In-Shoe Loading in Rearfoot and Non-Rearfoot Strikers during Running Using Minimalist Footwear

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1 Orthopedics & Biomechanics 1 In-Shoe Loading in Rearfoot and Non-Rearfoot Strikers during Running Using Minimalist Footwear Authors T. W. Kernozek 1, S. Meardon 2, C. N. Vannatta 1 Affiliation 1 Department of Health Professions, University of Wisconsin-La Crosse, La Crosse, WI, USA 2 Physical Therapy Department, East Carolina University, Greenville, NC, USA Key words pressure biomechanics kinetics metatarsal stress Abstract Recent trends promote a barefoot running style to reduce injury. Minimalist shoes are designed to mimic the barefoot running with some foot protection. However, it is unknown how minimalist shoes alter plantar loading. Our purpose was to compare plantar loads between rearfoot strikers and non-rearfoot strikers after 4 weeks of running in minimalist footwear. 30 females were provided Vibram Bikila shoes and instructed to gradually transition to running in these shoes. Plantar loading was measured using an in-shoe pressure sensor after the 4 weeks. Multivariate analysis was performed to detect differences in loading between rearfoot and nonrearfoot strikers in different plantar regions. Differences in plantar loading occurred between foot strike patterns running in minimalist footwear. Pressure and force variables were greater in the metatarsals and lower in the heel region in non-rearfoot strikers. Peak pressure for the whole foot was greater in non-rearfoot strikers while no difference was observed in maximum force or contact time for the whole foot between strike types. Allowing time for accommodation and adaptation to different stresses on the foot may be warranted when using minimalist footwear depending on foot strike pattern of the runner. accepted after revision February 10, 2014 Bibliography DOI /s Int J Sports Med 2014; 35: 1 6 Georg Thieme Verlag KG Stuttgart New York ISSN Introduction Running is a very common recreational activity that presents a considerable risk of injury. Van Gent et al. [36] reported that between % of runners sustained an injury in a given year. These injuries have largely persisted despite a greater understanding of biomechanics related to running injury and the advances of modern footwear designed to attenuate forces and modify lower extremity motions. According to Robbins and Hanna [29] the use of modern cushioned footwear may mask sensory feedback and thereby contribute to injury. In their later summary of the early work on lower extremity loading with and without footwear, they proposed that barefoot running or running in footwear that allows for adequate sensory feedback may minimize injury potential [28]. Other researchers have argued that in addition to improved sensory feedback, barefoot running reduces the peak vertical ground reaction force (pvgrf) during the initial impact phase of running. Lieberman et al. [21] reported that runners who naturally run barefoot and utilize a nonrearfoot strike running pattern experience a lower or even absent pvgrf during the initial stance phase of running when compared to traditionally shod runners who rearfoot strike. In addition, early work by Cavanagh and Lafortune [3] showed that rearfoot strikers displayed distinctly different ground reaction and center of pressure patterns using identical footwear. Together, this research suggests that a non-rearfoot strike pattern during running is associated with lower vertical loads in the early stance. Since greater pvgrf and rates of loading during the impact phase of running have been associated with injury [17], it has been postulated that utilizing a non-rearfoot strike, which may be facilitated with barefoot running, has the potential minimize injury occurrence. Runners display different foot strike patterns while running. It has been estimated that over 70 % of elite [15] and over 85 % of recreational runners [20] naturally utilize rearfoot strike patterns. The persistence of running related injury rates sustained despite the use of modern footwear coupled with recent publications [21, 23] has largely stimulated a growing interest in barefoot running, minimalistic footwear and strike Correspondence Dr. Thomas W. Kernozek Department of Health Professions University of Wisconsin-La Crosse 1300 Badger Street La Crosse, WI United States, Tel.: + 1/608/ Fax: + 1/608/ kernozek.thom@uwlax.edu patterns. Recently, Goss and Gross [11] reported

2 2 Orthopedics & Biomechanics that traditionally shod runners were 3.41 times more likely to report a running-related injury than individuals running in minimalistic footwear and that non-rearfoot strikers reported an annual 31 % incidence of running injury in one year compared to a 54 % incidence in rearfoot strikers. Daoud et al., [5] also reported runners using greater foot angle at contact and were 2.5 times more likely to report a mild to moderate repetitive stress injury in the lower extremity and low back. Interestingly, more than 50 % of the runners participating in Goss & Gross online survey of running-related injuries report changing their running shoe preference or foot strike pattern over a 12 month period [11]. Nearly every major running shoe manufacturer has now developed and marketed some type of minimalistic footwear targeted to the running public. Different models of minimalist footwear seem to target a different aspect of the barefoot running style, and each design may provide some benefit to the wearer [26]. In general, minimalistic shoes are designed to mimic the barefoot running experience by providing minimal cushioning across the entire plantar area, in particular the heel. These structures built into the shoe thought to modify natural foot motion with a reduced heel height, while maintaining some level of skin protection. Some of these designs have uppers similar to those of traditional athletic shoes while some lack lacing and have individual toe regions. Running in certain types of minimalistic footwear has resulted in kinematic and kinetic characteristics similar to barefoot running [22, 31]. However, some types of minimalistic footwear appears to result in subtle differences in many kinematic and kinetic variables [2]. Experienced runners, wearing minimalist footwear or running barefoot, reported using a more anterior foot strike pattern [11]. Due to the non-rearfoot strike pattern, loads to the mid and fore foot are likely to be different when running in minimalistic footwear. However, it is unknown if plantar loading differs between rearfoot strikers and non-rearfoot strikers with the use minimalist footwear. Therefore, our purpose was to compare plantar loads between rearfoot strikers and non-rearfoot strikers after a 4-week transition to running in minimalist footwear. Materials and Methods Over the period of 16-month period, we recruited 30 female recreational runners [23.1 years (SD 1.9), height 169 cm (SD 5.3), weight 61.7 kg (SD 1.9), average weekly mileage 36.4 km (SD 9.2)] interested in making the transition to minimalist footwear on a university campus. These runners self-reported running at least 24.1 km (15 mi) per week and had no history of running related injury 6 months prior to testing. Each participant provided informed consent prior to participation, and all methodology meets the ethical standards of the International Journal of Sports Medicine [14]. Fig. 1 Photo of the Vibram Bikila minimalistic shoe used in our investigation. Protocol On the first visit, runners were fitted with a pair of individually sized (size 38 or 40) Vibram Bikila (Vibram USA, Concord, MA, USA) minimalistic shoes ( Fig. 1). Next, each runner practiced for 5 min with the minimalistic shoes on a motorized treadmill (Cybex, CX445T, Boston, MA, USA) at her own pace. Participants were instructed to transition to minimalistic shoes slowly over a 4-week period by gradually increasing their mileage in the minimalistic shoes provided. They were also informed that it is not unusual for runners to report altered foot strike pattern and/or stride length with use of minimalistic shoes. A journal was provided for each runner in order to track compliance. Runners selfreported daily miles run and miles run with the minimalistic footwear. After a 4-week acclimation period, runners returned to the lab for assessment. Runners self-reported no injuries that stopped them from training during this period. There were 2 runners who dropped out of the study indicating that they lacked the time in their schedule to complete the training. All participants were weighed using a medical grade balance scale during the day of testing. A custom in-shoe sensor in an 82 sensor matrix configuration (Novel GMBH, Munich, Germany, Minneapolis/St. Paul, USA) was placed in their right shoe. A non-functional sensor was added to left shoe to accomodate for the small difference in height due to the approximate 2-mm changes that may occur in placing a sensor in one shoe. The sensor was sized to fit in the heel and extended slightly beyond the metatarsal heads. Due to the design of the minimalistic shoes studied, the toe areas of the shoes were not instrumented as this shoe has individual toes. This was not considered to be significant since the toe plantar regions have shown considerable variability during locomotor activities [18]. After a standardized 3-min treadmill warm-up, the Pedar X measurement system (Novel GMBH, Munich Germany, Minneapolis/St. Paul, USA) was used to sample this insole at 200 Hz, while each participant ran at a standardized speed (2.9 m/s or 6.5 mph) for approximately 60 s. Prior to testing, the 2-mm thick sensor insole was calibrated from 0 to 600 KPa in an air-bladder-based pressure chamber as described by Kernozek and Zimmer [19]. Calibration was verified prior to all testing sessions as described in the same investigation. Analysis Data from the first 30 steps of the testing session were extracted for analysis using Novel Scientific Analysis Software package (Novel GMBH, Munich Germany, Minneapolis/St. Paul, USA). To verify each runner s strike index during their treadmill running, we used the approach reported by Cavanagh and Lafortune [3]. Runners with an average center of pressure located in the posterior 33 % of a standard in-shoe sensor length (including the toe

3 Orthopedics & Biomechanics Fig. 2 Each runner s foot strike index was defined based on the average center of pressure (COP) location of 30 foot strikes during treadmill running. The box shows 33 % of a standard sensor length (24 cm). When the location of the COP at foot contact was within this region, the runner was classified as a rearfoot striker a. If the average COP at foot contact was outside this region they were classified as a non rearfoot striker b. region, which was omitted in the customized sensor) during foot contact were considered to be rearfoot strikers ( Fig. 2). Nonrearfoot strikers were defined by their center of pressure being in the remaining 66 % during foot contact. Data from the rearfoot and non-rearfoot strikers were further processed by masking the custom sensor into 5 regions corresponding to the heel, midfoot, medial forefoot, central forefoot and lateral forefoot regions ( Fig. 3). Mean total foot peak pressure (PP), peak force (PF) and contact time (CT) were calculated over 30 steps. In addition, regional PP, pressure time integral (PTI), PF and force time integral (FTI) were calculated for each region for 30 steps. PF and FTI data were normalized to each runner s body weight. A series of multivariate analysis of variance tests were used to examine the changes in these loading variables for each region independently using SPSS (Version 20, IBM, Armonk, NY). Alpha was set to Results By the end of the 4-week acclimation period, all runners were running their typical weekly mileage in the minimalistic shoes provided ( Table 1). 15 runners were confirmed as non-rearfoot strikers based on their foot strike index determined from in-shoe plantar loading data during treadmill running. Based on data from the overall plantar surface instrumented by the in-shoe sensor, total foot plantar loading was different between rearfoot strikers and non-rearfoot strikers (Wilks lambda = 0.04). Non-rearfoot strikers displayed nearly 25 % higher PP, while no differences occurred in maximum force normalized to body weight or contact time when compared to rearfoot strikers running in minimalist footwear. Fig. 3 Each runner s plantar loading pattern was divided into 5 regions: 1 was the heel, 2 was the midfoot region, 3 was the medial forefoot (MFF) region, 4 was the central forefoot (CFF) region and 5 was the lateral forefoot (LFF) region. Table 1 Mean percentage of mileage run (km) and standard deviation (SD) for rearfoot and non-rearfoot strikers during accommodation phase to minimalist footwear. All individuals ran at least 24.1 km (15 miles) per week. Rearfoot strikers Non-rearfoot strikers 22.1 % (2.5 %) 24.5 % (3.1 %) week % (3.1 %) 52.1 % (2.8 %) week % (2.9 %) 77.5 % (3.1 %) week % (1.5 %) 98.1 % (1.9 %) week (10.5) 34.6 (7.7) Mean (DS) total mileage (km) Plantar loading variables were different between rearfoot strikers and non-rearfoot strikers during treadmill running for all defined plantar regions (Wilks lambda < 0.05) ( Table 2). In the heel region, PP was 64 % greater, PTI was 54 % greater, PF was 67 % greater and FTI was 90 % greater for rearfoot strikers compared to non-rearfoot strikers. Smaller but significant changes were also seen across the midfoot and forefoot regions. For the midfoot, rearfoot strikers had a 23 % greater PP, 20 % greater PTI, 31 % 1

4 4 Orthopedics & Biomechanics Table 2 Mean and standard deviation (SD) for each plantar region of plantar loading variables for rearfoot strikers and non-rearfoot strikers during running in minimalist footwear. CFF = central forefoot, MFF = medial forefoot, LFF = lateral forefoot. PP (kpa) PF ( %BW) CT (ms) PTI (kpa*s) FTI (BW*s) FFS RFS FFS RFS FFS RFS FFS RFS FFS RFS heel (68.61)* (84.52) (17.84)* (23.25) (6.46)* (7.66) 2.12 (1.57)* 5.57 (2.09) midfoot (54.21) (62.73) (6.01)* (9.04) (6.40)* (5.75) 3.27 (0.47)* 4.65 (0.88) CFF (67.02)* (87.77) (8.22)* (6.64) (16.00)* (18.76) 8.76 (1.19)* 5.71 (1.39) MFF (43.17)* (59.51) (8.57)* (8.34) (14.29)* (14.46) 8.20 (1.47)* 7.02 (1.29) LFF (50.13)* (65.59) (8.11) (7.99) (19.88)* (13.96) 4.26 (1.16) 4.69 (0.97) (30.99)* (57.55) 186 (24) 188 (24) (20.59) (25.76) total foot (except toes) *Indicates a difference from the rearfoot strikers (p < 0.05) greater PF and 35 % greater FTI compared to non-rearfoot strikers. The loading in the 3 forefoot regions were generally greater for non-rearfoot strikers. Non-rearfoot strikers had a 32 % increase in PP, 44 % increase in PTI, and about a 42 % increase in PF and FTI for the medial forefoot when compared to rearfoot strikers. Central forefoot loading was also higher for non-rearfoot strikers where PP was greater by 28 %, PTI was greater by 37 %, PF was greater by 17 % and PTI was greater by 16 % in comparison. The lateral forefoot for non-rearfoot strikers was 15 % greater for PP, 14 % greater for PTI, but nearly 10 % less for PF and FTI when compared to rearfoot strikers. Discussion Plantar loading variables were different in all plantar regions of the foot between rearfoot and non-rearfoot strikers despite similar peak force and contact time obtained from the entire sensor during the stance phase of running in minimalist footwear. Expected increases in heel-loading were depicted for rearfoot strikers as compared to non-rearfoot strikers. In addition, metatarsal region loading was generally increased for non-rearfoot strikers when running in minimalist footwear. We did not observe any differences in contact time between rearfoot and non-rearfoot strikers, implying that cadence was similar between groups. Previous work has reported a decrease in contact time in barefoot runners who were assumed to have adopted a non-rearfoot strike pattern [6]. However, no studies were identified that specifically investigated differences between contact time between runners with different foot strike patterns. Peak force for the entire part of the foot instrumented was also similar between rearfoot and non-rearfoot strikers. Although the role of footwear and the attenuation of ground reaction forces remains a controversial issue, these results imply that subjects have similar total loading regardless of the foot strike type. Cavanagh and Lafortune [3] reported that average vertical ground reaction forces were similar between rear- and mid-foot strikers. Due to the reduced sampling rate of sensor insoles used in this study, we were incapable of measuring the heel strike transient typically seen in non-rearfoot strikers observed with force platform measures [3]. Nigg [27] theorized that heel-toe runners utilize impact forces as an input signal to muscles to pre-activate in order to control the soft tissue vibrations of the lower extremity due to impact loading. It has also been proposed that runners may have a preferred loading range and that they make subtle kinematic adjustments to their gait pattern to maintain their loading pattern within this preferred range [24, 28]. The results of our investigation appear to add some support to these ideas. Additionally, some authors have reported that although loading rates and plantar loading distributions may differ between types of footwear, running surfaces and with foot strike patterns, the overall management of loading is maintained largely by individual adaptations [6, 7, 8, 31]. Runners having a non-rearfoot strike pattern have been reported to reduce their passive loading while increasing their active loading [7, 21]. While further study appears warranted with respect to whether specifically such running changes are superior to others, it would appear that foot strike pattern may not be the sole contributor to overall peak loads, but will generally alter the distribution of regional loading. Running using a non-rearfoot strike may alter forefoot loading. Our in-shoe plantar loading data suggests that after an acclima-

5 Orthopedics & Biomechanics 5 tion period to minimalistic footwear, runners classified as nonrearfoot strikers sustained higher medial and central forefoot loads in terms of PP, PTI, PF and FTI compared to runners classified as rearfoot strikers. However, one variable, PP, was greater in the lateral forefoot region for non-rearfoot strikers in our investigation. The higher forefoot loads observed in the non-rearfoot runners may be relevant to metatarsal stress fractures. The apparent causes of metatarsal stress fracture include cyclical submaximal loading often associated with activities such as distance running [16]. Cadaveric simulations have reported that peak bone strain on the dorsal surface of the 2 nd metatarsal occurs during the push off phase of gait when Achilles tendon force and axial loading are at a maximum [9]. In addition using a mechanical model of the metatarsals, Gross and Bunch [13] found that the greatest bending strain and shear force during running occurs in the second metatarsal. The bending strain observed in these studies is likely due to combined bending and compressive loading occurring in the 2 nd metatarsal. Our data indicate that non-rearfoot strikers sustained higher metatarsal loading in the central forefoot region. This plantar region captures loads applied to the second and third metatarsals, which have been reported to account for % of the metatarsal fractures [4, 12, 33]. In military recruits, the use of boots with a more flexible outer sole has been associated with a greater incidence of 2 nd metatarsal stress fracture [1]. Salzer et al. [30] reported on 2 experienced runners who sustained metatarsal stress fractures after changing to minimalist footwear. It is possible that runners in the Salzer et al. [30] study had used a non-rearfoot strike pattern resulting in higher forefoot loading while using minimalist footwear. However, without prospective data, this cannot be affirmatively stated. Furthermore, studies are needed to determine if or how foot strike patterns are related to metatarsal stress fractures in runners. It is possible that non-rearfoot strikers utilize greater intrinsic plantar foot muscle activation to support the metatarsals. Ferris et al. [10] reported based on a cadaveric investigation, that there was an increase in strain measured at the second metatarsal with simulated toe plantar flexor fatigue. Similarly, Nagel et al. [25] showed the loading on the metatarsals increased while loads under the toes decreased during walking unshod after running a marathon. They suggested that the active contribution of the toes in a non-fatigued state may offload the metatarsals lowering the peak pressures. Likewise, less contribution of the toes when fatigued may contribute to greater plantar loads under the metatarsals. Unfortunately, in our investigation, the toes were not instrumented due to the nature of the minimalist shoes used and the shape of in-shoe pressure sensors, making the contribution of toe loading unknown. Therefore, further study surrounding these issues may be warranted. Data were collected in this study during the early portion of a treadmill run. Little is known about the stability of foot strike patterns and how loading may change during a distance run nor how running on a treadmill may have altered either the runner s chosen strike pattern or impact loads. Hasegawa et al. [15] reported that nearly 79 % of elite runners during a half marathon at the 15 km distance displayed a rearfoot strike pattern. Larson et al. [20] reported that 88 % of recreational and sub-elite runners at the 10 km mark of a marathon were rearfoot strikers. Interestingly, a greater frequency of rearfoot strikers was observed at the 32-km mark, suggesting that some runners may change their foot strike pattern to a rearfoot strike pattern with such a prolonged run. Even more interesting, they reported 55 of 936 runners displayed asymmetrical foot strike patterns and this percentage decreased at the further distance. Therefore, a runner s foot strike pattern may vary throughout a run resulting in changing forefoot and rearfoot loading. It has been shown that plantar loading may change during treadmill running based on the runner s state of fatigue. Willson and Kernozek [35] and Weist et al. [34] both showed a transition toward greater forefoot loading suggesting a more non-rearfoot strike pattern with fatigue. Both studies used similar methods of measuring plantar loading with similar data. Conflicting information from data collected in the field and treadmill running to fatigue make foot strike implications to injury difficult to assess. No attempt was made to account for foot type. It is possible that foot type may have an influence on regional plantar loading [4, 32]. It is unknown how foot type could have systematically influenced our findings and has yet to be explored in rearfoot and non-rearfoot strikers running in minimalist footwear. We cannot speculate how our accommodation timeframe may have altered the runner s technique since there was no data taken in these shoes at the study onset. Others have reported biomechanical and coordinative changes related to alterations in running technique during such a period [21, 28, 29]. It is possible that some runners may have not fully adapted their running style to the new running shoes or may have naturally transitioned to a non-rearfoot strike pattern that is typically associated with minimalistic footwear. Nonetheless, running in minimalistic shoes could impart novel loads to plantar aspects of the foot. Elevated mid and forefoot loads observed in forefoot runners should be considered foot structures when using minimalistic footwear especially in the metatarsal regions in runners using non-rearfoot strike patterns. Rearfoot strikers should also expect high heel loading. At a minimum, allowing time for accommodation and improved tolerance to these novel plantar loads during running is likely warranted when using minimalist footwear. Acknowledgements This work was supported by a University of Wisconsin-La Crosse Research Grant and Hi-Tech Funds by the State of Wisconsin. Conflict of interest statement: There are no personal or commercial relationships related to this work that would lead to a conflict of interest. References 1 Arndt A, Ekenman I, Westblad P, Lundberg A. Effects of fatigue and load variation on metatarsal deformation measured in vivo during barefoot walking. J Biomech 2002; 35: Bonacci J, Saunders PU, Hicks A, Rantalainen T, Vicenzino BGT, Spratford W. 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