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Limits of Stability and Adaptation to Wearing Rocker Bottom Shoes Edgar Ramos Vieira, Gerardo Guerrero, Daniel Holt, Monica Arreaza, Valentina Veroes and Denis Brunt Foot Ankle Int published online 7 April 2014 DOI: 10.1177/1071100714531227 The online version of this article can be found at: http://fai.sagepub.com/content/early/2014/04/07/1071100714531227

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FAIXXX10.1177/1071100714531227Foot & Ankle InternationalVieira et al

Clinical Research Article

Limits of Stability and Adaptation to Wearing Rocker Bottom Shoes

Foot & Ankle International® 1­–5 © The Author(s) 2014 Reprints and permissions: sagepub.com/journalsPermissions.nav DOI: 10.1177/1071100714531227 fai.sagepub.com

Edgar Ramos Vieira, PT, PhD1, Gerardo Guerrero, BSHS1, Daniel Holt, MS, ATC1, Monica Arreaza, BS, DPT1, Valentina Veroes, BS, DPT1, and Denis Brunt, PT, EdD1 Abstract Background: Stability and balance are fundamental during static and dynamic activities. The effects of wearing rocker bottom sole (RBS) shoes on the limits of stability (LOS) and adaptation to wearing RBS shoes need to be investigated. The objectives of this study were to evaluate the LOS when wearing RBS shoes, and to evaluate if people improve their stability while wearing RBS shoes over time. Methods: Eleven female subjects with no lower extremity impairments participated in the study. The LOS were tested at baseline and weeks 3 and 6 using a Neurocom SMART EquiTest equipment. Center of pressure (CoP) was determined using force plates, and the center of gravity (CoG) position was estimated from the CoP measures and subjects’ anthropometry. Subjects performed a series of tasks that involved leaning in different directions so as to move the vertical projection of their CoG. End-point excursions of the CoG floor projection were calculated as a percentage of the distance between the starting position and the target. Considering the body as an inverted pendulum, we recorded the average angular velocity of the inverted pendulum during the movements and quantified directional control as a percentage of movement toward versus away from the target. Shoe types were compared using paired t tests, and sessions were compared using repeated measures ANOVA. Results: The angular velocities of the inverted pendulum (ie, CoG velocity) were not significantly different between shoe conditions in the front and back directions at baseline (4 ± 3 with RBS vs 5 ± 2 deg/sec with regular shoes, and 4 ± 1 vs 6 ± 4 deg/sec). Front directional control of the CoG was significantly worse with RBS shoes at weeks 3 and 6 (P < .015). Front end-point excursions were also lower with RBS shoes both at baseline and week 6 (P < .014). There were no significant changes over time. Conclusion: The findings indicate that the LOS were negatively affected by wearing RBS shoes and that people do not improve their stability while wearing these shoes even after a 6-week period of use. Clinical Relevance: This study shows that wearing RBS shoes increase instability and the instability remains even after wearing these shoes for six weeks. Keywords: balance, shoes, adaptation, limits of stability Balance, posture, and stability are needed for standing and locomotion. Footwear sole geometry can influence these variables.4 Rocker bottom sole (RBS) shoes are unstable in the anterior-posterior direction and can increase postural sway, requiring adaptations of the ankle muscles’ activation patterns to maintain balance and control stability.4,9,13 Postural sway and the electromyographic activity of the tibialis anterior, peroneus longus, and vastus lateralis are higher when wearing RBS shoes compared to standard control shoes.1 Based on a review of the evidence in the literature, Nigg et al found that wearing unstable shoes (eg, RBS shoes) increases instability during standing and walking, and its use is associated with increased activity of the small ankle muscles, improved static balance in users whose balance skills are low, reduced forces in the lower extremity joints, and decreased knee and low back pain levels.11

Nigg et al found a reduction in lower limb pain after wearing RBS shoes due to decreased dorsi- and plantarflexion forces and joint loading.10 Landry et al found decreased hip flexion/extension and ankle inversion/eversion, and increased early stance dorsiflexion and internal rotation of the knee when wearing RBS shoes versus a stable control shoe.5 In addition, Ramstrand et al examined the effects of using RBS shoes on balance and stability of 31 women 1

Department of Physical Therapy, Florida International University, Miami, FL, USA Corresponding Author: Edgar Ramos Vieira, PT, PhD, Department of Physical Therapy, Florida International University, AHC3-430, 11200 SW 8th St, Miami, FL 33199, USA. Email: [email protected]

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more than 50 years old and found a small amount of balance improvement.12 A literature review of the physiological benefits associated with wearing RBS shoes found studies reporting increased lower limb muscle activity, reflex excitability, and increased venous return. The potential concerns mentioned in the review included the increased risk of fatigue and increased forefoot loading; the review also mentioned that further studies on the benefits and risks of wearing RBS shoes are required.7 RBS shoes have generated controversy as to whether or not the claims of improved posture, and “shaping” musculature are true. In May 2012, the US Federal Trade Commission sued a company producing an RBS shoe that claimed to increase lower extremity strength, weight loss, and muscle activation.2 The company had to pay $40 million to settle the charges that it deceived consumers with ads for “toning shoes.”2 RBS shoes result in an unstable foot-to-floor interface and may help train postural control and decrease pain. However, the effects of RBS shoes on the limits of stability (LOS) when wearing the shoes and potential adaptations over time to wearing them are not well understood. RBS shoes are becoming popular among young and older adults. People seem to want to try these shoes based on anecdotal reports of health and aesthetic benefits (eg, reduced lower limb pain, improved balance, and toned muscles). However, potential risks (eg, decreased stability that may increase the risk of falls) need to be investigated before any recommendation to wear or “prescribe” these shoes can be made to healthy or subjects with balance and/or foot and ankle problems. Therefore, the objectives of this study were to compare the LOS of subjects wearing RBS shoes versus regular shoes, and to evaluate if subjects improve their stability while wearing RBS shoes over time. It was hypothesized that the subjects would present decreased LOS when wearing the RBS shoes at baseline in comparison to when wearing regular shoes, and that the subjects would adapt over time to wearing RBS shoes so that the differences in LOS would no longer be significant after 6 weeks.

Methods Subjects Subjects with no lower extremity injuries, pain or balance disorders, who could partake in physical activity based on the Physical Activity Readiness Questionnaire,14 and that had not previously worn any type of RBS shoe were eligible to participate. Eleven volunteer female university students participated in the study after signing an informed consent form approved by the institutional review board. We included only female subjects on this initial study to reduce the potential risk of having sex differences as a confounder. Subjects’ characteristics (mean ± SD) were as

follows: age = 27 ± 5 years, height = 1.64 ± 0.04 m, mass = 60 ± 5 kg, and shoe size = 8 ± 0.3.

Design A longitudinal study was conducted with repeated LOS measures with RBS shoes and with regular shoes at baseline (first time ever wearing RBS shoes) and at weeks 3 and 6 from baseline (after starting to wear RBS shoes regularly). The RBS shoes used were from the Therashoes® (TheraShoe, Lake Worth, FL, USA) brand and the non-RBS tennis shoes that the subjects used on a regular basis constituted “regular shoes.” The participants were asked to wear the RBS shoes for 2 hours on the first day after the baseline testing, 3 hours on the second day after the baseline testing, 4 hours on the third day, and 6 hours per day after that until the completion of 6 weeks after the baseline testing. The testing order of RBS/regular shoes and testing conditions (LOS testing directions) were randomized.

Equipment and Measures The tests were performed with the subjects standing on a NeuroCom SMART EquiTest equipment with the software version 8.3.3. Center of pressure (CoP) position was determined based on the vertical ground-reaction forces measured using the NeuroCom dynamic force plates, and the center of gravity (CoG) position was estimated based on the CoP measures and subjects’ anthropometry.8 The CoG position was estimated by applying a moving average finiteduration impulse response filter to the CoP data with the movement about the ankle functioning as an inverted pendulum.8 Considering the starting point (neutral position) as a vertical (0 degrees) line between the CoG position and its floor projection, the theoretical anterior LOS was 6.25 degrees, the posterior LOS was 4.45 degrees, and the lateral LOS was 8 degrees to each side. The outcome measures were front and back end point excursions, CoG movement velocity, and CoG directional control at baseline and weeks 3 and 6. End-point excursions of the floor projection of the CoG were calculated as a percentage of the distance between the starting position and the target (theoretical LOS) (Figure 1). The average velocity of the inverted pendulum (CoG movement in relation to CoP) to get to the end point excursion was calculated in degrees per second (Figure 2), and directional control as a percentage of movement toward the target versus movement away from the target.8

Procedures All subjects were instructed to stand on the force plate with standardized foot placement: lateral border of the calcaneus aligned with anteroposterior lines and malleoli aligned with

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Figure 1.  Illustration of the trajectories of the center of gravity (CoG) projections during tests requiring to move the CoG from a central square representing the CoG position during neutral standing posture to target squares representing the theoretical limits of stability on different directions. Figure 3.  Standardized foot placement: lateral border of the calcaneus aligned with anteroposterior lines and malleoli aligned with lateromedial lines on the force plate.

lateromedial lines on the force plate (Figure 3). The test consisted of the subject maintaining the floor projection of her CoG in the middle of their base of support initially (starting position), then displacing it as quickly and accurately as possible toward 8 subsequent randomized targets in different directions (front, front right, front left, right, left, back, back right, and back left) without taking a step.8 The verbal instructions provided to the participants were “First, keep the marker on the center square. When you hear the beep, move the marker as quickly and accurately as possible toward the target that will be indicated on the screen, using your ankles, not your hip, and without taking a step.” A computer monitor displayed continuous visual feedback on the floor projection of her CoG position (“marker”). The subjects were instructed to initiate moving their CoG toward the targets (specific spots on the monitor) as soon as they heard an audio cue, and stop once the target was reached. A safety harness was used during testing to prevent injuries in the case of a fall. Each trial (directional testing) lasted 8 seconds, and the subjects performed 3 familiarization trials at baseline.

Data Analysis

Figure 2.  Illustration of a sagittal view of the anterior limits of stability, including ground reaction force vectors representing an inverted pendulum, and the angle (15 degrees) between the starting neutral position at 0.5 seconds and final position at 1.5 seconds (angular velocity = 15 degrees per second).

The data related to the front targets (front, front left, and front right) and back targets (back, back left, and back right) were grouped as compound front and back directions. Grouping the directions did not change the behavior of the data. Descriptive statistics were calculated. Paired t tests were used to compare performance with the 2 shoe types (RBS vs regular shoes). Repeated measures ANOVA was

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Discussion

Figure 4.  Front directional control (DC%) at baseline and at weeks 3 and 6.

Figure 5.  Front end point excursion (EPE%) at baseline and at weeks 3 and 6.

used to compare across time between sessions (baseline vs week 3 vs week 6). All tests were conducted using SPSS.

Results The angular velocities of the inverted pendulum (ie, CoG velocity) were not significantly different between shoe conditions in the front and back directions at baseline (4 ± 3 with RBS vs 5 ± 2 deg/sec with regular shoes, and 4 ± 1 vs 6 ± 4 deg/sec). Subjects exhibited decreased directional control when wearing RBS shoes at week 3 (P = .013) and 6 (P = .014) (Figure 4). For back directional control, the difference between the RBS shoes (69.1 ± 12.6%) and the regular shoes (72.0 ± 7.9%) was not significant (P = .225). Front end point excursions (Figure 5) were lower with the RBS shoes compared with the regular shoes at baseline and week 6 (P < .014). We found no significant decrease in front end point excursions with the regular shoes over time (P = .182). There were no statistically significant changes in directional control or in end point excursion over time among the testing sessions (baseline vs week 3 vs week 6).

We found that the LOS were negatively affected by wearing RBS shoes and that the subjects did not adapt/improve their LOS after 6 weeks of wearing RBS shoes. We think that it is unlikely that the stability reduction as a result of wearing RBS shoes would significantly increase the risk of falls among healthy young subjects performing simple tasks. However, the risk of falls may become more important among people with balance and/or foot and ankle problems and when performing tasks that challenge balance. A previous study using the NeuroCom found that older adult fallers had lower sensory organization test scores than nonfallers, and the sensory organization test scores of the fallers were highly correlated with their anterior LOS (r = .79, P = .006). Unfortunately the authors did not report the LOS values or the magnitude of the difference between fallers and nonfallers to allow a direct comparison with our findings.15 Another study found that a logistic regression model combining the LOS and CoG velocity were predictive of falls in women with multiple sclerosis (OR = 1.62, 95% CI 1.04-2.53, sensitivity = 65%, specificity = 73%).3 Therefore, increased risk of falls needs to be considered when deciding to wear or “prescribing” such shoes because it reduces stability, especially when considering populations at risk for falls.6 One example of where our findings may be particularly relevant is among hospital nurses. Considering that nurses often assist patients during transfers, wearing RBS shoes could represent a risk to both patients and staff. Even though we have no data to support this information, we heard and have observed that nurses often wear RBS shoes. We do not know if these shoes are specifically marketed to nurses. However, based on our observation of frequent use by nurses and the potential implications for safety, further investigation is required to evaluate potential risks. One of the limitations of our study is the small sample size and subject characteristics (young and healthy female participants). Studies using larger sample sizes and with different populations (eg, males, older adults) are recommended, and additional studies are necessary to further determine how wearing RBS shoes affects balance and stability during functional tasks. A previous study that compared balance and LOS with regular shoes before and after a 8-week period of wearing an RBS shoe as an intervention found no changes in LOS but improved directional control.12 We did not find changes over time in directional control with the regular shoes. The differences between our findings and the findings of Ramstrand et al12 may be due to the fact that they used a 8-week period while we used a 6-week period. In addition, they evaluated older women (>50 years old) while we evaluated younger women (

Limits of Stability and Adaptation to Wearing Rocker Bottom Shoes.

Stability and balance are fundamental during static and dynamic activities. The effects of wearing rocker bottom sole (RBS) shoes on the limits of sta...
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