Therefore, in this study, we (1) investigated whether running shoes equipped with motion control features modified injury risk in regular leisure-time runners and (2) if this influence depended on foot morphology. Our main hypothesis was that injury risk would be different when running in shoes with motion control compared with standard (neutral) shoes, while controlling for the potential confounders. Our secondary hypothesis was that the difference in injury risk would depend on foot morphology.

Foot posture is believed by some to be a risk factor for injury. 5 This often results in attempts to match footwear to a runner's foot morphology, despite an absence of evidence to suggest this approach will reduce injuries. 4–6 Specifically, motion control shoes are typically prescribed to runners with pronated feet, while neutral stability shoes are recommended to individuals with neutral feet and cushioned shoes to those with supinated feet. Several studies have been unable to demonstrate the benefit of the above-described prescription strategy, 7–10 although they were either inadequately powered, 9 or performed in a military population, 7 , 8 limiting the applicability of findings to distance runners. 4 Furthermore, a more pronated foot posture was reported not to be associated with injury risk in a prospective cohort study on over 900 novice runners, 11 questioning the use of running shoes featuring motion control systems designed to reduce foot pronation. Running in shoes equipped with that technology increased the risk of experiencing running-related pain. 9 Although these results are preliminary, it is worrying that a significant proportion of running shoes have motion control, a fact that many runners may not be aware of when buying their shoes.

Several hundred running shoe models are currently available in the market. Notwithstanding the increasing focus on running shoe design, technologies and function, running-related injury incidence has not changed noticeably over the last few decades. 1 Various biomechanical variables such as strike pattern, 2 impact forces, 3 foot posture or foot pronation, 4 , 5 have all been proposed as injury risk factors. Footwear features such as cushioning technology, stability and motion control systems (motion control shoes), have been designed to mitigate against these risk factors and are extensively used as selling points by shoe manufacturers.

Materials and methods

Participants and study design This randomised controlled trial (unregistered) recruited recreational runners, regardless of fitness level. Reporting of the study followed the CONSORT statement.12 Given an expected injury rate of 22% and 35% in the two groups,9 respectively, and a desired power of 0.8 and an α-level of 0.05, a total of 364 runners were required to test our main hypothesis. All participants received a full description of the study protocol and provided written informed consent for participation. All procedures were approved by the National Ethics Committee for Research (ref 201211/04). Participants were recruited via advertisements in local newspapers and on specialised internet sites from March to April 2014. Following online registration, participants were contacted by phone to verify inclusion criteria: healthy, aged 18–65 years, regular running (at least 1 session/week) for at least 6 months over the 12 months prior to the study, no contraindication to perform running activity, no prior (<12 months) surgery at the lower limbs or lower back region, and no use of orthopaedic insoles for running activities. Volunteers were also required to perform at least one running activity per week during the 6-month follow-up period (from June to December), to use the provided study shoes for all running activities, and to report, at least once per week, all sports activities and injury or pain experienced during the follow-up. Individuals reported to the laboratory for eligibility check and baseline assessment. A questionnaire gathered information about age, sex, running regularity over the previous 12 months (months of practice), running experience (years of regular practice) and previous injury to the lower back or lower limbs preventing normal running activity during the preceding 12 months. Foot posture was assessed using the six-item Foot Posture Index (FPI), previously proven to be valid.13 ,14 Normative values presented by Redmond et al15 were used as references to categorise each foot into one of the five categories (highly supinated, supinated, neutral, pronated and highly pronated) based on their FPI score. Two previously trained appraisers performed all evaluations and assessed the first 50 participants together to optimise consistency. Subsequently, high inter-rater agreement for classifying the runners in one of the five categories was found based on a further 40 participants, with a Cohen's κ coefficient of 0.86. Since the unit of analysis is the participant, the classification into one of the five categories was based on the foot with the most extreme score.

Study shoes characteristics Two versions (motion control and standard) of a running shoe model were provided for the trial by a renowned sport equipment manufacturer. Shoes were de-identified so the study participants did not know which brand they were given. The motion control shoes and standard shoes both had a heel-to-toe drop of 10 mm and were derived from a commercially available model. Motion control shoes were characterised by (1) a thermoplastic polyurethane structure located at the medial part of the midfoot and (2) a dual-density ethyl-vinyl-acetate (EVA) midsole located at the forefoot (figure 1). Apart from these features, the two versions were identical, so participants did not know which type of shoe they used. Figure 1 Illustration of the two technical features (coloured in black for illustration purposes) designed to limit the pronation movement of the runners. (A) Represents a piece of rigid plastic (thermoplastic polyurethane) located on the medial side, under the midfoot at the midsole edge. (B) Area of the harder midsole EVA (ethylene-vinyl acetate) foam. These elements were not recognisable on the shoe version distributed. A subset of each version was characterised regarding midsole hardness difference between the medial and the lateral part of the midsole, using an Asker-C durometer according to the standard JIS K 7312 protocol for hardness characterisation of viscoelastic polymers (average value of 5 independent measures/shoe; 12 shoes tested/model). The measure was taken perpendicularly to the frontal plan, with the shoes cut at the level of the first metatarsal head. Participants were randomly allocated to one of the two shoe models in accordance with stratification by potential confounders (age and body mass index (BMI); cut-off values are medians)16 and foot morphology. Participants and assessors involved in the shoe distribution and participant follow-up were both blinded regarding the shoe allocation. Each shoe pair was coded by a coworker not involved in the study prior to the distribution. The code was broken after completion of data collection.

Data collection during follow-up A dedicated internet-based platform (http://www.tipps.lu, Training and Injury Prevention Platform for Sports) was used to collect all information of the participants pertaining to their sports participation and any adverse events (injuries, pains and illnesses).17–19 Training sessions were characterised by the type of activity, context, duration, subjectively perceived intensity measured using the Borg CR-10 scale,20 distance covered, running surface and shoes worn. Whether participants experienced any pain during the session forcing him/her to reduce volume or intensity, or to interrupt the practice, was also reported. The response to each item was mandatory for every declared training session or competition and could be selected from a predefined list. Injury was self-reported and defined as any physical pain located at the lower limbs or lower back region, sustained during or as a result of running practice, and impeding planned running activity for at least 1 day (time-loss definition). The online injury questionnaire has been previously described.21 Injuries were classified according to consensus guidelines on sports injury surveillance studies.22 ,23 Self-reported data on every injury were systematically checked by the principle investigator for completeness and coherence. Participants who did not complete their entire running calendar with weekly information were contacted by one of the investigators to ensure that injury was not the reason for non-compliance. A participant was considered as dropping out of the study when no data had been uploaded in the system for more than 2 weeks despite an automatic email reminder and a phone call from the research team. At the end of the study (December 2014), the participants were invited for a final visit to check all injury data, compliance and shoe use.