Participants
Older women (aged 65 or over) were recruited via completing a mail-out using a database of people who had been attending the La Trobe University Podiatry Clinic for treatment of foot problems. From the mail-out, candidates were screened through a telephone call, after the screening process, 20 participants were recruited. Eligible participants needed to be female, over 65 years of age, able to walk household distances (more than 50 m) without a walking aid, capable of understanding the English language in verbal and written form, and not have a neurodegenerative condition (e.g., Parkinson’s disease), lower limb amputation, or have undergone foot and ankle surgery in the previous three months. Ethical approval was granted from the La Trobe University Human Ethics Committee (HEC22227), and written informed consent was obtained from all participants. This study was conducted as part of a larger series of studies [12, 13].
An a priori sample size calculation (using G*Power version 3.1.9.4, Kiel, Germany) estimated that 19 participants were required to provide 80% power to detect a large effect size (d = 0.70) between the two footwear conditions, with statistical significance for hypothesis tests set at p < 0.05 (two-tailed). The large effect size was justified on the basis of our previous footwear and balance study identifying large effect sizes for the difference between step width and end sway when wearing the first prototype balance-enhancing shoes compared to flat flexible shoes [12].
Questionnaire and clinical assessment
A self-completion questionnaire was administered which included basic participant, demographic and medical history data (age, a checklist of common medical conditions and medication usage), falls history in the previous 12 months, fear of falling (using the Falls Efficacy Scale International [23]), general health (using the Short Form-12 Version 2 survey [24]) and physical activity (using the Incidental and Planned Exercise Questionnaire). The presence and severity of foot pain was documented using the Manchester Oxford Foot Questionnaire [25], using the total index, pain, walking / standing and social interaction scores.
Falls risk assessment
Risk of falling was evaluated using the validated QuickScreen© tool, which consists of eight measures: (i) previous falls, (ii) total medications, (iii) use of psychoactive medications, (iv) visual acuity (using a 10% low contrast letter chart), (v) touch sensation (using a Semmes-Weinstein-type pressure aesthesiometer applied to the lateral malleolus), (vi) the sit to stand test (using a 430 mm high chair without armrests, five times as fast as possible with arms folded), (vii) the near tandem stand test (eyes closed, with feet separated laterally by 25 mm and the heel of the front foot 25 mm anterior to the great toe of the back foot) and (viii) the alternate step test (alternatively placing the whole left and right feet as fast as possible onto a 190 mm high and 400 mm deep step eight times). Each of these measures was dichotomised using established cut-points [26].
Balance and walking stability assessment
We measured area (in millimetres) of postural sway and walking stability using a wearable sensor (dimensions: 50×70×20 mm; mass: 35 g; Gyko, Microgate, Bolzano, Italy) which was attached to participants at the level of the thoracic spine using a special harness and documented movements up to 16 g and angular velocities of up to 2000°/sec with an acquisition frequency of 1000 Hz. The reliability of the Gyko system has been previously reported [27, 28]. We measured bipedal standing (floor and foam [460×460×130 mm], eyes open and closed), near-tandem standing (feet separated laterally by 25 mm and the heel of the front foot 25 mm anterior to the great toe of the back foot with eyes open), and walking on a treadmill, flat surface and irregular surface (foam plates randomly placed covered with artificial grass) (see Fig. 1). For the postural sway tests, we recorded for 30 seconds. For the treadmill walking, speed was set at 4 km/hour, which is the average speed of a 60 + year-old woman [29]. However, we found that three women (15%) were unable to comfortably walk at this speed, so we tested them at 2.2 km/hour and 1.2 km/hour respectively (two out of the three participants were tested at 1.2 km/hour). Treadmill walking was recorded for 60 seconds, and we allowed participants to walk at their own speed for the flat and irregular surface, of which four trials were recorded of each over an eight metre distance.
Footwear conditions
Participants performed each of the balance and gait assessments when wearing supportive and minimalist footwear. The Brannock device® was used to determine the appropriate size for the participants [30]. Order of testing was randomised (using Microsoft Excel, Microsoft Corp, Washington, USA) to avoid order (i.e. habituation or fatigue) effects. Figure 2 shows key features of the supportive and minimalist footwear.
The supportive footwear was based on an existing model (Zeira, Munro Footwear Group, Abbotsford, Australia) and was manufactured by Able Health (Sydney, Australia). The footwear had a firm (Shore A hardness 55 [31]) rubber sole of 20 mm thickness under the heel and 10 mm under the forefoot, laces plus Velcro® fastening, and a firm heel counter. The weight of the supportive footwear was 313 to 342 gm across the size range. The outersole had a 10 degree bevel into the heel region [32, 33], grooves perpendicular to the sole (1.2 mm deep and 2.4 mm wide) across the heel surface area [34], and perpendicular grooves (5 mm deep and 12 mm wide) across the rest of the sole [35, 36]. A textured insole was also constructed from 4 mm thick ethyl vinyl acetate (Shore A 25 [31]) with dome-shaped projections (3 mm high and 8 mm diameter, Shore A 85 [31]) placed across the forefoot in a 15 mm diamond pattern and along the lateral border, extending to the heel. The textured insole was informed by previous studies reporting improvements on balance in older people when similar insoles were worn [12, 37, 38]. The shoes were similar to our previous study [12] but lacked the high ankle collar and were manufacturered with the aim to be more aesthetically pleasing than the first prototype.
The minimalist footwear (Kmart, Wesfarmers, Perth, Australia) had a canvas upper and rubber sole of uniform 10 mm heel and 5 mm forefoot outersole thickness and lace fixation, and a hardness of Shore A 35 [31]. The weight of the minimalist footwear was 191 to 258 gm across the size range. The minimalist footwear was chosen as a control condition as it had no features deemed to be either beneficial or detrimental to balance. The footwear met the criteria to be considered ‘minimalist’ outlined by the Esculier et al [39] study, namely that it provided “minimal interference with the natural movement of the foot due to its high flexibility, low heel to toe drop, weight and stack height, and the absence of motion control and stability devices”.
Footwear assessment
After balance and walking assessment, both types of footwear were assessed using questions selected from the Monitor Orthopaedic Shoes questionnaire [40] scored on a 100 mm visual analog scale. The selected questions were: (i) please mark on the following line how attractive you think the shoes are (with the anchors “extremely unattractive” and “extremely attractive”), (ii) please mark on the following line how attractive you think other people would think the shoes are (with the anchors “extremely unattractive” and “extremely attractive”), (iii) please mark on the following line how comfortable you think the shoes are (with the anchors “extremely uncomfortable” and “extremely comfortable)”, (iv) please mark on the following line how well you think the shoes fit you (using the anchors “poorest fit possible” and “best fit possible”), (v) please indicate how easy it is for you to don the shoes on and off (using the anchors “most difficult as possible” and “as easy as imaginable”) and (vi) please indicate how heavy the shoes are (using the anchors “extremely light” and “extremely heavy”). Participants were also asked whether they felt more balanced in the supportive footwear (on a 100 mm visual analog scale), would consider wearing them if they found to be beneficial for balance (with the options yes, no, or maybe), and whether the design could be improved (open-ended response).
Footwear comfort for both types of footwear was also assessed using the comfort scale described by Műndermann et al [41] which enables the documentation of footwear comfort both overall and specific to heel cushioning, forefoot cushioning, medio-lateral control, arch height, heel cup fit, shoe heel width, shoe forefoot width, and shoe length. Participants were asked to rate the footwear on a 100 mm visual analog scale using the anchors “not comfortable at all” and “most comfortable condition imaginable”.
Statistical analysis
Statistical analysis was undertaken using SPSS Version 29.0 (IBM, Armonk, NY, USA). Differences between the two footwear conditions (supportive footwear and minimalist footwear) were evaluated using independent samples t-tests. Level of significance was set at 0.05. Effect sizes for between-group comparisons were calculated using Cohen’s d, and were interpreted as follows: ≤0.01 = very small, > 0.01 to 0.20 = small, > 0.20 to 0.50 = medium, > 0.50 to 0.8 = large, > 0.80 to 1.2 = very large, and > 1.20 = huge [42].