Effect of Rocker-Soled Shoes on Parameters of Knee Joint Load in Knee Osteoarthritis ELIZABETH G. MADDEN1,2, CRYSTAL O. KEAN1,3, TIM V. WRIGLEY1, KIM L. BENNELL1, and RANA S. HINMAN1 1 Centre for Health, Exercise and Sports Medicine, Department of Physiotherapy, Melbourne School of Health Sciences, University of Melbourne, Melbourne, Victoria, AUSTRALIA; 2School of Human, Health, and Social Sciences, Central Queensland University, Rockhampton, Queensland, AUSTRALIA; and 3School of Medical and Applied Sciences, Central Queensland University, Rockhampton, Queensland, AUSTRALIA
ABSTRACT
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MADDEN, E. G., C. O. KEAN, T. V. WRIGLEY, K. L. BENNELL, and R. S. HINMAN. Effect of Rocker-Soled Shoes on Parameters of Knee Joint Load in Knee Osteoarthritis. Med. Sci. Sports Exerc., Vol. 47, No. 1, pp. 128–135, 2015. Purpose: This study evaluated the immediate effects of rocker-soled shoes on parameters of the knee adduction moment (KAM) and pain in individuals with knee osteoarthritis (OA). Methods: Three-dimensional gait analysis was performed on 30 individuals (mean (SD): age, 61 (7) yr; 15 (50%) male) with radiographic and symptomatic knee OA under three walking conditions in a randomized order: i) wearing rocker-soled shoes (Skechers Shape-ups), ii) wearing non–rocker-soled shoes (ASICS walking shoes), and iii) barefoot. Peak KAM and KAM angular impulse were measured as primary indicators of knee load distribution. Secondary measures included the knee flexion moment (KFM) and knee pain during walking. Results: Peak KAM was significantly lower when wearing the rocker-soled shoes compared with that when wearing the non–rocker-soled shoes (mean difference (95% confidence interval), j0.27 (j0.42 to j0.12) NIm/BW Ht%; P G 0.001). Post hoc tests revealed no significant difference in KAM impulse between rocker-soled and non–rocker-soled shoe conditions (P = 0.13). Both peak KAM and KAM impulse were significantly higher during both shoe conditions compared with those during the barefoot condition (P G 0.001). There were no significant differences in KFM (P = 0.36) or knee pain (P = 0.89) between conditions. Conclusions: Rocker-soled shoes significantly reduced peak KAM when compared with non–rocker-soled shoes, without a concomitant change in KFM, and thus may potentially reduce medial knee joint loading. However, KAM parameters in the rocker-soled shoes remained significantly higher than those during barefoot walking. Wearing rocker-soled shoes did not have a significant immediate effect on walking pain. Further research is required to evaluate whether rocker-soled shoes can influence symptoms and progression of knee OA with prolonged wear. Key Words: SHOES, GAIT ANALYSIS, KNEE ADDUCTION MOMENT, KNEE, OSTEOARTHRITIS
O
Excessive knee loading plays an important role in OA pathogenesis and as such represents a potential target for treatment. The external knee adduction moment (KAM) is considered a valid and reliable indirect biomechanical marker of tibiofemoral compartment load distribution, such that a higher KAM indicates greater load in the medial compartment (36). Peak KAM has been shown to predict progression of radiographic disease and pain severity in people with knee OA (26) and has also been strongly correlated with pain and functional impairment during single-limb stance (18). A higher KAM angular impulse (a measure of the mean magnitude and duration of KAM) has been associated with greater loss of medial tibial cartilage in people with knee OA (2) and also increased knee pain independent of disease severity (19). Clinical guidelines for knee OA recommend that clinicians provide advice to patients regarding optimal footwear choices (8), given that wearing shoes significantly increases parameters of knee joint load compared with barefoot walking. However, there is little research evidence demonstrating which shoe types are best for those with knee OA. We have previously shown an average increase of 7.4% in peak KAM when people with knee OA walked in their own footwear compared with walking barefoot (14). Considerable individual variation was observed (with increases of up to 30%),
steoarthritis (OA) is the most common form of arthritis, and it frequently occurs in weight-bearing joints, particularly the knee. It is a leading cause of pain and impairment in older adults (11), with affected individuals reporting difficulty with many activities of daily living such as walking and stair climbing. Currently, there is no cure for OA, thus, determining effective conservative treatments to provide symptomatic relief and potentially slow disease progression is an important public health objective. Clinical guidelines (8) recommend conservative nondrug strategies, including advice about appropriate footwear, as the cornerstone of knee OA management.
Address for correspondence: Elizabeth Madden, M.Hlth.Sc., Department of Physiotherapy, Melbourne School of Health Sciences, University of Melbourne, Alan Gilbert Building 161 Barry St., Carlton Victoria 3053, Australia; E-mail: [email protected]. Submitted for publication October 2013. Accepted for publication April 2014. 0195-9131/15/4701-0128/0 MEDICINE & SCIENCE IN SPORTS & EXERCISEÒ Copyright Ó 2014 by the American College of Sports Medicine DOI: 10.1249/MSS.0000000000000384
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METHODS Thirty participants with radiographic and symptomatic knee OA were recruited as a convenience sample from a larger concurrent study within the Centre for Health, Exercise, and Sports Medicine at the University of Melbourne (3). All participants were Q40 yr of age, exhibited radiographic
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evidence of osteophytes in the tibiofemoral joint (Kellgren and Lawrence (KL) (13) grade, Q2), reported an average knee pain severity during the past week of Q4 on an 11-point numerical rating scale (0, no pain; 10, worst pain possible), and reported knee pain on most days of the previous month. Exclusion criteria included body mass index (BMI) Q36 kgImj2 (due to three-dimensional gait analysis difficulties), history of lower limb joint replacement, spinal surgery, or high tibial osteotomy, knee surgery or injection in the previous 6 months, history of knee fracture or malignancy, back pain 940 out of a 100-point pain scale, use of a gait aid, diagnosis of inflammatory joint or skin conditions (such as gout, fibromyalgia, or rheumatoid arthritis), history of stroke or neurological disorder (such polio or multiple sclerosis) or any peripheral nerve condition or neuropathy, and/or an inability to understand written or spoken English. All participants provided a informed written consent, and ethical approval was granted by the University of Melbourne human research ethics committee. Descriptive information. Demographic data included participant descriptors such as age, height, weight, and BMI. All participants underwent bilateral, semiflexed, weightbearing, posteroanterior knee radiographs to determine disease severity according to the KL scale and degree of joint space narrowing (JSN). The Western Ontario and McMaster Universities Osteoarthritis Index, a self-report questionnaire, was used to assess knee pain and functional ability for descriptive purposes. This is a valid and reliable disease-specific tool consisting of three subscales (pain, stiffness, and physical function). Each question is answered using a five-point Likert scale (with responses ranging from ‘‘none’’ to ‘‘extreme’’). Higher scores on each subscale indicate worse pain, stiffness, or physical dysfunction. Shoes. The rocker-soled shoes tested were the Skechers Shape-up Motivation (men) (Fig. 1A) and the Skechers Shape-up Fitness Junkie (women) shoe (Fig. 1B), which are relatively inexpensive and popular shoes easily available in Australia. The upper material of the shoe consisted of suede for the men’s shoe and synthetic for the women’s. Because specific shoe design features (such as heel height, arch supports, etc.) can influence KAM (38), we chose to use a standardized shoe as a control condition (rather than the individual’s own usual footwear) to minimize confounding effects. The non–rocker-soled shoes tested were standard walking shoes with a traditional (nonrocker) sole (ASICS Oceania Pty. Ltd.) (Fig. 1C and D). These shoes consisted of a leather upper and had no specific motion control properties either in sole density, upper construction, or last shape. Sole height was measured with a vernier caliper, and sole hardness was measured using a Shore A durometer, which is commonly used to determine hardness of materials such as vulcanized rubber and plastic products. Sole height in the rocker-soled shoe was approximately 56 mm at the rear foot and 35 mm at the forefoot for the men’s shoe and 46 mm at the rear foot and 35 mm at the forefoot for the women’s shoe. For both men’s and women’s rocker-soled shoes, the sole hardness
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suggesting that individual shoe features play a role in influencing KAM. In particular, shoes with greater heel heights and more rigid designs are associated with increases in KAM (15,17,37). Several recent studies have shown that innovative, specially designed footwear is an effective and pragmatic approach to reduce KAM parameters (3,37), however, there is limited research evaluating the effects of popular commercially available shoe styles. For health professionals to appropriately advise patients with knee OA about which shoe styles to wear (and conversely, which to avoid), further research is needed. Rocker-soled shoes are designed with an increase in sagittal plane sole convex curvature and reduced sole density in an attempt to mimic barefoot walking by creating an unstable platform (20,30). The exercise footwear market has heavily promoted rocker-soled shoes as a useful strategy to enhance exercise and weight loss by encouraging increased muscle activity to counteract the instability caused by the shoe. Accordingly, these shoes are increasing in popularity in the community because of their perceived health and fitness benefits. Although this style of shoe is manufactured by several different footwear companies, the limited available research predominantly evaluates the Masai Boot Technology (MBT) shoe and is further limited by the use of mostly healthy asymptomatic cohorts and a failure to consider KAM as a primary knee load parameter (4,29,31,40). It is also likely that variation in materials, shoe construction, and sole design across different rocker-soled shoe manufacturers may have differing effects on KAM. One study that has considered KAM has shown that rocker-soled shoes reduce KAM in overweight males likely because of a lateral shift in the center of pressure and a reduction in medial ground reaction force (6). The alterations in center of pressure and ground reaction force magnitude potentially result in a reduction in the knee frontal plane lever arm. Limited research, however, has been conducted on the biomechanical effects of rocker-soled shoes in people with knee OA. If effective in reducing KAM parameters in people with knee OA, rocker-soled shoes have the potential to be an effective and relatively inexpensive noninvasive strategy for managing OA symptoms and slowing disease progression. The primary aim of this study was to examine the immediate effect of rocker-soled shoes on parameters of KAM in people with radiographic and symptomatic knee OA. Secondary aims were to investigate changes in the knee flexion moment (KFM) and knee pain while walking with rocker-soled shoes.
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FIGURE 1—The rocker-soled men’s (A) and women’s shoe (B) and the men’s (C) and women’s non–rocker-soled shoe (D).
was 9 (T3) hardness (Ha) The non–rocker-soled shoe was approximately 35 mm at the rear foot and 22 mm at the forefoot, with a hardness scale of 54 (T3) Ha. Thus, the rockersoled shoes had a higher sole height and were much more compressible compared with the non–rocker-soled shoe. Outcome measures—gait analysis. All participants underwent three-dimensional gait analysis during three walking conditions administered in a random order: rocker-soled shoes, non–rocker-soled shoes, and barefoot. Before gait analysis, participants walked in their own shoes at a selfselected speed to establish an average walking speed (T5%) as determined by two photoelectric timing gates positioned 4 m apart within a 10-m walkway. This time was then used to control participants’ walking speed across testing trials and conditions to negate potential confounding effects of change in walking speed on KAM (27). Participants were given a 3- to 5-min acclimation period wearing the rocker-soled shoes to reduce any risk of falling or tripping. Kinetic and kinematic data were collected using a 12-camera Vicon MX motion analysis system operating at 120 Hz (Vicon, Oxford, United Kingdom) and three force plates at a sample rate of 1200 Hz (Advanced Mechanical Technology, Inc., Watertown, MA). Reflective markers (11 mm) were placed directly on the skin bilaterally, according to the Plug-in-Gait model on the anterior and posterior superior iliac spines, lateral thigh, lateral femoral condyle, lateral shank, and lateral malleolus, and on the foot or shoe over the base of the second metatarsal head and posterior calcaneus. Additional markers placed over the medial knees and ankles were included during an initial static standing trial to determine relative positioning of knee and ankle joint centers. For each condition, five trials with clean force plate strikes and within the set speed limits were recorded. Knee joint moments were calculated in the
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distal (tibial) coordinate system using inverse dynamics (PlugIn-Gait version 2; Vicon) and normalized by dividing by body weight times height and expressed as a percentage. For each trial, peak KAM and peak KFM (NIm/BW Ht%) during the first half of stance and positive KAM angular impulse (NImIs/BW Ht%) (representing the positive area under the KAM–time curve) were determined, and the five trials were averaged for each condition. The KFM should be considered when examining the effects of treatments on KAM because concomitant increases in KFM may negate joint loading reductions resulting from decreases in KAM (41). Outcome measures—knee pain during walking. An 11-point numerical rating scale was used to assess knee pain severity during each walking condition. Immediately after each condition, participants were asked to rate their knee pain on a scale ranging from 0 (no pain) to 10 (worst pain possible). Sample size calculation. Our previous research has shown that to detect a minimum 7.5% change in peak KAM, an absolute change of approximately 0.3 (NIm/BW Ht%) is required (22). A change of this magnitude can be considered clinically relevant (26) and has been shown to be achievable with foot-related interventions (10). Assuming an SD of approximately 0.4 NIm/BW Ht% (16), sample size calculation using G*Power indicated that a minimum of 21 participants would be required to detect a change of 0.3 NIm/BW Ht% with 90% statistical power using a repeated-measures design. Statistical analysis. Data analyses were completed using PASW Statistics version 20 (SPSS, Inc., Chicago, IL). Data were tested for normality using the Kolmorgorov–Smirnov statistic to ensure appropriate use of parametric tests. Differences in gait variables, walking speed, and pain between walking conditions were examined using one-way repeated-measures ANOVA, with statistical significance set at an alpha level of
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TABLE 1. Participant characteristics. Mean (SD) (n = 30) Age (yr) Sex (n, %) Male Female Height (m) Body mass (kg) BMI (kgImj2) WOMAC Pain (score range, 0–20) Stiffness (score range, 0–8) Function (score range, 0–68) KL grade (n, %) Grade 2 Grade 3 Grade 4 Medial JSN 0 1 2 3 Lateral JSN 0 1 2 3
61.0 (7.3) 15 15 1.67 79.5 28.3
(50) (50) (0.07) (12.7) (3.7)
7.6 (3.1) 4.0 (1.6) 25.6 (10.2) 9 (30) 10 (33) 11 (37) 7 6 10 7
(23.3) (20) (33.3) (23.3)
22 4 1 3
(73.3) (13.3) (3.3) (10)
For the JSN, higher scores indicate greater JSN; for the WOMAC, lower scores indicate less pain/stiffness and better physical function; KL, KL x-ray grading. WOMAC, Western Ontario and McMaster Universities Arthritis Index.
0.05. When significant differences were obtained, post hoc comparisons were performed using Bonferroni post hoc tests. The analysis was repeated with the removal of participants with predominantly lateral radiographic disease compared with medial disease.
Descriptive characteristics are presented in Table 1. Most participants had medial tibiofemoral OA on x-ray with medial osteophytes and/or medial JSN. Six participants had worse disease in the lateral compartment. Mean (SD) peak KAM, KAM impulse, peak KFM, walking speed, and pain during each walking condition are presented in Table 2. Results from the one-way repeated-measures ANOVA indicated a significant difference in peak KAM between walking conditions (P G 0.001). Post hoc testing revealed that when wearing the rocker-soled shoes, peak KAM was significantly less than that when wearing the non–rocker-soled shoes (mean difference (95% confidence interval (CI), j0.27 (j0.42 to j0.12) NIm/BW Ht%; P G 0.001). However, peak KAM when wearing the rocker-soled shoes remained significantly higher
DISCUSSION This study examined the effects of rocker-soled shoes on parameters of KAM in a group of individuals with symptomatic and radiographic knee OA. Results show that, within this cohort and using our selected rocker-soled shoe, a significant reduction in peak KAM was evident compared with that in non–rocker-soled shoes. Despite this reduction, peak KAM
TABLE 2. Mean (SD) peak KAM, KAM angular impulse, KFM, walking speed, and pain during each walking condition. Condition Variable Peak KAM (NIm/BW Ht%) KAM angular impulse (NIm/BW Ht%) Peak KFM (NIm/BW Ht%) Walking speed (mIsj1) Pain (0–10)
Rocker-Soled Shoes 3.76 1.21 3.48 1.32 3.3
T T T T T
1.31*,** 0.52** 1.95 0.22 2.4
Non–Rocker-Soled Shoes 4.04 1.24 3.53 1.33 3.3
T T T T T
1.33** 0.49** 1.93 0.23 2.2
Barefoot
ANOVA P Values
T T T T T
G0.001 G0.001 0.36 0.06 0.89
3.56 1.09 3.44 1.31 3.3
1.29 0.48 1.96 0.23 2.3
*Significantly different from non–rocker-soled shoes (P G 0.05). **Significantly different from barefoot (P G 0.05).
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RESULTS
compared with that during barefoot walking (mean difference (95% CI), 0.20 (0.01–0.39) NIm/BW Ht%; P = 0.03). Peak KAM when wearing the non–rocker-soled shoe was also significantly higher compared with that during barefoot walking (mean difference (95% CI), 0.47 (0.31–0.63) NIm/BW Ht%; P G 0.001). The reduction in peak KAM in the rocker-soled shoes compared with that in the non–rocker-soled shoes corresponded to a mean (SD) change of j7.1% (9.2%) (range, j28.3% to 9.6%). Percentage changes in peak KAM for each individual between the rocker-soled shoe and non–rocker-soled shoe are plotted in Fig. 2A. Results remained unchanged when the six participants with predominantly lateral radiographic disease were excluded from the analysis. Results from the one-way repeated-measures ANOVA also indicated a significant difference in KAM impulse between walking conditions (P G 0.001). Post hoc testing revealed no significant difference between the two shoe conditions (mean difference (95%CI), j0.02 (j0.6 to 0.00) NIm/BW Ht%; P = 0.13). There was a large variability in the individual percentage changes in KAM impulse between the rocker-soled shoe and non–rocker-soled shoe, which ranged from a decrease of approximately 31% to an increase of 9%, as shown in Fig. 2B. The KAM impulse was significantly higher during both shoe conditions compared with that during barefoot walking (mean difference (95%CI), 0.11 (0.07–0.16) NIm/BW Ht%, and P G 0.001 in the rocker-soled shoe; mean difference (95%CI), 0.14 (0.10–0.18) 33 NIm/BW Ht%, and P G 0.001 in the non–rocker-soled shoe). Results remained unchanged when the six participants with predominantly lateral radiographic disease were excluded from the analysis. There was no significant difference in either peak KFM or walking speed across walking conditions (Table 2). Similarly, there was no significant difference in knee pain ratings across walking conditions (P = 0.89). Results remained unchanged when the six participants with predominantly lateral radiographic disease were excluded from the analysis.
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FIGURE 2—Individual changes in peak KAM (A) and KAM (B) angular impulse when walking in rocker-soled shoes, reported as a percentage change from the non–rocker-soled shoe condition. Each individual’s severity of OA (according to KL radiographic scores (where 2, mild; 3, moderate; and 4, severe)) is also indicated.
remained significantly higher when walking in the rockersoled shoes compared with that during barefoot walking. The KAM impulse was not reduced during the rocker-soled shoe condition compared with that during non–rocker-soled shoe condition and remained significantly higher compared with that during barefoot walking. There were no changes in peak KFM with the shoes, nor did participants report any immediate change in knee pain severity between walking conditions. Several footwear styles have been shown to significantly influence peak KAM in both the cohort with knee OA (3,38) and healthy (9) cohort, with certain design features such as heel height widely recognized as significantly increasing KAM (15). A significantly higher peak KAM (ranging from 7% to 15%) has been observed in people with knee OA when walking in clogs and stability shoes compared with that when wearing flip-flops and flat walking shoes (37). Parameters of KAM seem to approximate barefoot walking when people with knee OA wear either flip-flops or flat walking shoes. Several studies investigating special shoes custom-designed to reduce KAM have demonstrated reductions of between
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6% and 8% in peak KAM (3,7,9,39). The reductions in peak KAM with the rocker-soled shoe observed in the present study (approximately 7%) are similar to those noted in these studies of custom-designed shoes. In contrast to previous research on high-heeled shoes (17), the increased heel height of the rocker-soled shoe (compared with that of the non– rocker-soled shoe) did not increase peak KAM in our study. Future research is needed to determine whether reduced sole density, sole curvature, or both may explain the reductions in peak KAM observed in our study. There have only been two peer-reviewed publications exploring the biomechanical and neuromuscular effects of Skechers rocker-soled shoes, and these have evaluated the Shape-up Sandal (32,33). These publications report changes in gait, muscle activity, and instability with the rocker-soled sandals in a cohort of 15 healthy females but did not evaluate parameters of KAM. Thus, it is not possible to compare our findings of the effect of the Skecher rocker-soled shoe on KAM parameters with any other study. In contrast to the Skecher rocker-soled shoe, the MBT rocker-soled shoe has been more widely evaluated. Many of
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during testing. In the only randomized controlled trial (RCT) (n = 123) on people with knee OA to date, 12-wk use of MBT rocker-soled shoes did not significantly reduce pain compared with a standardized walking shoe (30). Another more recent RCT (n = 115) examining the effect of MBT shoes on low back pain showed no significant difference in pain after 12 months when compared with flat walking shoes (23). Given that there are several differences in shoe construction across manufacturers of rocker-soled shoes, including variations in shoe curvature and low-density sole thickness placement within the sole, further RCT testing different models of rocker-soled shoes are needed in large samples of people with symptomatic knee OA before firm conclusions about the effect of these shoe types on symptoms can be drawn. Although the present study suggests that rocker-soled shoes are effective in reducing parameters of KAM, there are little data available about how these shoes may influence balance ability in older individuals. The ‘‘unstable’’ nature of these shoes is a marketed feature and is purported to provide balance training and subsequently improve balance (31). It is therefore important to consider the fact that people experiencing knee pain have impaired balance (12) and are at an increased risk of falls (1) compared with those without knee pain. Previous research has shown that numerous footwear features can affect balance in older adults. These include increased heel height, decreased sole density, and increased curvature of midsole geometry (25,34). Most of these features are apparent in the structural design of rockersoled shoes. Given the premise that rocker-soled shoes invoke instability and thus could compromise balance, the effects of these shoes on balance need to be evaluated within an OA population who typically have impaired balance associated with their condition (12). It is important to ensure that rocker-soled shoes have no detrimental effect on balance before they could be advised for use in the clinical management of people with knee OA. Our study did not aim to determine the mechanisms by which rocker-soled shoes can influence KAM. It is possible that rocker-soled shoes may lead to small changes in parameters such as walking speed (although not significantly different in our study), frontal plane trunk lean, and/or foot position/centre of pressure, for example, all of which may influence the frontal plane knee joint lever arm and, thus, the KAM. Further research is required to evaluate the mechanisms by which rocker-soled shoes influence KAM. Future research should also be directed toward evaluating the effects of these types of shoes on cumulative knee load. Although we noted that KAM impulse was higher when wearing rocker-soled and non–rocker-soled shoes compared with that during barefoot walking, it is not clear if this is due to a reduced KAM magnitude or a reduction in ground contact time. It is possible that cumulative loading (number of steps taken multiplied by KAM impulse) (24) during barefoot walking may be higher than that when walking with shoes if reductions in stride length occur when barefoot. Although KAM impulse may be reduced with barefoot walking, a
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the studies, however, are limited by the use of small and/or asymptomatic cohorts or did not measure KAM parameters (21,28,29,35,39). Several recent studies that have examined the effects of MBT shoes in comparison with those of a non– rocker-soled shoe on KAM have done so with varying results. Landry et al. (21) examined the effects of the MBT on gait variables (including peak KAM) over a 6-wk period in 23 healthy participants and observed no significant change in peak KAM. However, studies have noted reductions in peak KAM between MBT shoes and conventional footwear in overweight males (6) and in young and old adults without any knee pathology (5). We observed an average 7% reduction in peak KAM with our chosen rocker-soled shoes in people with knee OA, which was much less than the 13% reduction observed in overweight males with MBT shoes compared with participants’ own footwear (6), although KAM impulse was not investigated. We used a standardized non–rocker-soled shoe for comparison across participants, which may explain the variation in study findings. The 4.7% reduction in peak KAM observed with MBT shoes (compared with conventional sports shoes) in younger and older adults (5) is consistent with our findings. The data from the present study (Fig. 2) show that the individual response to rocker-soled shoes is quite variable, with seven (23.3%) of our participants actually demonstrating an increase in peak KAM even though a mean reduction was observed relative to non–rocker-soled shoes. In our cohort, the effect of the shoes on peak KAM ranged from a 31% decrease in KAM to a 9% increase. This suggests that individual participant characteristics may influence the effect of the shoes on peak KAM. A similar variation in response to footwear has been reported by other authors (14,30). It is unclear at present which participant factors mediate response to rocker-soled shoes, but BMI, hip and/or knee alignment and/or torsion, or foot posture may play a role. Further research is required to elucidate the role of such factors in determining biomechanical responses to rocker-soled shoes in people with knee OA. We assessed KAM as a surrogate indicator of medial compartment load. Although peak KAM positively correlates with medial knee joint compartment force (42), it is not the only contributor to medial knee joint load, which may also be affected by other factors such as increased walking speed, increased KFM, and alterations in muscle activity. Using a patient with an instrumented knee replacement, it has been shown that gait patterns effective at reducing KAM can result in a concomitant increase in peak KFM and lead to net increase in medial compartment force (41). Therefore, we also measured peak KFM as a secondary outcome in the present study. We observed no increase in peak KFM with the rocker-sole shoes, which increases the likelihood of a reduction in medial knee contact forces with rocker-soled shoes. We observed no significant immediate change in pain with the rocker-soled shoes. This is probably not unexpected, given the short duration of shoe wear used in this study and the relatively low levels of pain experienced by participants
reduced stride length would necessitate additional steps to cover a set distance and may actually result in increased cumulative loading relative to shod walking conditions. Further investigation is required to fully elucidate the effects of rocker-soled shoes on knee joint loading. Strengths of this study include the randomization of test conditions, the generalizability of the study cohort with equal numbers of both males and females with knee OA tested, and a similar spread of radiographic disease severity across both sexes. The participation of people with symptomatic knee OA is a particular strength of this study, given that most of similar research has studied healthy people. Although the study was powered to detect a clinically relevant change in peak KAM, the results found regarding KAM impulse imply that a larger sample size may be needed to detect significant changes with the rocker-soled shoes to elucidate more statistically significant results. Although speed was controlled across conditions, with nonsignificant differences observed, these small changes may be partly responsible for changes in KAM observed in the rocker-soled shoe. Prolonged periods of wear may also influence the significance found with KAM impulse, and examination after intervention may also show more meaningful outcomes. Other limitations of this study include the short time provided to participants to acclimatize to the rocker-soled shoe, no
assessment of whether OA pain was predominantly of patellofemoral origin, the assessment of only immediate changes in biomechanics and walking pain, and the lack of longer-term use of the shoes. Results of this study revealed that the rocker-soled shoes significantly reduced peak KAM during walking in a group of individuals with radiographic and symptomatic knee OA when compared with non–rocker-soled shoes. However, peak KAM still remained elevated when compared with that during barefoot walking. Clinically, footwear is a conservative, practical, and economical strategy for managing knee OA. Rocker-soled shoes may present a useful management strategy for people with knee OA, given the role of excessive knee joint loading in disease pathogenesis. However, given the unstable nature of this shoe style, further examination of the effects of rocker-soled shoes on balance is needed before recommending these shoes to older people with knee OA.
This study was funded by a National Health and Medical Research Council Program grant (#631717). K. B. is partly supported by an Australian Research Council future fellowship (#FT0991413). R. H. is partly supported by an Australian Research Council future fellowship (#FT130100175). There are no conflicts of interest to disclose. The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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30. Nigg BM, Emery C, Hiemstra LA. Unstable shoe construction and reduction of pain in osteoarthritis patients. Med Sci Sports Exerc. 2006;38(10):1701–8. 31. Nigg BM, Federolf P, Landry SC. Gender differences in lower extremity gait biomechanics during walking using an unstable shoe. Clin Biomech (Bristol, Avon). 2010;25(10):1047–52. 32. Price C, Smith L, Graham-Smith G, Jones R. The effect of unstable sandals on instability in gait in healthy female subjects. Gait Posture. 2013;38(3):410–15. 33. Price C, Smith L, Graham-Smith G, Jones R. The effect of unstable sandals on single-leg standing. Footwear Sci. 2013;5(3):147–54. 34. Robbins S, Waked E, Allard P, McClaran J, Krouglicof N. Foot position awareness in younger and older men: the influence of footwear sole properties. J Am Geriatr Soc. 1997;45(1):61–6. 35. Romkes J, Rudmann C, Brunner R. Changes in gait and EMG when walking with the Masai Barefoot Technique. Clin Biomech (Bristol, Avon). 2006;21(1):75–81. 36. Shakoor N, Lidtke RH, Sengupta M, Fogg LF, Block JA. Effects of specialized footwear on joint loads in osteoarthritis of the knee. Arthritis Rheum. 2008;59(9):1214–20. 37. Shakoor N, Sengupta M, Foucher KC, Wimmer MA, Fogg LF, Block JA. Effects of common footwear on joint loading in osteoarthritis of the knee. Arthritis Care Res (Hoboken). 2010; 62(7):917–23. 38. Sto¨ggl T, Haudum A, Birklbauer J, Murrer M, Mu¨ller E. Short and long term adaptation of variability during walking using unstable (Mbt) shoes. Clin Biomech (Bristol, Avon). 2010;25(8):816–22. 39. Taniguchi M, Tateuchi H, Takeoka T, Ichihashi N. Kinematic and kinetic characteristics of Masai Barefoot Technology footwear. Gait Posture. 2012;35(4):567–72. 40. Walter JP, D’Lima DD, Colwell CW, Fregly BJ. Decreased knee adduction moment does not guarantee decreased medial contact force during gait. J Orthop Res. 2010;28(10):1348–54. 41. Zhao D, Banks SA, Mitchell KH, D’Lima DD, Colwell CW, Fregly B. Correlation between the knee adduction torque and medial contact force for a variety of gait patterns. J Orthop Res. 2007;25(6):789–97.
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20. Landry SC, Nigg BM, Tecante KE. Standing in an unstable shoe increases postural sway and muscle activity of selected smaller extrinsic foot muscles. Gait Posture. 2010;32(2):215–9. 21. Landry SC, Nigg BM, Tecante KE. Walking in an unstable Masai Barefoot Technology (MBT) shoe introduces kinematic and kinetic changes at the hip, knee and ankle before and after a 6-week accommodation period: a comprehensive analysis using principal component analysis (PCA). Footwear Sci. 2012;4(2):101–14. 22. Lim BW, Hinman RS, Wrigley TV, Sharma L, Bennell KL. Does knee malalignment mediate the effects of quadriceps strengthening on knee adduction moment, pain, and function in medial knee osteoarthritis? A randomized controlled trial. Arthritis Rheum. 2008; 59(7):943–51. 23. MacRae CS, Lewis JS, Shortland AP, Morrissey M, Critchley D. Effectiveness of rocker sole shoes in the management of chronic low back pain: a randomized clinical trial. Spine (Phila Pa 1976). 2013;38(22):1905–12. 24. Maly MR, Robbins SM, Stratford PW, Birmingham TB, Callaghan JP. Cumulative knee adductor load distinguishes between healthy and osteoarthritic knees—a proof of principle study. Gait Posture. 2013; 37(3):397–401. 25. Menant JC, Steele JR, Menz HB, Munro BJ, Lord SR. Effects of footwear features on balance and stepping in older people. Gerontology. 2008;54(1):18–23. 26. Miyazaki T, Wada M, Kawahara H, Sato M, Baba H, Shimada S. Dynamic load at baseline can predict radiographic disease progression in medial compartment knee osteoarthritis. Ann Rheum Dis. 2002;61(7):617–22. 27. Mundermann A, Dyrby CO, Hurwitz DE, Sharma L, Andriacchi TP. Potential strategies to reduce medial compartment loading in patients with knee osteoarthritis of varying severity: reduced walking speed. Arth Rheum. 2004;50(4):1172–8. 28. Nigg B. Biomechanical considerations on barefoot movement and barefoot shoe concepts. Footwear Sci. 2009;1(2):73–9. 29. Nigg B, Hintzen S, Ferber R. Effect of an unstable shoe construction on lower extremity gait characteristics. Clin Biomech (Bristol, Avon). 2006;21(1):82–8.
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MADDEN, E. G., C. O. KEAN, T. V. WRIGLEY, K. L. BENNELL, and R. S. HINMAN. Effect of Rocker-Soled Shoes on Parameters of Knee Joint Load in Knee Osteoarthritis. Med. Sci. Sports Exerc., Vol. 47, No. 1, pp. 128–135, 2015. Purpose: This study evaluated the immediate effects of rocker-soled shoes on parameters of the knee adduction moment (KAM) and pain in individuals with knee osteoarthritis (OA). Methods: Three-dimensional gait analysis was performed on 30 individuals (mean (SD): age, 61 (7) yr; 15 (50%) male) with radiographic and symptomatic knee OA under three walking conditions in a randomized order: i) wearing rocker-soled shoes (Skechers Shape-ups), ii) wearing non–rocker-soled shoes (ASICS walking shoes), and iii) barefoot. Peak KAM and KAM angular impulse were measured as primary indicators of knee load distribution. Secondary measures included the knee flexion moment (KFM) and knee pain during walking. Results: Peak KAM was significantly lower when wearing the rocker-soled shoes compared with that when wearing the non–rocker-soled shoes (mean difference (95% confidence interval), j0.27 (j0.42 to j0.12) NIm/BW Ht%; P G 0.001). Post hoc tests revealed no significant difference in KAM impulse between rocker-soled and non–rocker-soled shoe conditions (P = 0.13). Both peak KAM and KAM impulse were significantly higher during both shoe conditions compared with those during the barefoot condition (P G 0.001). There were no significant differences in KFM (P = 0.36) or knee pain (P = 0.89) between conditions. Conclusions: Rocker-soled shoes significantly reduced peak KAM when compared with non–rocker-soled shoes, without a concomitant change in KFM, and thus may potentially reduce medial knee joint loading. However, KAM parameters in the rocker-soled shoes remained significantly higher than those during barefoot walking. Wearing rocker-soled shoes did not have a significant immediate effect on walking pain. Further research is required to evaluate whether rocker-soled shoes can influence symptoms and progression of knee OA with prolonged wear. Key Words: SHOES, GAIT ANALYSIS, KNEE ADDUCTION MOMENT, KNEE, OSTEOARTHRITIS
O
Excessive knee loading plays an important role in OA pathogenesis and as such represents a potential target for treatment. The external knee adduction moment (KAM) is considered a valid and reliable indirect biomechanical marker of tibiofemoral compartment load distribution, such that a higher KAM indicates greater load in the medial compartment (36). Peak KAM has been shown to predict progression of radiographic disease and pain severity in people with knee OA (26) and has also been strongly correlated with pain and functional impairment during single-limb stance (18). A higher KAM angular impulse (a measure of the mean magnitude and duration of KAM) has been associated with greater loss of medial tibial cartilage in people with knee OA (2) and also increased knee pain independent of disease severity (19). Clinical guidelines for knee OA recommend that clinicians provide advice to patients regarding optimal footwear choices (8), given that wearing shoes significantly increases parameters of knee joint load compared with barefoot walking. However, there is little research evidence demonstrating which shoe types are best for those with knee OA. We have previously shown an average increase of 7.4% in peak KAM when people with knee OA walked in their own footwear compared with walking barefoot (14). Considerable individual variation was observed (with increases of up to 30%),
steoarthritis (OA) is the most common form of arthritis, and it frequently occurs in weight-bearing joints, particularly the knee. It is a leading cause of pain and impairment in older adults (11), with affected individuals reporting difficulty with many activities of daily living such as walking and stair climbing. Currently, there is no cure for OA, thus, determining effective conservative treatments to provide symptomatic relief and potentially slow disease progression is an important public health objective. Clinical guidelines (8) recommend conservative nondrug strategies, including advice about appropriate footwear, as the cornerstone of knee OA management.
Address for correspondence: Elizabeth Madden, M.Hlth.Sc., Department of Physiotherapy, Melbourne School of Health Sciences, University of Melbourne, Alan Gilbert Building 161 Barry St., Carlton Victoria 3053, Australia; E-mail: [email protected]. Submitted for publication October 2013. Accepted for publication April 2014. 0195-9131/15/4701-0128/0 MEDICINE & SCIENCE IN SPORTS & EXERCISEÒ Copyright Ó 2014 by the American College of Sports Medicine DOI: 10.1249/MSS.0000000000000384
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METHODS Thirty participants with radiographic and symptomatic knee OA were recruited as a convenience sample from a larger concurrent study within the Centre for Health, Exercise, and Sports Medicine at the University of Melbourne (3). All participants were Q40 yr of age, exhibited radiographic
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evidence of osteophytes in the tibiofemoral joint (Kellgren and Lawrence (KL) (13) grade, Q2), reported an average knee pain severity during the past week of Q4 on an 11-point numerical rating scale (0, no pain; 10, worst pain possible), and reported knee pain on most days of the previous month. Exclusion criteria included body mass index (BMI) Q36 kgImj2 (due to three-dimensional gait analysis difficulties), history of lower limb joint replacement, spinal surgery, or high tibial osteotomy, knee surgery or injection in the previous 6 months, history of knee fracture or malignancy, back pain 940 out of a 100-point pain scale, use of a gait aid, diagnosis of inflammatory joint or skin conditions (such as gout, fibromyalgia, or rheumatoid arthritis), history of stroke or neurological disorder (such polio or multiple sclerosis) or any peripheral nerve condition or neuropathy, and/or an inability to understand written or spoken English. All participants provided a informed written consent, and ethical approval was granted by the University of Melbourne human research ethics committee. Descriptive information. Demographic data included participant descriptors such as age, height, weight, and BMI. All participants underwent bilateral, semiflexed, weightbearing, posteroanterior knee radiographs to determine disease severity according to the KL scale and degree of joint space narrowing (JSN). The Western Ontario and McMaster Universities Osteoarthritis Index, a self-report questionnaire, was used to assess knee pain and functional ability for descriptive purposes. This is a valid and reliable disease-specific tool consisting of three subscales (pain, stiffness, and physical function). Each question is answered using a five-point Likert scale (with responses ranging from ‘‘none’’ to ‘‘extreme’’). Higher scores on each subscale indicate worse pain, stiffness, or physical dysfunction. Shoes. The rocker-soled shoes tested were the Skechers Shape-up Motivation (men) (Fig. 1A) and the Skechers Shape-up Fitness Junkie (women) shoe (Fig. 1B), which are relatively inexpensive and popular shoes easily available in Australia. The upper material of the shoe consisted of suede for the men’s shoe and synthetic for the women’s. Because specific shoe design features (such as heel height, arch supports, etc.) can influence KAM (38), we chose to use a standardized shoe as a control condition (rather than the individual’s own usual footwear) to minimize confounding effects. The non–rocker-soled shoes tested were standard walking shoes with a traditional (nonrocker) sole (ASICS Oceania Pty. Ltd.) (Fig. 1C and D). These shoes consisted of a leather upper and had no specific motion control properties either in sole density, upper construction, or last shape. Sole height was measured with a vernier caliper, and sole hardness was measured using a Shore A durometer, which is commonly used to determine hardness of materials such as vulcanized rubber and plastic products. Sole height in the rocker-soled shoe was approximately 56 mm at the rear foot and 35 mm at the forefoot for the men’s shoe and 46 mm at the rear foot and 35 mm at the forefoot for the women’s shoe. For both men’s and women’s rocker-soled shoes, the sole hardness
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suggesting that individual shoe features play a role in influencing KAM. In particular, shoes with greater heel heights and more rigid designs are associated with increases in KAM (15,17,37). Several recent studies have shown that innovative, specially designed footwear is an effective and pragmatic approach to reduce KAM parameters (3,37), however, there is limited research evaluating the effects of popular commercially available shoe styles. For health professionals to appropriately advise patients with knee OA about which shoe styles to wear (and conversely, which to avoid), further research is needed. Rocker-soled shoes are designed with an increase in sagittal plane sole convex curvature and reduced sole density in an attempt to mimic barefoot walking by creating an unstable platform (20,30). The exercise footwear market has heavily promoted rocker-soled shoes as a useful strategy to enhance exercise and weight loss by encouraging increased muscle activity to counteract the instability caused by the shoe. Accordingly, these shoes are increasing in popularity in the community because of their perceived health and fitness benefits. Although this style of shoe is manufactured by several different footwear companies, the limited available research predominantly evaluates the Masai Boot Technology (MBT) shoe and is further limited by the use of mostly healthy asymptomatic cohorts and a failure to consider KAM as a primary knee load parameter (4,29,31,40). It is also likely that variation in materials, shoe construction, and sole design across different rocker-soled shoe manufacturers may have differing effects on KAM. One study that has considered KAM has shown that rocker-soled shoes reduce KAM in overweight males likely because of a lateral shift in the center of pressure and a reduction in medial ground reaction force (6). The alterations in center of pressure and ground reaction force magnitude potentially result in a reduction in the knee frontal plane lever arm. Limited research, however, has been conducted on the biomechanical effects of rocker-soled shoes in people with knee OA. If effective in reducing KAM parameters in people with knee OA, rocker-soled shoes have the potential to be an effective and relatively inexpensive noninvasive strategy for managing OA symptoms and slowing disease progression. The primary aim of this study was to examine the immediate effect of rocker-soled shoes on parameters of KAM in people with radiographic and symptomatic knee OA. Secondary aims were to investigate changes in the knee flexion moment (KFM) and knee pain while walking with rocker-soled shoes.
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FIGURE 1—The rocker-soled men’s (A) and women’s shoe (B) and the men’s (C) and women’s non–rocker-soled shoe (D).
was 9 (T3) hardness (Ha) The non–rocker-soled shoe was approximately 35 mm at the rear foot and 22 mm at the forefoot, with a hardness scale of 54 (T3) Ha. Thus, the rockersoled shoes had a higher sole height and were much more compressible compared with the non–rocker-soled shoe. Outcome measures—gait analysis. All participants underwent three-dimensional gait analysis during three walking conditions administered in a random order: rocker-soled shoes, non–rocker-soled shoes, and barefoot. Before gait analysis, participants walked in their own shoes at a selfselected speed to establish an average walking speed (T5%) as determined by two photoelectric timing gates positioned 4 m apart within a 10-m walkway. This time was then used to control participants’ walking speed across testing trials and conditions to negate potential confounding effects of change in walking speed on KAM (27). Participants were given a 3- to 5-min acclimation period wearing the rocker-soled shoes to reduce any risk of falling or tripping. Kinetic and kinematic data were collected using a 12-camera Vicon MX motion analysis system operating at 120 Hz (Vicon, Oxford, United Kingdom) and three force plates at a sample rate of 1200 Hz (Advanced Mechanical Technology, Inc., Watertown, MA). Reflective markers (11 mm) were placed directly on the skin bilaterally, according to the Plug-in-Gait model on the anterior and posterior superior iliac spines, lateral thigh, lateral femoral condyle, lateral shank, and lateral malleolus, and on the foot or shoe over the base of the second metatarsal head and posterior calcaneus. Additional markers placed over the medial knees and ankles were included during an initial static standing trial to determine relative positioning of knee and ankle joint centers. For each condition, five trials with clean force plate strikes and within the set speed limits were recorded. Knee joint moments were calculated in the
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distal (tibial) coordinate system using inverse dynamics (PlugIn-Gait version 2; Vicon) and normalized by dividing by body weight times height and expressed as a percentage. For each trial, peak KAM and peak KFM (NIm/BW Ht%) during the first half of stance and positive KAM angular impulse (NImIs/BW Ht%) (representing the positive area under the KAM–time curve) were determined, and the five trials were averaged for each condition. The KFM should be considered when examining the effects of treatments on KAM because concomitant increases in KFM may negate joint loading reductions resulting from decreases in KAM (41). Outcome measures—knee pain during walking. An 11-point numerical rating scale was used to assess knee pain severity during each walking condition. Immediately after each condition, participants were asked to rate their knee pain on a scale ranging from 0 (no pain) to 10 (worst pain possible). Sample size calculation. Our previous research has shown that to detect a minimum 7.5% change in peak KAM, an absolute change of approximately 0.3 (NIm/BW Ht%) is required (22). A change of this magnitude can be considered clinically relevant (26) and has been shown to be achievable with foot-related interventions (10). Assuming an SD of approximately 0.4 NIm/BW Ht% (16), sample size calculation using G*Power indicated that a minimum of 21 participants would be required to detect a change of 0.3 NIm/BW Ht% with 90% statistical power using a repeated-measures design. Statistical analysis. Data analyses were completed using PASW Statistics version 20 (SPSS, Inc., Chicago, IL). Data were tested for normality using the Kolmorgorov–Smirnov statistic to ensure appropriate use of parametric tests. Differences in gait variables, walking speed, and pain between walking conditions were examined using one-way repeated-measures ANOVA, with statistical significance set at an alpha level of
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TABLE 1. Participant characteristics. Mean (SD) (n = 30) Age (yr) Sex (n, %) Male Female Height (m) Body mass (kg) BMI (kgImj2) WOMAC Pain (score range, 0–20) Stiffness (score range, 0–8) Function (score range, 0–68) KL grade (n, %) Grade 2 Grade 3 Grade 4 Medial JSN 0 1 2 3 Lateral JSN 0 1 2 3
61.0 (7.3) 15 15 1.67 79.5 28.3
(50) (50) (0.07) (12.7) (3.7)
7.6 (3.1) 4.0 (1.6) 25.6 (10.2) 9 (30) 10 (33) 11 (37) 7 6 10 7
(23.3) (20) (33.3) (23.3)
22 4 1 3
(73.3) (13.3) (3.3) (10)
For the JSN, higher scores indicate greater JSN; for the WOMAC, lower scores indicate less pain/stiffness and better physical function; KL, KL x-ray grading. WOMAC, Western Ontario and McMaster Universities Arthritis Index.
0.05. When significant differences were obtained, post hoc comparisons were performed using Bonferroni post hoc tests. The analysis was repeated with the removal of participants with predominantly lateral radiographic disease compared with medial disease.
Descriptive characteristics are presented in Table 1. Most participants had medial tibiofemoral OA on x-ray with medial osteophytes and/or medial JSN. Six participants had worse disease in the lateral compartment. Mean (SD) peak KAM, KAM impulse, peak KFM, walking speed, and pain during each walking condition are presented in Table 2. Results from the one-way repeated-measures ANOVA indicated a significant difference in peak KAM between walking conditions (P G 0.001). Post hoc testing revealed that when wearing the rocker-soled shoes, peak KAM was significantly less than that when wearing the non–rocker-soled shoes (mean difference (95% confidence interval (CI), j0.27 (j0.42 to j0.12) NIm/BW Ht%; P G 0.001). However, peak KAM when wearing the rocker-soled shoes remained significantly higher
DISCUSSION This study examined the effects of rocker-soled shoes on parameters of KAM in a group of individuals with symptomatic and radiographic knee OA. Results show that, within this cohort and using our selected rocker-soled shoe, a significant reduction in peak KAM was evident compared with that in non–rocker-soled shoes. Despite this reduction, peak KAM
TABLE 2. Mean (SD) peak KAM, KAM angular impulse, KFM, walking speed, and pain during each walking condition. Condition Variable Peak KAM (NIm/BW Ht%) KAM angular impulse (NIm/BW Ht%) Peak KFM (NIm/BW Ht%) Walking speed (mIsj1) Pain (0–10)
Rocker-Soled Shoes 3.76 1.21 3.48 1.32 3.3
T T T T T
1.31*,** 0.52** 1.95 0.22 2.4
Non–Rocker-Soled Shoes 4.04 1.24 3.53 1.33 3.3
T T T T T
1.33** 0.49** 1.93 0.23 2.2
Barefoot
ANOVA P Values
T T T T T
G0.001 G0.001 0.36 0.06 0.89
3.56 1.09 3.44 1.31 3.3
1.29 0.48 1.96 0.23 2.3
*Significantly different from non–rocker-soled shoes (P G 0.05). **Significantly different from barefoot (P G 0.05).
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RESULTS
compared with that during barefoot walking (mean difference (95% CI), 0.20 (0.01–0.39) NIm/BW Ht%; P = 0.03). Peak KAM when wearing the non–rocker-soled shoe was also significantly higher compared with that during barefoot walking (mean difference (95% CI), 0.47 (0.31–0.63) NIm/BW Ht%; P G 0.001). The reduction in peak KAM in the rocker-soled shoes compared with that in the non–rocker-soled shoes corresponded to a mean (SD) change of j7.1% (9.2%) (range, j28.3% to 9.6%). Percentage changes in peak KAM for each individual between the rocker-soled shoe and non–rocker-soled shoe are plotted in Fig. 2A. Results remained unchanged when the six participants with predominantly lateral radiographic disease were excluded from the analysis. Results from the one-way repeated-measures ANOVA also indicated a significant difference in KAM impulse between walking conditions (P G 0.001). Post hoc testing revealed no significant difference between the two shoe conditions (mean difference (95%CI), j0.02 (j0.6 to 0.00) NIm/BW Ht%; P = 0.13). There was a large variability in the individual percentage changes in KAM impulse between the rocker-soled shoe and non–rocker-soled shoe, which ranged from a decrease of approximately 31% to an increase of 9%, as shown in Fig. 2B. The KAM impulse was significantly higher during both shoe conditions compared with that during barefoot walking (mean difference (95%CI), 0.11 (0.07–0.16) NIm/BW Ht%, and P G 0.001 in the rocker-soled shoe; mean difference (95%CI), 0.14 (0.10–0.18) 33 NIm/BW Ht%, and P G 0.001 in the non–rocker-soled shoe). Results remained unchanged when the six participants with predominantly lateral radiographic disease were excluded from the analysis. There was no significant difference in either peak KFM or walking speed across walking conditions (Table 2). Similarly, there was no significant difference in knee pain ratings across walking conditions (P = 0.89). Results remained unchanged when the six participants with predominantly lateral radiographic disease were excluded from the analysis.
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FIGURE 2—Individual changes in peak KAM (A) and KAM (B) angular impulse when walking in rocker-soled shoes, reported as a percentage change from the non–rocker-soled shoe condition. Each individual’s severity of OA (according to KL radiographic scores (where 2, mild; 3, moderate; and 4, severe)) is also indicated.
remained significantly higher when walking in the rockersoled shoes compared with that during barefoot walking. The KAM impulse was not reduced during the rocker-soled shoe condition compared with that during non–rocker-soled shoe condition and remained significantly higher compared with that during barefoot walking. There were no changes in peak KFM with the shoes, nor did participants report any immediate change in knee pain severity between walking conditions. Several footwear styles have been shown to significantly influence peak KAM in both the cohort with knee OA (3,38) and healthy (9) cohort, with certain design features such as heel height widely recognized as significantly increasing KAM (15). A significantly higher peak KAM (ranging from 7% to 15%) has been observed in people with knee OA when walking in clogs and stability shoes compared with that when wearing flip-flops and flat walking shoes (37). Parameters of KAM seem to approximate barefoot walking when people with knee OA wear either flip-flops or flat walking shoes. Several studies investigating special shoes custom-designed to reduce KAM have demonstrated reductions of between
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6% and 8% in peak KAM (3,7,9,39). The reductions in peak KAM with the rocker-soled shoe observed in the present study (approximately 7%) are similar to those noted in these studies of custom-designed shoes. In contrast to previous research on high-heeled shoes (17), the increased heel height of the rocker-soled shoe (compared with that of the non– rocker-soled shoe) did not increase peak KAM in our study. Future research is needed to determine whether reduced sole density, sole curvature, or both may explain the reductions in peak KAM observed in our study. There have only been two peer-reviewed publications exploring the biomechanical and neuromuscular effects of Skechers rocker-soled shoes, and these have evaluated the Shape-up Sandal (32,33). These publications report changes in gait, muscle activity, and instability with the rocker-soled sandals in a cohort of 15 healthy females but did not evaluate parameters of KAM. Thus, it is not possible to compare our findings of the effect of the Skecher rocker-soled shoe on KAM parameters with any other study. In contrast to the Skecher rocker-soled shoe, the MBT rocker-soled shoe has been more widely evaluated. Many of
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FOOTWEAR AND KNEE LOAD IN KNEE OSTEOARTHRITIS
during testing. In the only randomized controlled trial (RCT) (n = 123) on people with knee OA to date, 12-wk use of MBT rocker-soled shoes did not significantly reduce pain compared with a standardized walking shoe (30). Another more recent RCT (n = 115) examining the effect of MBT shoes on low back pain showed no significant difference in pain after 12 months when compared with flat walking shoes (23). Given that there are several differences in shoe construction across manufacturers of rocker-soled shoes, including variations in shoe curvature and low-density sole thickness placement within the sole, further RCT testing different models of rocker-soled shoes are needed in large samples of people with symptomatic knee OA before firm conclusions about the effect of these shoe types on symptoms can be drawn. Although the present study suggests that rocker-soled shoes are effective in reducing parameters of KAM, there are little data available about how these shoes may influence balance ability in older individuals. The ‘‘unstable’’ nature of these shoes is a marketed feature and is purported to provide balance training and subsequently improve balance (31). It is therefore important to consider the fact that people experiencing knee pain have impaired balance (12) and are at an increased risk of falls (1) compared with those without knee pain. Previous research has shown that numerous footwear features can affect balance in older adults. These include increased heel height, decreased sole density, and increased curvature of midsole geometry (25,34). Most of these features are apparent in the structural design of rockersoled shoes. Given the premise that rocker-soled shoes invoke instability and thus could compromise balance, the effects of these shoes on balance need to be evaluated within an OA population who typically have impaired balance associated with their condition (12). It is important to ensure that rocker-soled shoes have no detrimental effect on balance before they could be advised for use in the clinical management of people with knee OA. Our study did not aim to determine the mechanisms by which rocker-soled shoes can influence KAM. It is possible that rocker-soled shoes may lead to small changes in parameters such as walking speed (although not significantly different in our study), frontal plane trunk lean, and/or foot position/centre of pressure, for example, all of which may influence the frontal plane knee joint lever arm and, thus, the KAM. Further research is required to evaluate the mechanisms by which rocker-soled shoes influence KAM. Future research should also be directed toward evaluating the effects of these types of shoes on cumulative knee load. Although we noted that KAM impulse was higher when wearing rocker-soled and non–rocker-soled shoes compared with that during barefoot walking, it is not clear if this is due to a reduced KAM magnitude or a reduction in ground contact time. It is possible that cumulative loading (number of steps taken multiplied by KAM impulse) (24) during barefoot walking may be higher than that when walking with shoes if reductions in stride length occur when barefoot. Although KAM impulse may be reduced with barefoot walking, a
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the studies, however, are limited by the use of small and/or asymptomatic cohorts or did not measure KAM parameters (21,28,29,35,39). Several recent studies that have examined the effects of MBT shoes in comparison with those of a non– rocker-soled shoe on KAM have done so with varying results. Landry et al. (21) examined the effects of the MBT on gait variables (including peak KAM) over a 6-wk period in 23 healthy participants and observed no significant change in peak KAM. However, studies have noted reductions in peak KAM between MBT shoes and conventional footwear in overweight males (6) and in young and old adults without any knee pathology (5). We observed an average 7% reduction in peak KAM with our chosen rocker-soled shoes in people with knee OA, which was much less than the 13% reduction observed in overweight males with MBT shoes compared with participants’ own footwear (6), although KAM impulse was not investigated. We used a standardized non–rocker-soled shoe for comparison across participants, which may explain the variation in study findings. The 4.7% reduction in peak KAM observed with MBT shoes (compared with conventional sports shoes) in younger and older adults (5) is consistent with our findings. The data from the present study (Fig. 2) show that the individual response to rocker-soled shoes is quite variable, with seven (23.3%) of our participants actually demonstrating an increase in peak KAM even though a mean reduction was observed relative to non–rocker-soled shoes. In our cohort, the effect of the shoes on peak KAM ranged from a 31% decrease in KAM to a 9% increase. This suggests that individual participant characteristics may influence the effect of the shoes on peak KAM. A similar variation in response to footwear has been reported by other authors (14,30). It is unclear at present which participant factors mediate response to rocker-soled shoes, but BMI, hip and/or knee alignment and/or torsion, or foot posture may play a role. Further research is required to elucidate the role of such factors in determining biomechanical responses to rocker-soled shoes in people with knee OA. We assessed KAM as a surrogate indicator of medial compartment load. Although peak KAM positively correlates with medial knee joint compartment force (42), it is not the only contributor to medial knee joint load, which may also be affected by other factors such as increased walking speed, increased KFM, and alterations in muscle activity. Using a patient with an instrumented knee replacement, it has been shown that gait patterns effective at reducing KAM can result in a concomitant increase in peak KFM and lead to net increase in medial compartment force (41). Therefore, we also measured peak KFM as a secondary outcome in the present study. We observed no increase in peak KFM with the rocker-sole shoes, which increases the likelihood of a reduction in medial knee contact forces with rocker-soled shoes. We observed no significant immediate change in pain with the rocker-soled shoes. This is probably not unexpected, given the short duration of shoe wear used in this study and the relatively low levels of pain experienced by participants
reduced stride length would necessitate additional steps to cover a set distance and may actually result in increased cumulative loading relative to shod walking conditions. Further investigation is required to fully elucidate the effects of rocker-soled shoes on knee joint loading. Strengths of this study include the randomization of test conditions, the generalizability of the study cohort with equal numbers of both males and females with knee OA tested, and a similar spread of radiographic disease severity across both sexes. The participation of people with symptomatic knee OA is a particular strength of this study, given that most of similar research has studied healthy people. Although the study was powered to detect a clinically relevant change in peak KAM, the results found regarding KAM impulse imply that a larger sample size may be needed to detect significant changes with the rocker-soled shoes to elucidate more statistically significant results. Although speed was controlled across conditions, with nonsignificant differences observed, these small changes may be partly responsible for changes in KAM observed in the rocker-soled shoe. Prolonged periods of wear may also influence the significance found with KAM impulse, and examination after intervention may also show more meaningful outcomes. Other limitations of this study include the short time provided to participants to acclimatize to the rocker-soled shoe, no
assessment of whether OA pain was predominantly of patellofemoral origin, the assessment of only immediate changes in biomechanics and walking pain, and the lack of longer-term use of the shoes. Results of this study revealed that the rocker-soled shoes significantly reduced peak KAM during walking in a group of individuals with radiographic and symptomatic knee OA when compared with non–rocker-soled shoes. However, peak KAM still remained elevated when compared with that during barefoot walking. Clinically, footwear is a conservative, practical, and economical strategy for managing knee OA. Rocker-soled shoes may present a useful management strategy for people with knee OA, given the role of excessive knee joint loading in disease pathogenesis. However, given the unstable nature of this shoe style, further examination of the effects of rocker-soled shoes on balance is needed before recommending these shoes to older people with knee OA.
This study was funded by a National Health and Medical Research Council Program grant (#631717). K. B. is partly supported by an Australian Research Council future fellowship (#FT0991413). R. H. is partly supported by an Australian Research Council future fellowship (#FT130100175). There are no conflicts of interest to disclose. The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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