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1 This article was downloaded by: [Songning Zhang] On: 08 November 2012, At: 01:58 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK Footwear Science Publication details, including instructions for authors and subscription information: An unstable rocker-bottom shoe alters lower extremity biomechanics during level walking Songning Zhang a, Maxime R. Paquette a, Clare E. Milner a, Carolyn Westlake a, Erin Byrd a & Lucas Baumgartner a a The University of Tennessee, Biomechanics/Sports Medicine Lab, Department of Kinesiology, Recreation and Sports Studies, 1914 Andy Holt Ave., Knoxville, United States To cite this article: Songning Zhang, Maxime R. Paquette, Clare E. Milner, Carolyn Westlake, Erin Byrd & Lucas Baumgartner (2012): An unstable rocker-bottom shoe alters lower extremity biomechanics during level walking, Footwear Science, 4:3, To link to this article: PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

2 Footwear Science Vol. 4, No. 3, September 2012, An unstable rocker-bottom shoe alters lower extremity biomechanics during level walking Songning Zhang*, Maxime R. Paquette, Clare E. Milner, Carolyn Westlake, Erin Byrd and Lucas Baumgartner The University of Tennessee, Biomechanics/Sports Medicine Lab, Department of Kinesiology, Recreation and Sports Studies, 1914 Andy Holt Ave., Knoxville, United States (Received 7 May 2012; final version received 26 September 2012) Biomechanical data for gait in unstable rocker bottom shoes reported in the literature is not comprehensive across available shoe types. Purpose: The objective of this study was to comprehensively evaluate centre of pressure (COP), ground reaction force (GRF), joint kinematics and kinetics, and electromyography (EMG) of selected muscles while walking in an unstable shoe compared to a control shoe. Methods: Fifteen subjects performed five walking trials at 1.3 m/s and 1.8 m/s in control and unstable shoes. Kinematic, GRF and EMG data were simultaneously collected. Results: The unstable shoe caused an increased mediolateral COP displacement. Greater loading rate of initial peak vertical GRF, reduced ankle plantarflexion range of motion (ROM) and greater total sagittal-plane ankle ROM were also observed for the unstable shoe compared to the control shoe. Peak dorsiflexion and plantarflexion moments, peak knee flexion moment as well as EMG activation of tibialis anterior and rectus femoris were reduced in the unstable shoe. Finally, the peak inversion moment and hip abduction moment were increased in the unstable shoe. Conclusion: These results suggest that the unstable shoe presents challenges to the body to maintain mediolateral stability and therefore helps improve involved ankle and hip muscles. Furthermore the demands for the lower extremity muscles related to the sagittal-plane motions were reduced in the unstable shoe. Keywords: rocker-bottom shoe; unstable shoe; gait; joint moment; COP; EMG Introduction Unstable rocker-bottom shoes pioneered by the Masai Barefoot Technology (MBT, Switzerland) feature an anterior-posterior curved rocker-bottom sole that has a drastic heel slope and a more graduate forefoot slope. These shoes were originally intended to simulate barefoot walking on uneven surfaces (Romkes et al. 2006). Proposed benefits of this type of footwear include a training of lower extremity muscles and joint proprioception (Nigg et al. 2006a, Romkes et al. 2006) and promoting a more upright posture during walking (Romkes et al. 2006). Its initial popularity in Europe has recently extended into North American markets and other parts of the world. Until recently, biomechanical studies on MBT and other rocker-bottom unstable shoes were scarce (Nigg et al. 2006a, Romkes et al. 2006). In the past few years, there have been an increased number of studies in the literature investigating joint kinematics, kinetics, and electromyography (EMG) of selected muscles and plantar pressure characteristics of MBT shoes in gait (Nigg et al. 2006a, Romkes et al. 2006, Nigg et al. 2010, van Engelen et al. 2010, Roberts et al. 2011, Buchecker et al. 2012). In addition, training benefits for knee osteoarthritis (Nigg et al. 2006b), and adaptation and variability in walking (Stoggl et al. 2010) and balance in unstable shoes have been examined (Stewart et al. 2007, Landry et al. 2010, Ramstrand et al. 2010, Turbanski et al. 2011). During walking, participants have been shown to have reduced cadence, stride time and length, step length, walking speed, and single support time, dorsiflexion at toe-off and, increased contact dorsiflexion angle in MBT compared to a control shoe at a self-selected speed (Romkes et al. 2006). Nigg et al. (2006a) also found a significantly more dorsiflexed ankle position in early stance in MBT shoes compared to a control shoe. However, they did not report other significant changes in ankle kinematic or kinetic patterns in MBT shoes compared to a control shoe. The lack of differences between MBT and a control shoe in ankle motion during level-walking is consistent *Corresponding author. szhang@utk.edu ISSN print/issn online ß 2012 Taylor & Francis

3 244 S. Zhang et al. with findings of other studies (Nigg et al. 2010, Roberts et al. 2011, Buchecker et al. 2012). Walking in MBT shoes has also been shown to reduce peak knee and hip flexion, peak hip extension, and knee and hip range of motion (ROM) (Romkes et al. 2006). In addition, it was shown that MBT reduced first peak knee external adduction moment but did not alter the peak external flexion moment of over-weight subjects wearing MBT compared to a control shoe (Buchecker et al. 2012). In terms of muscle activation, Nigg et al. (2006a) showed no significant differences in EMG intensity for tibialis anterior (TA) and biceps femoris (BF) when walking in MBT compared to control shoes. However, Romkes et al. (2006) showed increased TA activity for the whole swing phase whereas gastrocnemius activation increased from terminal swing to the initial contact. The authors suggested that the co-activation of these muscles coupled with the observed changes in ankle kinematics were possibly related to a need to stabilise the ankle in the unstable MBT shoes. Although several studies on MBT shoes have used ground reaction force (GRF) and centre of pressure (COP) data to compute other joint variables (e.g., moments), no GRF and COP data have been reported in the literature. Therefore, the effects of anteriorposterior unstable shoes on GRF and COP data during level-walking have not been established. Several studies have employed a self-selected walking speed to compare unstable and control shoes (Romkes et al. 2006, Buchecker et al. 2012). Romkes et al. (2006) reported a slower walking speed in MBT compared to control shoes. It is, hence, more difficult to compare biomechanical outcomes of the unstable shoe with a control shoe when walking speed is not controlled. In addition, control shoes used in previous studies were not standardised (Nigg et al. 2006a, Romkes et al. 2006, Buchecker et al. 2012). Furthermore, the majority of studies focused only on MBT shoes (Nigg et al. 2010) although several other brands of unstable shoes (e.g., Shape-Ups of Skechers, Easy Tone of Reebok, TrueBalance of New Balance, and Active Balance of Dockers) are available on the market. Very limited research evidence on biomechanical efficacy of other unstable shoe models is available. Due to recent applications of these unstable shoes in patient populations (e.g., knee osteoarthritis, foot problems and diabetes) it has become even more important to provide biomechanical data on the effects of other available unstable shoes during walking. Therefore, the objective of this study was to comprehensively evaluate COP, GRF, joint kinematics and kinetics, and EMG of selected muscles of the lower extremity joints of an unstable shoe compared to a standardised control shoe at controlled normal and fast speeds during walking. Due to the increased heel and forefoot slopes of the unstable shoes used in this study, we expected increased efforts of the lower extremity in the initial weight acceptance phase but decreased efforts during the push-off of the gait cycle. Thus, it was hypothesised that unstable shoes would yield greater initial peak vertical GRF, dorsiflexion at contact and a smaller ankle dorsiflexion moment during early stance, along with smaller push-off vertical GRF, smaller plantarflexion ROM and smaller peak plantarflexion moment during late stance compared to the control shoe. We also expected that the EMG of plantarflexors and dorsiflexors would also decrease in unstable shoes. Methods Participants Fifteen healthy males (Age: yrs; Height: m; Mass: kg) free of any injuries at the time of testing were recruited to participate in this study from the campus of the university. The participants provided written informed consent approved by the Institutional Review Board at The University of Tennessee prior to testing. Instrumentation A flat-bottom control shoe (Glacier, Genesco, Inc., Figure 1a) and an anterior-posterior unstable shoe designed primarily for men (Active Balance TM, Genesco, Inc., Figure 1b) were used in the current study. The slope for the heel and forefoot regions of the unstable and control shoes was quantified using the following equations: S FT ¼ L ft f 100 S HL ¼ L hl r 100 FT ¼ t HL ¼ h ð2þ f r where S FT : forefoot slope ratio, L FT : forefoot slope length, f: forefoot length, S HL : heel slope ratio, L HL : heel slope length, r: heel length, FT : forefoot slope angle, HL : heel slope angle, t: toe height, and h: heel height (Figure 1b). The mean heel slope ratio (S HL ) was 108.8% and 104.0% for testing and control shoes, respectively. The mean forefoot slope ratio (S FT ) was 105.5% and 102.9% for the same two shoes. The heel slope angle ( HL ) was 23.4 and 8.1 deg for testing and control shoes, respectively. The forefoot slope angle ( FT ) was 17.1 and 12.7 deg for the same two shoes. During walking trials, three-dimensional (3D) kinematics (240 Hz, Vicon Motion Analysis Inc., ð1þ

4 Footwear Science 245 Figure 1. Two shoes used in the study, (a) control shoe and (b) unstable shoe.

5 246 S. Zhang et al. Oxford, UK), ground reaction forces (1200 Hz, Advanced Mechanical Technology Inc., Watertown, MA, USA), and electromyography (EMG) activity of selected muscles (1200 Hz, MyoSystem 1400a, Noraxon USA, Inc., Scottsdale, AZ, USA) were collected simultaneously using the Vicon system. Before the experimental walking trials, the EMG electrodes were first applied to the long head of BF, rectus femoris (RF), medial gastrocnemius (MG), and TA according to existing guidelines for EMG electrode locations (Freriks et al. 1999, Konrad 2005). The electrode attachment sites were shaved, lightly abraded with a fine sand paper, and cleaned with an alcohol swab before application of the EMG electrodes. The inter-electrode distance was maintained at a fixed distance of 2.0 cm using dual electrodes. The anatomical markers were then attached to the trunk and pelvis and the right thigh, shank and foot to define joint centres during the static trials. Four tracking markers were attached to the inferior-posterior aspect of the trunk, posterior-lateral pelvis, and lateral thigh and shank via thermoplastic shells and neoprene wraps to track the segmental motions during the dynamic movement trials. Tracking markers for the foot were placed on the posterior, lateral and medial heel counter of the shoes (Zhang et al. 2006, Zhang et al. 2009). A separate static calibration trial was collected prior to the data capture of the dynamic movement trials for the unstable and control shoes. Experimental protocol Participants were first asked to walk in the unstable shoe for 10 minutes in an indoor hallway to become familiar with the shoe. For the control shoe, the acclimation period was during the practice trials prior to the testing. During the experimental testing, the participant performed five successful level walking trials in each of four testing conditions: control (SH1_N) and unstable (SH2_N) shoe at normal speed (1.3 m/s) and control (SH1_F) and unstable (SH2_F) shoe at fast speed (1.8 m/s). A walking trial was successful when the participant contacted the force platform without targeting and when the walking speed was within the desired ranges: (5%) m/s for normal speed or (5%) m/s for fast speed. The walking speed was monitored by two pairs of photo cells (63501IR, Lafayette Instrument Company, Lafayette, IN, USA) and an electronic timer (54035A, Lafayette Instrument Company, Lafayette, IN, USA). The shoe conditions were first randomised followed by the randomisation of the two walking speeds within each shoe condition. The averages of the five trials were used in the statistical analyses. Data analyses Visual3D software suite (C-Motion, Inc., Germantown, MD, USA) was used to obtain the 3D kinematic and kinetic computations of the trunk, and lower extremity joints and EMG data. An X-Y-Z Cardan rotation sequence was used to compute joint angles. The right-hand rule was used to determine the polarity of joint kinematics and kinetics. Therefore, a positive value indicates ankle dorsiflexion, knee extension and hip flexion in the sagittal-plane, and ankle inversion, and knee and hip adduction in the frontalplane. The COP displacements were expressed in the foot coordinate system that was projected onto the force platform surface. Positive values indicate medial and anterior COP displacements within the foot coordinate system. GRFs were normalised to body weight (BW) and internal joint moments were normalised to body mass (Nm/kg). Customised computer programs (VB_V3D and VB_Table, MS, Visual Basics) were used to determine critical events of the 3D kinematic and kinetic variables of interest interactively and organise the discrete critical events for statistical analyses. The EMG signals were band-pass filtered with cutoff frequencies of 20 and 450 Hz. The signals were fullwave rectified and smoothed with a root-mean-square (RMS) filter with a moving window of 70 milliseconds (ms). To normalise the EMG signals during walking trials, the maximum RMS value of walking trials at the normal speed in the control shoe was identified for each muscle of each participant. The EMG signals in all walking conditions were normalised to this maximum EMG value for each participant to obtain the normalised EMG signals. The normalised EMG signals were then integrated from heel-strike (HS) to toeoff (TO) during the stance phase to obtain the integrated EMG (IEMG) value for each muscle. A22 (Shoe x Speed) repeated measures analysis of variance (ANOVA) was used to detect differences between the two shoes and two speeds (18.0, SPSS, Chicago, IL, USA). Post hoc comparisons were performed using a paired t-test. An alpha level of 0.05 was set a priori. Results The main purpose of this study was to investigate differences in lower extremity biomechanical variables

6 Footwear Science 247 Table 1. Centre of pressure and ground reaction forces: mean STD. Control shoe Unstable shoe Variables Normal speed Fast speed Normal speed Fast speed ST (s)* # ML COP (m)* AP COP (m) &*# ! !$ F1_Z (BW) &# $ ! $ Loading Rate (BW/s)* # F2_Z (BW) & * # $ ! !$ BRK_Y (BW) & * # $ ! !$ PRP_Y (BW) # Note: ST: Stance Time, ML COP: Mediolateral COP displacement during stance, AP COP: Anteroposterior COP displacement during stance, F1_Z: Initial peak vertical GRF, F2_Z: Push-off peak vertical GRF, Loading Rate: Loading rate of F1_Z, BRK_Y: Peak braking peak GRF, PRP_Y: Peak propulsive GRF. & : significant interaction of shoe and speed, *: significant shoe main effect, # : significant speed main effect,! : significant difference between shoes at same speed in post hoc comparison; $ : significant difference between speeds for same shoe in post hoc comparison. between shoe conditions and only shoe main effects and shoe and speed interactions are presented. Centre of pressure and ground reaction force The stance time showed a shoe main effect (p ¼ 0.001, Table 1). It was shorter in the control compared to the unstable shoe. The average mediolateral (ML) and anteroposterior (AP) COP curves for both shoes at normal speed are provided in Figure 2. Results showed significantly greater ML COP displacement for the unstable shoe compared to the control shoe (p , Table 1). A significant shoe and speed interaction of AP COP displacement was found (p ¼ 0.022). The post hoc comparisons showed smaller AP COP displacement in the unstable shoe than the control shoe at both normal (p ) and fast (p ) speeds. However, the AP COP displacement was greater at fast speeds compared to normal speeds only for the unstable shoes (p ¼ 0.002). A significant shoe and speed interaction (p ¼ 0.001) was observed for the initial peak vertical GRF (Table 1). The post hoc comparison showed that the peak GRF was smaller for the unstable shoe at the normal speed than the control shoe (p ¼ 0.021). It was also greater at the faster speed in both the control (p ) and unstable (p ) shoes. The loading rate of this peak was significantly greater in the unstable shoe compared to the control shoe (p ¼ 0.001, Table 1). The push-off peak vertical GRF also showed a significant shoe and speed interaction (p ¼ 0.037). The post hoc comparison indicated smaller peaks at normal (p ¼ 0.008) and fast (p ) speeds for the unstable shoe compared to the control shoe. The peak was greater at fast speed compared to normal speed in both the control (p ) and unstable (p ) shoes with a greater increase in the unstable shoe. In addition, the peak AP braking GRF showed a significant shoe and speed interaction (p ¼ 0.011). The post hoc comparison indicated that it was greater at normal (p ¼ 0.027) and fast (p ) speeds in the unstable shoe compared to the control shoe. A greater peak at fast speed compared to normal speed was found in both the control (p ) and unstable (p ) shoes with a greater increase for the unstable shoe. However, no significant shoe and speed interaction and shoe effects were found for the peak propulsive GRF (Table 1). Joint kinematics The average sagittal-plane joint angle curves for both shoes at normal speed are provided in Figure 3. The ankle contact angle was more dorsiflexed in the unstable shoe compared to the control shoe (p ¼ 0.029) (Table 2). The plantarflexion ROM during early stance was smaller for the unstable shoe compared to the control shoe (p ). In addition, the total sagittal ankle ROM (heel strike to toe-off) was greater for the unstable shoe compared to the control shoe (p ) (Table 2). The frontal-plane kinematic results showed that the ankle eversion ROM was greater for the unstable shoe compared to the control shoe (p ¼ 0.016, Table 2). The hip joint abduction ROM was greater for the unstable shoe compared to the control shoe (p ¼ 0.001).

7 248 S. Zhang et al. Figure 2. Average centre of pressure (COP) curves during stance phase at normal speed for the control (solid line) and unstable (dot line) shoes: a) ML COP and b) AP COP. Joint kinetics The sagittal-plane joint kinetic results showed that the unstable shoe had smaller peak dorsiflexion (p ) and plantarflexion (p ¼ 0.01) moments compared to the control shoe (Table 3). The peak knee extension moment showed a significant shoe and speed interaction (p ¼ 0.002). The post hoc comparisons demonstrated that the unstable shoe had a greater knee extension moment compared to the control shoe (p ¼ 0.049) at the fast speed only. The peak knee extension moment was also greater at the fast speed compared to the normal speed in both control (p ) and unstable (p ) shoes. The peak knee flexion moment was smaller for the unstable shoe compared to the control shoe (p ). The average frontal-plane joint moment curves for both shoes at normal speed are provided in Figure 4. The ankle joint showed a greater peak inversion moment in the unstable shoes compared to control shoes (p ¼ 0.006, Table 3). The peak knee abduction moment was only greater at the fast walking speed compared to the normal speed (p ). The peak hip abduction moment was greater in the unstable shoe compared to the control shoe (p ¼ 0.001). Electromyography (EMG) The TA IEMG values were smaller in the unstable shoe compared to the control shoe (p ¼ 0.003, Table 4). The RF IEMG values were also smaller for the unstable shoe compared to the control shoe (p ¼ 0.005). Discussion The primary objective of this study was to compare COP, GRF, lower extremity joint kinematics and kinetics, and EMG during walking between the unstable shoe and the control shoe. The ML COP displacement was greater in the unstable shoe compared to the control shoe (Figure 2a), indicating greater ML displacement introduced by the unstable shoe. This greater COP movement may introduce ML instability wearing the shoe and may appear to be due to the softer midsole in the unstable shoe compared to the control shoe, although a material testing on midsole/ sole mechanical properties was not conducted. The unstable shoe has a 46.2% greater heel slope ratio and 187.8% greater heel slope angle compared to the control shoe. Therefore, the greater heel slope of the unstable shoe may cause more anterior heel contact and could therefore be responsible for the shorter

8 Footwear Science 249 Figure 3. Average sagittal-plane joint angle curves at normal speed during stance phase for the control (solid line) and unstable (dot line) shoes: (a) ankle, (b) knee and (c) hip. AP COP displacement during stance (Figure 2b). In addition, the greater heel slope may also be responsible for the greater ankle dorsiflexion angle at contact in the unstable shoes. We believe this greater heel slope facilitates anterior rolling of the foot during stance which may explain the smaller initial plantarflexion ROM in the unstable compared to control shoes. Although the initial peak vertical GRF was smaller in the unstable compared to the control shoes, the loading rate of the peak in the unstable shoes was greater than the control shoe. The increased loading rate may be also a result of the greater heel slope and the time needed to reach the peak vertical GRF. This finding was supported by the greater loading rates of the initial peak GRF seen when walking in a short-leg walker with different heel slopes added compared to normal street shoes (Adamczyk et al. 2006). An earlier transient vertical GRF peak was also observed during walking in short-leg walkers, which was not observed during walking in a control running shoe (Zhang et al. 2006, Keefer et al. 2008). Finally, the greater heel slope may also be responsible for the increased peak braking GRF in the unstable shoes compared to the control shoes as the sloped heel provides a contact surface that is at an angle to the contact surface during heel strike compared to the flatter heel surface of the control shoes. In addition to the increased contact dorsiflexion angle, early stance plantarflexion ROM was smaller in the unstable shoe compared to the control shoe. Again, these differences are likely related to the increased heel slope of the unstable shoe. These results are supported by previous findings on MBT shoes (Nigg et al. 2006a, Romkes et al. 2006). These changes were also reflected in the reduced initial peak ankle dorsiflexion moment. The reduced ankle dorsiflexion moment showed the reduced effort of the dorsiflexors to eccentrically control the initial plantarflexion of the ankle in the unstable shoe. A reduction in dorsiflexion moment in early stance may benefit individuals with reduced dorsiflexor capacity such as anterior compartment syndrome (Varelas et al. 1993) and tibial muscular dystrophy (Udd et al. 1998). In addition to the changes in ankle moments during early stance, a greater total ankle sagittal ROM in stance was observed for the unstable shoe. The greater contact time in unstable shoes may have allowed more time to perform total ankle ROM compared to control shoes. In addition, the increased total ankle ROM may be caused by the larger forefoot slope of the unstable shoe (increases of 25.3% of forefoot slope ratio and 34.8% of forefoot slope angle) and, greater contact dorsiflexion angle for the unstable shoe compared to the control shoe. This increased anterior slope of the unstable shoe seems to improve the rolling effect from mid-stance to toe-off to reduce the need for plantarflexion as reflected in the reduced peak plantarflexor moment, peak knee flexor moment, and peak propulsive vertical GRF in the unstable shoe during push-off. The reduced push-off peak GRF and ankle and knee concentric involvement in late stance suggest that the unstable shoes require less overall lower extremity effort compared to control shoes to maintain the same walking speed. This benefit may have implications in improving walking performance. Romkes et al. (2006) did not report any GRF and joint kinetic variables and their subjects walked at self-selected speeds. Nigg and his colleagues (2006a)

9 250 S. Zhang et al. Table 2. Sagittal- and frontal-plane joint kinematic variables: mean STD. Control shoe Unstable shoe Joints Variables Normal speed Fast speed Normal speed Fast speed Ankle CONT (deg)* # ROM_PF (deg)* # Max_DF (deg) # ROM_Total (deg)* # ROM_EV (deg)* Knee ROM_FL (deg) # ROM_ADD (deg) Hip ROM_EX (deg) # ROM_ADD (deg) *# Note: CONT: Sagittal plane ankle contact angle, ROM_PF: ROM from contact to peak plantarflexion, Max_DF: Peak ankle dorsiflexion, ROM_Total: Sagittal plane ankle ROM from contact to toe-off, ROM_EV: Eversion ROM from contact to peak eversion, ROM_FL: Knee flexion ROM from contact to peak flexion, ROM_ADD: Knee/hip adduction ROM from contact to peak adduction, ROM_EX: Hip extension ROM from contact to peak extension. *: significant shoe main effect, and # : significant speed main effect. Table 3. Peak sagittal- and frontal-plane joint moments: mean STD. Control shoe Unstable shoe Joints Variables Normal speed Fast speed Normal speed Fast speed Ankle MMT_DF (Nm/kg)* # MMT_PF (Nm/kg)* # MMT_INV (Nm/kg)* Knee MMT_EX (Nm/kg) &# $ !$ MMT_FL (Nm/kg)* # MMT_ABD (Nm/kg) # Hip MMT_EX (Nm/kg) # MMT_FL (Nm/kg) # MMMT_ABD (Nm/kg)* # Note: MMT_DF: peak ankle dorsiflexor moment in early stance, MMT_PF: peak ankle plantarflexor moment in late stance, MMT_INV: peak ankle inversion moment, MMT_EX: peak knee/hip extension moment, MMT_FL: peak knee/hip flexion moment, MMT_ABD: peak knee/hip abduction moment. & : significant interaction of shoe and speed, * : significant shoe main effect, and # : significant speed main effect,! : significant difference between shoes at same speed in post hoc comparison, $ : significant difference between speeds for same shoe in post hoc comparison. conducted one of the first studies on the MBT shoes and reported no differences between the MBT and control shoes in the angular impulses of the ankle, knee and hip, but did not examine peak joint moments. However, participants wore their own street shoes as the control shoe. Self-selected walking speeds and control shoes could introduce variability that is not related to the true biomechanical differences between the testing and control shoes. In addition to the changes in the sagittal-plane, the ankle joint showed greater eversion ROM, which may be related to the greater ML COP displacement in the unstable shoe compared to the control. The increased heel slope of the unstable shoe not only moved the heel contact point more anterior but also more laterally to increase the ML COP displacement (Figure 2a). Although material testing was not performed on the midsoles of the two shoes, the midsole of the unstable shoe appears to be slightly more compliant than that of the control shoe. A more compliant midsole may also cause a shift of the COP laterally at heel contact in the unstable shoe. Future studies should consider incorporating midsole material testing. This increased ML displacement appears to cause the increased ankle

10 Footwear Science 251 Figure 4. Average frontal-plane joint moment curves at normal speed during stance phase for the control (solid line) and unstable (dot line) shoes: (a) ankle, (b) knee and (c) hip. Table 4. Muscles IEMG values during stance: mean STD. Control shoe Normal speed Fast speed Unstable shoe Normal speed Fast speed TA (% s)* # MG (% s) RF (% s)* # BF (% s) Note: & : significant interaction of shoe and speed, * : significant shoe main effect, and # : significant speed main effect,! : significant difference between shoes at same speed in post hoc comparison, $ : significant difference between speeds for same shoe in post hoc comparison. inversion moment during early stance in the unstable shoe. Additionally, these increased frontal-plane variables at the ankle were accompanied by increased hip adduction ROM and abduction moment during stance. The significant changes found at the ankle and hip joints may be a coordinated and coupled effort to stabilise the body s centre of mass displacement in the frontal-plane. These results provide evidence that the unstable shoes may be useful in improving dynamic balance in walking due to the increased ML COP displacement and eversion ROM. Specifically, the increase ankle inversion and hip abduction moments may help increase frontal-plane ankle and hip stability. In fact, previous studies have shown that MBT shoes improve static (Stewart et al. 2007, Landry et al. 2010, Ramstrand et al. 2010, Turbanski et al. 2011) and dynamic (Stoggl et al. 2010, Turbanski et al. 2011) balance. Studies investigating frontal-plane ankle and hip muscle strength following an unstable shoe intervention may be needed to confirm the effect of such shoes on muscle strength and frontal-plane stability. The observed ankle and knee joint movement patterns in the unstable shoe are further supported by the activation patterns of the TA and RF muscles. The TA muscle had smaller IEMG values in the unstable shoe suggesting a reduced effort of the dorsiflexors in early stance. This is in agreement with the reduced initial ankle dorsiflexion moment and unchanged late plantarflexion moment for the unstable shoe during stance. The increased heel slope reduces the demand for the eccentric action of the dorsiflexors (i.e., TA) in early stance. The greater rolling effect of the anterior sloped sole may help reduce the need for the plantarflexor moment, which is reflected in the unchanged medial gastrocnemius activation and reduced TA activation. Romkes et al. (2006) also found a decreased TA activation in early stance but an overall increased TA activation for the MBT shoe. However, Nigg et al. (2006a) showed a trend of reduced TA activity in the MBT shoe during walking. For the activity of the RF, IEMG values were reduced in the unstable shoe compared to the control shoe. This result supports the reduced peak knee flexor moment in late stance for the unstable shoe during stance. Romkes et al. (2006) reported an increased RF activation in mid-stance but a reduced activation in stance-to-swing transition in the MBT. Furthermore, the unchanged muscle activation level of BF supports the result that the peak hip extensor moment in early stance was not different for the unstable shoe compared to the control shoe. This result is supported by the finding of unchanged semitendinosus activity in the MBT shoe (Romkes et al. 2006).

11 252 S. Zhang et al. Most previous studies on MBT shoes used only one walking speed (Nigg et al. 2006a, Romkes et al. 2006, van Engelen et al. 2010, Roberts et al. 2011, Buchecker et al. 2012). No study has examined effects of increased speed on kinematic and kinetic characteristics and EMG activity during walking in an unstable shoe. In addition to the standardised normal walking speed, our participants also walked at a fast speed of 1.8 m/s. The results from this study showed that the effect of the unstable shoe on most of the variables at the normal speed was maintained or increased at the fast speed. In addition, the knee flexion ROM, hip extension ROM, knee peak abduction moment, and peak hip extension and flexion moments were increased at the fast speed. These results suggest that an increase in speed may also amplify the effects of the unstable shoe on these knee and hip variables. However, the effects of these changes on joint variables in the long term are unknown. It is important to note that the participants in several previous MBT shoe studies had an acclimatisation period with the MBT shoes that lasted for two or more weeks before the biomechanical assessment was conducted (Nigg et al. 2006a, Romkes et al. 2006, Boyer and Andriacchi 2009). In the current study, the participants were asked to walk in the unstable shoe in a hallway for 10 minutes prior to the biomechanical assessment. It is not clear if the observed changes of GRF, kinematic and kinetic variables and EMG activities would persist if a longer acclimatisation period was used. It is possible that some of the changes seen after an acute acclimatisation may diminish or disappear. Further studies on the effects of acclimatisation in the unstable shoes on gait variables are warranted. Finally, the results from this study were obtained from only one brand of unstable shoes and thus, it is unknown whether the observed effects can be generalised to other brands/models. Conclusion The unstable shoe causes an increased ML COP displacement but a decreased AP COP displacement during walking. The unstable shoe also increased loading rate and reduced initial ankle plantarflexion ROM but increased total sagittal-plane ankle ROM. In addition, peak dorsiflexion and plantarflexion moments as well as the peak knee flexion moment were reduced in the unstable shoes. These results were further supported by the reduced EMG activation of the TA and RF muscles. The increased ML COP displacement was coupled with the increased peak inversion moment and peak hip abduction moment. These results suggest that the unstable shoe requires greater frontal-plane ankle and hip involvements in maintaining ML stability, which may have implications for potential training benefits. In addition, the demands for lower extremity muscles related to the sagittal-plane motions are reduced suggesting that walking in the unstable shoe may result in reduced loading to lower extremity joints compared to walking in the regular control shoe. However, greater frontal plane demands noted above must be considered. Therefore, future studies on the effects of unstable shoes should also focus on patients with other lower extremity problems. Acknowledgements The project was supported in part by funding from Genesco Inc. References Adamczyk, P.G., Collins, S.H., and Kuo, A.D., The advantages of a rolling foot in human walking. Journal of Experimental Biology, 209 (20), Boyer, K.A. and Andriacchi, T.P., Changes in running kinematics and kinetics in response to a rockered shoe intervention. Clinical Biomechanics (Bristol, Avon), 24 (10), Buchecker, M., et al., Lower extremity joint loading during level walking with Masai barefoot technology shoes in overweight males. Scandinavian Journal of Medicine and Science in Sports, 22 (3), Freriks, B., et al., The recommendations for sensors and sensor placement procedures for surface electromyography. In: H.J. Hermens, et al., eds. European recommendations for surface electromyography. Enschede, Netherlands: Roessingh Research and Development. Keefer, M., et al., Effects of modified short-leg walkers on ground reaction force characteristics. Clinical Biomechanics (Bristol, Avon), 23 (9), Konrad, P., Muscle Map. The ABC of EMG: A practical introduction to kinesiological electromyography. Scottsdale, AZ: Noraxon. Landry, S.C., Nigg, B.M., and Tecante, K.E., Standing in an unstable shoe increases postural sway and muscle activity of selected smaller extrinsic foot muscles. Gait & Posture, 32 (2), Nigg, B.M., Emery, C., and Hiemstra, L.A., 2006b. Unstable shoe construction and reduction of pain in osteoarthritis patients. Medicine & Science in Sports & Exercise, 38 (10), Nigg, B., Hintzen, S., and Ferber, R., 2006a. Effect of an unstable shoe construction on lower extremity gait characteristics. Clinical Biomechanics (Bristol, Avon), 21 (1),

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