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J R Soc Med. 2006 January; 99(1): 32–37.
PMCID: PMC1325079

Do shock absorbing insoles in recruits undertaking high levels of physical activity reduce lower limb injury? A randomized controlled trial

Abstract

Objectives: To assess the benefits, if any, of the use of shock absorbing insoles in reducing lower limb injury among Air Force recruits, and to assess the differences, if any, in the efficacy of two commonly available shock absorbing insoles.

Design: Randomized controlled trial. Setting: RAF Halton, UK. Site of all basic training for RAF personnel.

Participants: 1205 recruits participating in basic training between 17 September 2003 and 7 April 2004.

Interventions: Participants were randomized to receive either standard issue Saran non-shock absorbing insoles, or shock absorbing Sorbothane or Poron insoles, on a 1:1:1 basis.

Main Outcome Measures: The primary outcome measure was withdrawal from training for lower limb injury. The two primary comparisons were shock absorbing insole versus non-shock absorbing insole, and Sorbothane versus Poron (comparison of different shock absorbing insoles). Secondary outcomes were medical withdrawals for reasons other than those qualifying for the primary outcome measure.

Results: When comparing the non-shock absorbing insole to the shock absorbing insoles 72/401 participants (18.0%) allocated to Saran insoles were removed from training because of a qualifying lower limb injury, compared with 149/804 (18.5%) allocated to the shock absorbing insole (Sorbothane or Poron), odds ratio 1.04 (95% CI 0.75 to 1.44; P=0.87). When comparing the two shock absorbing insole 73/421 participants (17.3%) randomized to Sorbothane were removed from training because of a qualifying lower limb injury, compared with 76/383 for Poron (19.8%), odds ratio 0.85 (95% CI 0.58 to 1.23; P=0.37).

Conclusions: Similar rates of lower limb injuries were observed for all insoles (shock absorbing and non-shock absorbing) in the trial. The trial provides no support for a change in policy to the use of shock absorbing insoles for military recruits.

INTRODUCTION

Sports injuries are a common cause of musculoskeletal morbidity in modern western societies.1 Physical capability, technique and psychological dedication are required to attain high level physical performance.2 However, the activity necessary to achieve desired fitness levels can result in injury,3 as observed following a national increase in childhood and adolescent participation in organized physical activity.4 Overuse injuries may result from the accumulation of repetitive musculoskeletal loading forces, each lower than the threshold inducing acute injury. Bloomfield et al.4 suggest contributing factors include: poor technique; an excessive number of attempts at an activity; inappropriate footwear; musculoskeletal immaturity; and/or a body shape and stature that may predispose to injury.

Military personnel require high levels of fitness.2 RAF Halton is the site of Phase 1 training for all noncommissioned RAF (Royal Air Force) personnel. During the 9-week course, Phase 1 RAF recruits undergo standardized intensive physical activity including running, marching, recreational team sports, swimming and gymnasium-based activities such as circuit training. Military recruits are at significant risk of lower limb injury during their initial training.5 As injuries affect military training and operational output, and their treatment is often difficult, time-consuming and expensive, Parkkari et al.1 suggest that preventative strategies may be justified both on clinical and economical grounds.

Professional sporting organizations and the military have started to utilize shock absorbing insoles (SAIs) in an attempt to prevent overuse injury. SAIs may protect against injury by reducing the magnitude and rate of loading of the peak forces generated at foot strike during running and walking, reducing the ground reaction forces across the foot, and the loads transmitted to the skeleton.6

UK military personnel currently use three principal types of commercially produced insole:

  • Saran (a flat 3 mm coarse weave dark green plastic overlaid with a transparent top sheet of nylon nonwoven mesh) is the RAF standard issue at a military cost of £0.50 per pair. As Saran is principally adopted for its insulating, rather than shock-absorbing properties, and is RAF standard issue, it was considered a suitable control.
  • Sorbothane SAI (3 mm grey polyurethane foam moulded into a shaped foot-bed, with 1 mm red viscoelastic sorbothane polymer inset into the areas under the heel and ball of the foot) was developed to reduce foot strike impact forces.7 The military commonly uses Sorbothane as a second-line insole for individuals experiencing lower limb discomfort, but they cost the military £10.00 per pair.
  • Poron SAI (bright green shaped flat sheets of open-cell polyurethane high-density foam) is available in 1.5 mm, 3 mm or 6 mm thicknesses. A 3 mm insole was selected for this trial to match the sorbothane polyurethane foam. Poron is increasingly used to relieve symptoms due to skeletal shocks at heel strike8 and is favoured by the Royal Marines because of its shock absorption, durability and weight properties9 at a military cost of £2.70 per pair.

Running produces a high proportion of lower back and lower limb injuries due to the velocity-dependent ground impact forces at each heel-strike which range from 1.2 times body weight when walking to 3.6 times body weight whilst sprinting.4 Biomechanics, such as joint angular velocities, influence impact forces more than anatomical factors.10,11 Heel-strike impact forces induce soft tissue resonance, however such vibrations are small or unrecordable.12 The amount of work done during a physical activity such as locomotion is dependent upon the ground-foot interface,13 and has a direct bearing upon performance.14 Different combinations of materials in shoes soles, insoles or sports surfaces dissipate different amounts of energy,13 change muscle activity, and affect the body's musculoskeletal work capability and subsequent injury rates.15 Nigg et al.12 demonstrated that wearing viscoelastic rather than solid heels during exercise reduced oxygen requirement (as a measure of work) by up to 2%.

Unpublished RAF data indicated that between 1996-1997 48% of recruits sustained injuries during phase 1 training: 80% of these injuries affected the lower limbs; 80% of all reported injuries occurred during high impact training in weeks 2 and 3, and of these, 35% of injuries affected the shin and foot (PL White personal communication). Each intake of approximately 70 personnel would lose 250 training days as a result of injury, representing 1.5% total training time available. Between 1997-1998, a local study of Sorbothane SAIs was undertaken, reporting a 28% reduction in the overall injury rate, with injuries occurring in weeks 2 and 3 reducing from 80% to 30%. A 38% reduction in training days lost was achieved. As a result, Sorbothane SAIs were issued to all RAF recruits undergoing phase 1 training. However, as the study was a non-randomized ‘before and after comparison’, the differences could be due to bias such as the Hawthorne (participation in a study) effect or the related placebo effect.

The relative merits of SAIs have usually been judged retrospectively. Systemic review provided insufficient evidence to suggest SAIs significantly reduce the incidence of overuse injury.17 As the review included studies of only ‘low to moderate’ quality, the authors acknowledged more data are required. Thacker et al.18 also proposed rigorous research to address common sports medicine problems, as the absolute and relative effectiveness of many interventions remain poorly understood.19

No RCTs have compared differing SAIs within a UK military clinical setting, and there is currently no consolidated UK military policy concerning the use of SAIs. As RAF recruits are drawn from a cross-section of the British population, males and females of differing socioeconomic class, ethnic background and levels of initial physical ability were included.

METHODS

Participants

Recruits who were scheduled for basic training at RAF Halton were eligible for inclusion in the study. Participants were males or females aged between 16-35 years (the boundaries of age permitted for entry to RAF phase 1 training). Participants had all undergone medical screening examinations at Armed Forces Careers Offices, and been found medically fit for military service. Participants were not excluded on the grounds of their height, weight or body mass index.

Exclusion criteria were: failure to provided informed consent; past medical history of lower limb injury; any ongoing medical problems; current pregnancy.

Participants were included from 11 consecutive intakes at RAF Halton between 17 September 2003 and 4 February 2004. Follow up was conducted between 17 September 2003 and 7 April 2004.

Interventions

Participants were randomized to receive Saran (control), Sorbothane or Poron SAIs by 1:1:1 proportion. Those meeting the inclusion and exclusion criteria above were issued their randomized insole appropriate on the day of basic kitting at the beginning of their basic training.

Objectives

The objective of the study was to estimate the beneficial effects, if any, of the use of newer and more costly SAIs on the rate of lower limb injury in participants undergoing high levels of physical activity associated with RAF basic training. There were two primary analyses. First, to see if the use of either Poron or Sorbothane SAIs would reduce the rate of qualifying lower limb injury in comparison with Saran insoles. The second was to estimate the difference, if any, in the rate of qualifying lower limb injuries in participants using either Sorbothane or Poron SAIs. Secondary objectives included examining the rate of other, non-qualifying injuries sustained during basic training, and examining the influence of RAF Flight allocation (grouping for basic training) on the protective effects, if any, of different insoles.

Outcomes

The primary outcome measure was defined as any lower limb injury diagnosed by the Regional Medical Centre doctors, nurses or physiotherapists from participants who presented to the Medical Centre requesting withdrawal from training on account of injury. Clinical staff at RAF Halton, very experienced in dealing with lower limb injury, received additional training in the classification of injury in the context of the trial. Secondary outcomes were other debilitating injuries resulting in withdrawal from training.

Sample size

Separate sample size calculations were performed for each primary comparison. Since there are two non-independent primary analyses in this study, the available statistical power was divided between the two analyses to preserve the overall critical α level of 5% (two-sided).20 For the comparison of SAIs (Sorbothane or Poron) versus Saran, randomizing 400 patients to each group, and thus comparing 800 participants using SAIs with 400 participants using Saran insoles, was sufficient to detect a 11.5% difference as statistically significant, with 90% power (17β) at the allocated critical α level of 1.67%, making the conservative assumption that 50% of participants would experience a primary outcome.

For the comparison of different SAIs (Sorbothane versus Poron) randomizing 400 patients to each group was sufficient to detect an 11.5% difference as statistically significant, with 90% power (17β) at the allocated critical α level of 3.33%, making the conservative assumption of a rate of primary outcome in the less effective insole group of 43.5%.

Therefore the intention was to randomize 400 participants in each group. As each intake to RAF Halton for basic training includes about 100 potential participants, recruitment was continued offering participation in the study to all eligible participants in each intake until more than 1200 participants had been recruited.

Randomization

A randomization code was generated using a simple three-way randomization algorithm programmed in Microsoft Access, and stratified by flight. Random allocation was through a concealed process, in which details of all recruits expected to attend RAF Halton were sent to the randomization centre at the University of Birmingham during the week before their planned arrival. Provisional allocation for each recruit was generated in Birmingham and communicated with the study personnel at RAF Halton. Participants were recruited in Halton by staff blinded to the allocation of participants. The allocated insoles were supplied to participants during ‘kitting’ on the second day of their phase 1 training.

Blinding

The three insoles utilized in this study are not identical in design, and thus blinding of investigators and participants was not possible.

Statistical methods

The primary analyses used Fisher's exact test, providing odds ratios and 95% confidence intervals. P values are also provided and were used to assess statistical significance at the higher level required due to the alpha spending procedure dividing available statistical power between related comparisons in this three-arm trial. In addition, absolute risk differences and 95% confidence intervals were provided to enable assessment of the practical importance of the observed differences between the groups.

The primary analyses were conducted using the intention to treat principle.21 Participants who did not complete phase 1 training were included in the study until they left the service.

A generalized linear model with a logit link and binomial error was used to assess the affect of flight upon the primary outcome (analogous to the inclusion of treatment centre in a multi-centre trial). Overdispersion (extra binomial variability) was accounted for by inflating the scale factor by the ratio of the residual deviance and the degrees of freedom, on the appropriate stratum.

Ethical approval

Ethical approval for the study was provided by the RAF Experimental Medical Ethics Committee.

RESULTS

Participant flow

During the study period a total of 1205 participants were randomized to one of the three study groups. 401 participants were randomized to receive Saran insoles, 421 to receive Sorbothane SAIs and 383 to receive Poron SAIs. During the period of the trial 98 participants withdrew from basic training. These participants were included until they left the service, at which point they were declared medically fit. The flow of participants through the trial is described in Figure 1.

Figure 1
Participant flow (solid lines indicate inclusions; dotted lines indicate exclusions)

Baseline characteristics

As anticipated, the participants in this trial were largely males in their late teens and early twenties. The baseline characteristics by group are described in Table 1.

Table 1
Baseline characteristics of included participants

Withdrawal from training for medical reasons

During the study there were 221 withdrawals from training that met the criteria for primary outcome events. These are described in Table 2. In addition, there were 68 withdrawals from training for medical reasons other than those qualifying for the primary endpoint in the study. These are described in Table 3.

Table 2
Qualifying reasons for primary endpoint withdrawal from service
Table 3
Medical reasons for withdrawal from active service other than primary endpoints

Saran versus shock absorbing insole

Primary outcome

Overall 72/401 participants (18.0%) allocated to Saran insoles were removed from training because of a qualifying lower limb injury, compared with 149/805 (18.5%) allocated to a SAI (Sorbothane or Poron), odds ratio 1.04 (95% CI 0.75 to 1.44; P=0.87). The difference in withdrawal per group was small in absolute terms, risk difference 70.6 (95% CI 75.1 to 4.2%), number needed to treat (NNT) with a SAI to avoid a withdrawal is NNT=174_harm (95% CI 20_harm to 24_benefit).

Affect of flight on primary comparison of Sorbothane and Poron versus Saran

The influence of flight membership on the outcome was examined using a logistic model in which flight was included as a classification variable. This provided similar results to the unstratified analysis: odds ratio 1.04 (95% CI 0.76 to 1.42; P=0.81).

Other non-qualifying withdrawals

Overall 14/401 participants (3.5%) allocated to Saran insoles withdrew from training for medical reasons other than qualifying lower limb injuries. These injuries included lower back pain, groin pain, hip pain and multiple conditions. By contrast, 54/804 (6.7%) withdrew for these reasons in the SAIs group, odds ratio 1.99 (1.07 to 3.93; P=0.024). In absolute terms, this represented 3.2% more withdrawals with Sorbothane or Poron compared with Saran (95% CI 0.5% to 5.7%), and number needed to treat with SAIs (NNT)=32_harm (18_harm to 196_harm).

Sorbothane versus Poron

Primary outcome

Overall 73/421 participants (17.3%) randomized to Sorbothane were removed from training because of a qualifying lower limb injury, compared with 76/383 for Poron (19.8%), odds ratio 0.85 (95% CI 0.58 to 1.23; P=0.37). The difference in withdrawal per group was small in absolute terms, risk difference for Poron minus Sorbothane is 72.5% (95% CI 2.9% to 77.9%). This equates to a number needed to treat with Sorbothane rather than Poron of 40_benefit (95% CI 35_harm to 13_benefit).

Affect of flight on primary comparison of Sorbothane versus Poron

The influence of flight membership on the outcome was examined using a logistic model in which flight was included as a classification variable. This provided similar results to the unstratified analysis: odds ratio 0.85 (95% CI 0.59 to 1.21; P=0.36)

Other non-qualifying withdrawals

Overall 26/421 (6.2%) participants randomized to Sorbothane withdrew from training for medical reasons other than qualifying lower limb injuries. By contrast 28/383 (7.3%) allocated to Poron withdrew for these reasons, odds ratio 0.85 (95% CI 0.47 to 1.52; P=0.58). In absolute terms, this represents 1.1% fewer withdrawals due to reasons other than primary outcome in the Sorbothane group (95% CI 72.4% to 4.8%).

COMMENT

In a large randomized trial, we sought evidence to support a policy decision to switch to the use of Sorbothane SAIs to avoid serious lower limb injuries necessitating withdrawal from training in physically active servicemen and women. Our study provided no evidence to support such a switch in policy. Indeed, it provided no evidence for the advantage of SAIs over standard issue Saran insoles. In addition, the precision of our results most likely excludes the possibility that clinically important differences in injury rates exist.

This trial shows that SAIs do not lessen the risk of lower limb injury for military personnel. This result may be generalizable to civilian groups involved in physically demanding exercise, such as police forces, firemen, nurses, dancers, amateur recreational sportsmen and women; and professional individual athletes and sporting teams at local, regional, national and international level. Despite mechanistic theory that SAIs may augment innate muscle tuning in reducing heel-strike impact forces and lessening intra-compartmental vibrations, both Sorbothane and Poron have failed to influence injury rates in a military setting.

Our study was a large-scale randomized trial, providing good evidence on the relative effectiveness of the insoles examined. The study was conducted in military recruits and the extent to which the findings may be directly extrapolated to other potential users of shock absorbing insoles is unclear. Given that we found no evidence to support the use of shock absorbing insoles, it may seem unlikely that benefits would be observed in other similarly active populations. However there may be situations outside the scope of our work in which this may be the case.

Such a result carries financial implications for the military. This trial provides no justification for the continued funding of disproportionately expensive SAIs over standard insoles. In the military setting, the additional expenditure conveys no benefit to training or operational output, and should not be sustained. Additional savings may be achieved by extending such a policy across the wider RAF, Army and Royal Navy.

Investment in further research into risk management and injury reduction within the military training environment is proposed to augment the evidence base against which to target resources cost-effectively.

Acknowledgments

acknowledgment This study was funded in part by the Defence Postgraduate Medical Deanery.

Notes

Competing interests The authors declare no conflicts of interest.

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Articles from Journal of the Royal Society of Medicine are provided here courtesy of Royal Society of Medicine Press