Control conditions for footwear insole and orthotic research
Introduction
Footwear insoles/orthotics are commonly utilized as an intervention to prevent, treat or manage a variety of musculoskeletal disorders [1], [2], [3]. Typically, the desired objective of the insole is to modify an individual's gait so as to alter kinetic variables that are believed to be injurious. For example, in osteoarthritis research, an investigator may attempt to reduce the peak external knee adduction moment during gait [4], [5], [6]; in patellofemoral pain research, it may be desirable to modify the external knee adduction (or internal knee abduction) angular impulse [7]; for ankle sprains, the ankle inversion moment might be reduced [8]; and for tibial stress syndrome, a reduced maximum vertical ground reaction force loading rate may be desired [9]. Each of these kinetic variables during gait can be modified by specialized footwear insoles or orthotics [6], [10], [11], [12].
Clinically, the effectiveness of footwear insoles or orthotics generally show mixed results, where some trials show pain/symptom improvement and some trials do not [13], [14]. In one recent systematic review on insoles for knee osteoarthritis, it was proposed that a potential reason as to why trials show differing clinical results is that different control conditions are often used across studies [15]. Specifically, in trials that used a flat insole as a control condition, no clinical benefit of specialized insoles or orthotics was observed; however, trials that used the participant's own shoe as a control condition did find clinical benefits. The authors assumed that flat insoles are biomechanically inert, in the way that a sugar pill might be chemically inert for a drug trial, and therefore speculated that the discrepancy in clinical effectiveness between studies could be attributed to placebo effects in studies that did not use a flat control insole.
While flat insoles and the participant's own shoe remain popular choices for control conditions in insole and orthotic research, researchers have also used a standardized shoe across participants, or a standardized shoe across participants with the addition of a flat insole [5], [10], [12]. Clearly, a number of options exist for a control condition, yet there does not appear to be a consensus on the most appropriate option for clinically-oriented biomechanics research.
Recently, in response to the systematic review by Parkes et al. [14], an alternate explanation as to why clinical benefit depended on control condition was proposed: that the flat insole control condition was in fact not biomechanically inert [16]. Or in other words, it is possible that flat insole conditions actually cause changes to participant's biomechanics such that they induce clinical responses in control groups, making the detection of differences between control and experimental groups difficult. Surprisingly little research is available on footwear control conditions to support or refute this theory. In one study by McCormick et al. [17], it was shown that plantar pressure distributions beneath the foot are altered during gait with the addition of a flat control insole, lending support to the theory that perhaps the flat control insert and other control conditions alter the individual's biomechanics relative to what they normally experience with their own shoes. This is important, since the goal of a biomechanical control condition in clinical research should be to ensure the individual's biomechanics are unmodified relative to what they normally experience to prevent a clinical response from occurring.
The purpose of the present study was to conduct a comparison of commonly utilized control conditions in the footwear insole and orthotic literature to identify if kinetic variables at the ground, ankle and knee that are associated with musculoskeletal injury, are altered relative to what a participant would normally experience while wearing their own shoe. These footwear conditions included the participant's own shoe, the participant's own shoe with a flat insole, a standardized shoe, and a standardized shoe with a flat insole. Given previous research found flat insoles can modify plantar pressure distributions [17], it was hypothesized that peak external knee adduction moments, external knee adduction angular impulses, peak internal ankle inversion moments, and maximum vertical loading rates – variables associated with musculoskeletal injury that are modifiable through insole interventions [4], [7], [8], [9]–would be significantly altered in each of the proposed control conditions relative to the participant's own shoe.
Section snippets
Participants
A sample of 15 healthy individuals were recruited to participate in this exploratory study (8 male, 7 female, mean age of 22 (SD 1.7) years, height of 176.4 (SD 10.7) cm, and mass of 73.6 (SD 11.5) kg). All participants were free from any lower extremity pain or injury at the time of data collection, and all participants gave written informed consent prior to data collection, in accordance with the University's health research ethics board. While this was a younger, healthy population, an older
Results
Percent changes to each biomechanical variable are shown for each participant during walking and running in Fig. 2, Fig. 3, respectively. As can be seen, the percent change directions and magnitudes differed dramatically across participants and footwear conditions. These data are shown in Table format in the Supplementary File.
In the Supplementary File, Tables can be found that show the mean inter-trial variability for each biomechanical variable and participant. These also show the cutoff
Discussion
The purpose of this study was to identify the most suitable control condition, from a biomechanical perspective, for use in future randomized controlled trials. It was hypothesized that biomechanics would be significantly altered in each of the conditions relative to the participant's own shoe. This hypothesis was supported since there was a significantly larger proportion of individuals than the expected proportion of 20% who experienced a relevant biomechanical change for many of the footwear
Conclusion
This study evaluated four different commonly used control conditions for insole and orthotic studies, and found that while consistent directional differences across conditions were not observed, a large proportion of individuals experienced biomechanically relevant changes relative to what they would normally experience in their own shoe. Based on these findings, it is recommended that the individual's own shoe be utilized for future insole studies as the control condition where biomechanical
Contributions
RTL, JTW and DJS conceived of the study, and the design was established by RTL. RTL prepared control interventions, recruited participants, collected data, analyzed the data, and drafted the manuscript. JTW and DJS critically reviewed the manuscript and all authors approved the final version. RTL takes full accountability for the article.
Acknowledgements
RTL was funded by scholarships from the a Vanier Canada Graduate Scholarship Program at the Canadian Institutes of Health Research, the Natural Sciences & Engineering Research Council of Canada CREATE Program, the Killam Trusts, and the Alberta Innovates Health Solutions MD-PhD Studentship Program. Funding agencies played no role in study design, data analysis or writing of this manuscript.
Conflicts of interest
The authors declare no conflicts of interest.
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