available, off-the-shelf, shock-absorbing insoles tested, an insole made from open-cell polyurethane foam shaped into a footbed with an ethyl vinyl acetate heel cup was the most effective.8,9 During running with military boots, these insoles attenuated the peak pressures generated at heel strike by 36% and during forefoot loading by 28%.8 They were the only insoles that reduced the loading rate of the initial impact force and the only insoles that enhanced movement kinematics by reducing peak ankle dorsiflexion during ground contact in running. In addition, the performance of these insoles was well maintained after a period of mechanical degradation.9

Although these insoles performed well in previous tests and were undisputedly better than the other insoles tested, further assessment of the insoles was required before the insoles were issued to all Royal Marine (RM) recruits, to ensure that they were comfortable and suitable for prolonged military use. Although the insoles maintained their shock absorbency after a period of mechanical degradation, such degradation is not completely representative of wear. Mechanical degradation simulates wear only in terms of the impact force applied in one direction and does not include factors such as shearing and static compressive forces. Furthermore, mechanical wear does not account for the effects on the insoles of sweat and repeated wetting and drying. Therefore, a user trial was undertaken to determine the comfort and performance of the insoles during and after use by RM recruits for 18 weeks during training.

RM recruits and other military personnel are currently issued Saran insoles, which consist of a 3-mm-thick base of coarse weave plastic and a top sheet of nylon nonwoven fabric. These insoles were not designed to be shock absorbing but were intended to improve insulation and hygiene inside the boots. Switching to a foam shock-absorbing insole that is likely to retain water could reduce insulation inside the boots and increase the risk of personnel sustaining cold injuries to their feet. In an attempt to quantify this risk, the insulation provided by Saran insoles and the shock-absorbing insoles when wet and dry was measured with a thermal foot manikin. It was hypothesized that the shock-absorbing insoles (a) would reduce the peak pressures generated during running in military boots (relative to the no-insole, control condition) when new and after wear by RM recruits; (b) would reduce peak deceleration measured with a mechanical drop-test (relative to the no-insole, control condition) when new and after wear by RM recruits; (c) would remain comfortable during wear, if initially comfortable when issued; and (d) would provide less insulation than the Saran insole when wet.

Methods

User Trial

Subjects

Thirty-eight RM recruits who had completed the first 12 weeks of their training volunteered for the trial. Their written informed consent was gained in accordance with the Declaration of Helsinki.10 The recruits were issued the insoles at week 12 of training, rather than at the start of training, to try to maximize the number of subjects completing the trial and because many of the activities before this time are undertaken in training shoes rather than boots.

Procedure

The subjects were matched into pairs according to body mass and were then randomly issued a pair of shock-absorbing insoles of either 3-mm or 6-mm thickness. The sizing was checked to ensure that the insoles fit the subjects' boots. The subjects were asked to wear the insoles in their boots for all of the activities they undertook in boots. These activities included field exercises and speed marches amounting to a total distance of approximately 270 km plus assault and endurance course runs and general wear around the camp. The subjects were told that they could remove the insoles and terminate their participation in the trial at any time.