- Email: [email protected]
Lower-limb dynamics and clinical outcomes for habitually shod runners who transition to barefoot running
Accepted Manuscript Lower-limb dynamics and clinical outcomes for habitually shod runners who transition to barefoot running Rami Hashish, Sachithra D. Samarawickrame, Susan Sigward, Stanley P. Azen, George J. Salem PII:
S1466-853X(16)30209-7
DOI:
10.1016/j.ptsp.2016.12.003
Reference:
YPTSP 792
To appear in:
Physical Therapy in Sport
Received Date: 12 August 2015 Revised Date:
17 November 2016
Accepted Date: 7 December 2016
Please cite this article as: Hashish, R., Samarawickrame, S.D., Sigward, S., Azen, S.P., Salem, G.J., Lower-limb dynamics and clinical outcomes for habitually shod runners who transition to barefoot running, Physical Therapy in Sports (2017), doi: 10.1016/j.ptsp.2016.12.003. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Lower-limb dynamics and clinical outcomes for habitually shod runners who transition to barefoot running Rami Hashish1, Sachithra D Samarawickrame1, Susan Sigward1, Stanley P Azen2, George J Salem1
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Author Affiliations: Division of Biokinesiology and Physical Therapy University of Southern California Los Angeles, CA Division of Preventive Medicine
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University of Southern California
Corresponding Author: Rami Hashish, PhD, DPT
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Los Angeles, CA
Division of Biokinesiology and Physical Therapy University of Southern California
1540 Alcazar Street, Los Angeles, CA 90033
Phone: 206.226.4402
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Email: [email protected]
Running Title: Transition to Barefoot Running
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Key Words: Footwear, Injuries, Energetics, Loading Rate, Ankle Acknowledgements: The authors would like to thank Kush Gaur for his assistance in data
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collection. This study was partially funded by a Southern California CTSI pilot grant (122176-4124).
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Abstract Recent investigations have revealed lower vertical loading rates and knee energy absorption amongst experienced barefoot runners relative to those who rear-foot strike (RFS). Although this has led to an adoption of barefoot running amongst many recreational shoe runners, recent
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investigations indicate that the experienced barefoot pattern is not immediately realized. Accordingly, we examined lower-extremity dynamics and clinical outcomes for 26 RFS shod runners who performed an 8-10 week transition to barefoot running. Foot-strike patterns, vertical load rates, and joint energetics were evaluated before and after the transition using inverse dynamics.
Clinical assessments were conducted throughout the transition by two licensed
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clinicians. Eighteen of the 26 runners successfully completed the transition: 7 maintained a RFS, 8 adopted a mid-foot strike (MFS), and 3 adopted a forefoot strike (FFS) during novice
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barefoot running. Following the transition, novice MFS/FFS runners often demonstrated reversions in strike-patterns and associated reductions in ankle energetics. We report no change in loading rates and knee energy absorption across transition time points. Importantly, there were no adverse events other than transient pain and soreness. These findings indicate that runners do not innately adopt the biomechanical characteristics thought to lower injury risk
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in-response to an uninstructed barefoot running transition.
ACCEPTED MANUSCRIPT 2|Page Introduction Upwards of 75% of habitual shoe rear-foot strike (RFS) runners sustain an annual overuse injury (Hespanohl, et al., 2013; van Gent, et al., 2007). Largely attributed to evolutionary adaptations in the human leg and foot, some researchers contend that Homo – the genus of
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hominids that includes the modern human (Homo sapiens) – may actually be predisposed to contact the ground with a mid-foot strike (MFS) or forefoot strike (FFS) (Bramble & Lieberman, 2004; Lieberman, et al., 2010). This is corroborated by recent investigations that indicate to a propensity to employ such a strike-pattern amongst experienced barefoot runners (Lieberman,
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et al., 2010; Squadrone & Gallozzi, 2009).
Nearly 90% of habitual shoe runners employ a RFS (Larson, et al., 2011). Similar findings have
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been reported for inexperienced runners from habitually barefoot populations (Hatala, et al., 2013; Pontzer, et al., 2014). Employing a RFS when barefoot exposes the calcaneus to the ground without a mechanism to soften the collision force. In contrast, employing a MFS or FFS allows for the posterior ankle muscles and tendons to slowly lower the body over the compliant ankle-foot complex. Possibly as a result of the altered impact pattern, barefoot FFS running is associated with lower vertical loading rates (Lieberman, et al., 2010; Shih, Lin, & Shiang, 2013), knee joint mechanical demand (Williams, et al., 2012), and knee contact forces (Kulmala, et al.,
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2013), relative to when rearefoot striking.
Recent investigations have demonstrated variability in foot-strike patterns amongst novice barefoot runners (Hamill, et al., 2011; Hashish, et al., 2015; Nunns, et al., 2013; Willy & Davis, Reports of higher vertical loading rates amongst novice RFS minimalist footwear
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2014).
runners indicates a heightened risk of lower-extremity injury (Willy & Davis, 2014). Corroborating this premise, a recent study conducted by Ryan and colleagues (2014) reported a
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higher incidence of injury amongst runners following a 12-week transition from traditionally shod to minimalist footwear running (Ryan, et al., 2014). This suggests habitual shoe runners do not innately employ a FFS and the biomechanical characteristics consistent with lower injury risk when acutely transitioning to barefoot or minimalist footwear running.
Researchers have attempted to examine the effects of transitioning to minimalist footwear on running dynamics. Three separate research groups have examined the influence of 4- (Warne, et al., 2013; Warne & Warrington, 2012), 6- (Lieberman, et al., 2010), and 12-week (McCarthy et al., 2013) transitions to Vibram FiveFingers on lower-extremity dynamics. Following the
ACCEPTED MANUSCRIPT 3|Page transitions, habitual shoe runners demonstrated a higher propensity to FFS and a more plantarflexed ankle at contact (Lieberman, et al., 2010; McCarthy et al., 2013; Warne, et al., 2013).
The aforementioned findings notwithstanding, these investigations examined changes between
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shod and minimally shod running, as opposed to pure barefoot running. However, recent examinations have demonstrated differences between running in minimalist footwear and purely barefoot (Bonacci, et al., 2013; Divert, et al., 2008; Squadrone & Gallozzi, 2009). Accordingly, the use of minimalist footwear as a surrogate for bare feet presents a potential source of confusion. In light of this, the current understanding of barefoot dynamics in response to the
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transition is limited. Furthermore, investigations that have examined clinical outcomes in response to the transition to minimalist footwear have either warned against the effects of
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rearfoot striking (McCarthy, et al., 2013), or did not examine lower-limb dynamics (Ridge, et al., 2013; Ryan, et al., 2014).
A previous investigation conducted by the authors examined changes in lower-extremity dynamics between shod and novice barefoot running (Hashish, et al., 2015). The purpose of the present investigation is to expand upon those findings and explore changes in lower-limb dynamics and pain scores for habitually shod runners who undergo an 8-10-week transition to
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pure barefoot running without instruction regarding foot-strike patterns. We hypothesized that in response to the transition, shod RFS runners would adopt a consistent FFS contact pattern, resulting in a shift in mechanical demand from the knee to the ankle and an attenuation of the vertical loading rate. Attributed to the change in loading dynamics, we expected favorable
Methods
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reports regarding pain from participants involved in the transition.
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An a priori power analysis was conducted using data from pilot work for this study. The pilot study consisted of three habitually shod RFS runners, all of whom adopted a FFS following an 8-week transition. Using the variable with the highest standard deviation (vertical loading rate), it was revealed that 7 subjects were required to adequately power the study (effect size = 1.08, α =0.05, β=0.20). To account for potential variability in foot-strike patterns, twenty-six (11 male, 15 female) recreational runners, who ran between 15-40 km.wk-1, participated in this investigation. Their mean age, height, weight and running distance were 26.3±3.6 years, 1.69±0.13 m, 64.9±12.7 kg, 22.4±6.2 km.wk-1, respectively. To qualify, each runner was required to be a habitually shod RFS runner (Cavanagh & Lafortune, 1980); between the age of 19 and
ACCEPTED MANUSCRIPT 4|Page 40 years; free of injury for the past 6 months; and have no previous experience in barefoot, or minimalist shoe running (including participation in barefoot sports). Minimalist footwear was operationally defined as shoes with a heel-to-toe drop less than or equal to 8mm. Participants were recruited from the University Health Sciences campus via posted paper fliers, and Informed consent was obtained from all participants on a format
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classroom presentations.
approved by the Institutional Review Board for the University Health Sciences Campus.
The protocol consisted of two visits to the University’s Research Laboratory. Prior to and following the transition, three-dimensional motion analysis data were collected using an 11Ground reaction forces were
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camera system (Qualisys, Gothenburg, Sweden) at 250 Hz.
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collected at 3000 Hz using a ground-embedded force platform (AMTI, Newton, MA). Design
During the initial laboratory visit, skin-mounted markers and tracking clusters were affixed to the pelvic, thigh, shank, and foot segments bilaterally. The foot was modeled with a dorsal tracking plate (Hashish, et al., 2014). Following instrumentation, a standing calibration trial was recorded. Each subject then completed 4-6 successful over-ground shod, and novice barefoot running trials in the laboratory at their self-selected speed.
The laboratory path was
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approximately 9 meters in length. The surface of the ground-embedded force platform was consistent with that of the running path. Timing gates were used to determine running velocity along this path. All successful running trial velocities were within a range of 5% of one another. No other instruction (e.g. foot-strike pattern, cadence, etc.) was provided. A successful trial was
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defined as a running trial in which the foot of the dominant limb was entirely on the force plate and running speed was within the stated range. After 8-10 weeks, the participants underwent
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the same protocol with the exception that shod running was not evaluated. Transition Protocol
Participants incrementally increased their weekly barefoot running percentage over a period of 8-10 weeks, until all running was performed barefoot. Participants were asked to maintain typical weekly running volume throughout the transition period and to perform all running on either a paved track or sidewalk. Participants began by performing 10% of weekly running volume barefoot during the first week. They incrementally increased weekly barefoot running percentage by 10-20% per week every week thereafter until all running was performed barefoot. Instruction regarding preferred foot-strike pattern was not provided. A successful transition was
ACCEPTED MANUSCRIPT 5|Page defined as the ability to complete the pre-transition weekly running volume, entirely barefoot, free of injury, in no more than 10 weeks. Compliance was monitored via a web-based tracker, in which the participants would log their daily running volume, and the percentage of which was
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performed barefoot.
Throughout the transition period, the PI and co-PI – a Doctor of Physical Therapy and Medical Doctor, respectively – contacted the participants on a bi-weekly basis. If participants reported musculoskeletal pain greater than or equal to 4/10 according to a numeric rating scale (indicating moderate pain), standard physical therapy assessments were conducted to deduce
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the cause and severity (Williams & Hoggart, 2004). The progressive nature of the transition was altered according to the runner’s presentation. Upon successful completion of the transition,
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participants completed the post-transition biomechanics testing.
Data Analysis
Three-dimensional marker coordinates during the running trials were reconstructed using Qualisys Track Manager Software (Qualisys, Gothenburg, Sweden). Visual 3D (C-motion, Rockville, MD) was used to process the raw coordinate data and compute segmental kinematics and kinetics for the dominant lower limb. Trajectory data were filtered with a fourth-order zero
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lag Butterworth 12 Hz low-pass filter. The local coordinate systems of the pelvis, thigh, shank, and foot were derived from the standing calibration trial. Joint kinematics were calculated using the joint coordinate system approach with a six-degree of freedom model. Net joint moments were calculated using inverse dynamics equations and were normalized to body mass. Joint
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powers were the product of the angular velocity and moment for each joint (Heiderscheit, et al., 2011). Initial contact was defined as the first instance when the vertical ground reaction force (GRF) exceeded 20 Newtons. The loading phase of running was operationally defined as initial
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contact to 13% of stance (Willy, et al., 2008). The absorption phase was operationally defined as initial contact to peak knee flexion (Heiderscheit, et al., 2011). All outcome variables were averaged over 4-6 successful trials.
Outcome Measures
Foot-Strike Patterns and Ankle Contact Angles: The length of the foot was determined from the standing calibration and was tracked during movement using a dorsal triad (Hashish, et al., 2014). Foot-strike patterns were categorized into a RFS, MFS, or FFS, according to the strike index method (Cavanagh & Lafortune, 1980). As
ACCEPTED MANUSCRIPT 6|Page indicated by a center of pressure analysis, initial contact made with the rear one-third of the foot was categorized as a RFS, initial contact made with the middle third was categorized as a MFS, and initial contact made with the anterior third was categorized as a FFS. This methodology is consistent with previous investigations that have examined foot-strike patterns in shod and
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barefoot runners (Hamill, et al., 2011; Hashish, et al., 2015). Ankle angle was also determined at initial contact.
Joint Energetics: Ankle and knee energy absorptions were determined by integrating the negative net joint power for that respective joint during the absorption phase. Ankle and knee
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energy generations were determined by integrating the positive net joint power for that
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respective joint during the stance phase (Derrick, et al., 1998; Heiderscheit, et al., 2011).
Vertical Loading Rate: The peak vertical loading rate was determined by taking the maximum, instantaneous derivative of the vertical GRF during the loading phase (Cheung & Rainbow, 2014). Statistical Analysis
Runners were grouped according to their novice barefoot foot-strike pattern (i.e. RFS, MFS, or
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FFS). A 3x2 Fixed-Effects Mixed Model ANOVA (strike-pattern [3] x transition [2]) was used to compare the change in outcome measures across strike-patterns and transition period (p ≤ 0.05). In the event of a significant F ratio, Bonferonni post hoc tests were used to analyze pairwise comparisons (p ≤ 0.05). When there was a significant interaction, or main effect of
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transition, post hoc comparisons using paired t-tests were conducted between pre- and posttransition conditions (p ≤ 0.05). When data violated Mauchley’s test of sphericity, a GreenhouseGeisser correction was used. P values between 0.05 and 0.10 were considered evidence of
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a statistical trend. Cohen’s d effect size (ES) are reported with the associated p value for the post hoc comparisons. All statistical calculations were conducted using PASW Version 18.0 (IBM Corporation; New York, USA).
Results
Eight of the 26 participants who completed baseline testing (i.e. novice barefoot running) did not complete the intervention. Three participants dropped-out due to scheduling conflicts and 5 were unable to adhere to the program (Table 1). No participants left the study because of injury. The mean age, height, weight, and running distance for the 18 participants (6 male, 12 female)
ACCEPTED MANUSCRIPT 7|Page included in the analysis were 25.2±2.9 years, 1.69±0.14 m, 62.4±12.3 kg, 23.2 ± 6.2 km.wk-1, respectively. There were 6 reported ‘adverse events,’ all of which were attributed to transient pain or soreness.
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Upon clinical assessment, 4 of the adverse events were attributed to delayed onset muscle soreness and thus no changes were made to the transition protocol. Delayed onset muscle soreness was defined according to the criteria put forth by the American College of Sports Medicine (Braun & Sforzo, 2011). All reported instances of delayed onset muscle soreness
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reduced to 3/10 pain or less within 3 days.
Two participants reported adverse events that warranted modifications. One participant
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presented with pain 8/10 due to blistering of the soft tissue along their metatarsal phalangeal joints, bilaterally. Following a 5-day suspension from running, this participant reported pain 3/10 on their dominant foot and 4.5/10 on their non-dominant foot. Following an additional 3-day suspension from running, pain resolved to 2/10 bilaterally; the participant was then progressed such that all running was performed barefoot within 10 weeks. Another participant reported pain 7/10 to their plantar fascia, bilaterally. Following a 3-day period of limited barefoot running volume (from 80% to 25%), pain resolved to 3/10 bilaterally. This participant was progressed
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and fully habituated within 8 weeks (Table 2). Weekly follow-ups with this participant for the remainder of the transition revealed pain less than 2/10 and a one-month follow-up revealed
Running Velocity
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that this participant was pain free.
There were no differences in running velocity between groups or running conditions (Table 3).
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Foot-Strike Patterns and Ankle Initial Contact Angles Of the 18 RFS shod runners, 3 adopted a FFS, 8 adopted a MFS, and 7 maintained a RFS during novice barefoot running. Following the transition, the 3 FFS runners reverted to a MFS. Of the 8 MFS runners, 6 maintained a MFS, and 2 reverted to a RFS following the transition. Of the 7 RFS runners, 6 maintained a RFS and 1 adopted a MFS. Results regarding ankle initial contact angles are presented in Table 3.
Joint Energetics
ACCEPTED MANUSCRIPT 8|Page There was a significant strike-pattern x transition interaction for ankle energy absorption (p=.012) and significant main effects of strike-pattern (p<.001) and transition (p=.001). There were also significant differences between FFS and RFS runners (132%; p<.001) and MFS and RFS runners (88%; p=.001). FFS and MFS runners demonstrated significant reductions in ankle | MFS: ∆= -1.9 ± 2.2 J.kg-1; ES=0.92; 1-β=1.0; p=0.044).
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energy absorption following the transition (FFS: ∆= -3.8 ± 1.3 J.kg-1; ES=3.07; 1-β=1.00; p=.034
There was a significant main effect of strike-pattern for knee energy absorption (p=0.003). RFS
p=.021) and MFS runners (40%; p=.005).
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runners presented with significantly higher knee energy absorption than FFS runners (44%;
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There was a significant strike-pattern x transition interaction for ankle energy generation (p=0.045) and a significant main effect of transition (p<0.001). There was also a non-significant trend towards an effect of strike-pattern (p=0.064). All groups of runners demonstrated significant reductions in ankle energy generation following the transition (FFS: ∆= -5.6 ± 1.4 J.kg; ES=3.95, 1-β=1.0; p=0.021 | MFS: ∆= -3.8 ± 1.7 J.kg-1; ES=2.26; 1-β=1.0; p<0.001 | RFS: ∆= -
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2.2 ± 2.1 J.kg-1; ES=1.04; 1-β=1.0; p=0.034). There were no significant differences in knee
Vertical Loading Rate
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energy generation (Fig. 1).
There was a significant main effect of strike-pattern for loading rate (p=0.019). RFS runners
2; Table 3).
Discussion
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presented with significantly higher vertical loading rates than FFS runners (166%; p=0.026) (Fig
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This is the first prospective study to examine the transition to pure barefoot running in habitually shod RFS runners. Importantly, no runner in this investigation presented with injury other than transient soft-tissue pain/soreness to the plantar fascia and/or soleus. Furthermore, no participant presented with pain (other than general calf muscle soreness) following the 8-10 week transition. The consistent anatomical location of soreness across participants indicates training effects – and plausibly, morphological adaptations – to the muscles and connective tissues that control and support the ankle and foot. Similarly, Hryviak et al (2014) reported that in a survey of 509 runners who tried barefoot running, most respondents either did not sustain new injuries, or reported diminished symptoms of pre-existing injuries (Hryvniak, et al., 2014). In
ACCEPTED MANUSCRIPT 9|Page contrast, Ridge et al (2013) reported an increase in foot bone marrow edema amongst recreational runners who transitioned to minimalist footwear running (Ridge, et al., 2013). In light of these cumulative findings, in order to prevent any possible adverse events, a slower progression, and/or introductory exercises that would prepare the ankle and foot for the altered
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mechanical demands is proposed.
An important finding from this investigation was that no runner demonstrated a FFS following the transition period. These findings are in contrast with studies that have examined transitions to minimalist footwear running. Lieberman et al (2010) reported significantly more plantar-flexion
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at initial contact and substantially more forefoot strikes in response to a 6-week transition (Lieberman, et al., 2010). Warne et al (2012) reported similar findings in response to a 4-week
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transition (Warne, et al., 2013). In contrast, McCarthy et al (2013) reported non-significant changes in ankle contact angles for female runners who underwent a 12-week transition (McCarthy, et al., 2013). The studies by Warne et al (2012) and McCarthy et al (2013) are fundamentally different from the present investigation as they provided guidance regarding technique. These findings nonetheless highlight the variability in how habitually shod runners respond to a transition to barefoot or minimalist footwear running. Furthermore, it appears that if
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a FFS is desired, explicit instruction may be necessary.
The reversion in foot-strike patterns following the 8-10 week transition period likely reduced the demand to the plantar-flexors, plantar aponeurosis, and Achilles tendon. This is consistent with our findings of reduced ankle, but increased knee absorption following the transition period We also found significant reductions in ankle energy generation
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amongst these runners.
amongst all strike-pattern groups following the transition. It is likely that runners reverted their strike-patterns, or continued to RFS, due to their preferred motor pattern (Nigg, 2001), or as a
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mechanism to offload the posterior compartment of their calf and foot. These findings have important clinical implications. Barefoot running is often promoted based on its associative reduction in knee mechanical demand (Bonacci, et al., 2014).
Yet, we demonstrate that
following a transition period, runners may shift mechanical demand from their ankle to their knee, potentially increasing the likelihood of injury at this joint. This reversion may be the result of soreness in the plantar fascia and soleus reported by participants in this study. It is thus plausible that continuing to barefoot run could result in significant injury to the lower-extremity. This indeed is supported by the findings of Ryan et (2014) who reported a higher incidence of
ACCEPTED MANUSCRIPT 10 | P a g e injury amongst minimalist shoe runners, relative to those who ran in standard running shoes over a 12-week transition (Ryan, et al., 2014).
Contrary to our hypotheses, there was no significant difference in vertical loading rates between
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transition time points. We also clearly demonstrate that runners do not inherently (i.e. without instruction) adopt a FFS and a more plantar-flexed ankle at initial contact. These findings oppose the premise that running unshod will result in the adoption of a more forefoot contact pattern and a reduction in loading rate (Lieberman, et al., 2010). Furthermore, there does not appear to be a pattern between clinical outcome measures and calculated vertical load rates.
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Accordingly, we encourage further research to investigate the contributing patterns to high
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loading rates, as well as the influence of such patterns on actual injury risk.
We recognize limitations of this study. The 8-10 week transition may have been insufficient in duration to alter movement patterns, explaining the variability in lower-extremity dynamics amongst participants. Similarly, the running path was short in length (approximately 9 meters), likely hindering the ability to achieve steady-state running mechanics. This may in part explain the lack of change in strike patterns across transition time points. Furthermore, without a control group, we cannot definitively conclude whether the stated changes were attributed to the
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transition. The FFS group was also quite small, limiting the generalizability of these findings. It is also important to note that 8 of the 26 runners who completed the baseline testing did not complete the transition. Reasons for dropout included discomfort when managing street surfaces, and a general dislike for barefoot running (due to it being “inconvenient” or “different”).
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Whilst runners who presented with pain scores ≤ 3 (out of 10) were allowed to participate during the transition period, it is plausible that injuries could have developed if they continued to run
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long-term. The aforementioned findings should be interpreted accordingly.
Conclusion
The present study indicated variability in foot-strike patterns and stance phase dynamics amongst habitually shod runners who perform an 8-10 week uninstructed transition to barefoot running. The transition resulted in a reduced reliance upon the ankle regardless of preferred foot-strike pattern. Furthermore, runners demonstrated an inability to reduce vertical load rates and knee energy absorption following the transition period. These findings indicate that performing a barefoot transition without explicit instruction by a clinician and/or trainer may be deleterious. This may explain the high attrition rate amongst participants in this study, and
ACCEPTED MANUSCRIPT 11 | P a g e contributed to the fact that 4 of the 18 runners presented with moderate pain at some point during the transition. Long-term prospective studies are required in order to determine whether a slower, guided progression, and/or introductory exercises that prepare the ankle and foot for
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Disclosure statement No potential conflicts of interest are reported by the authors.
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the altered mechanical demands help to alleviate injury potential.
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Larson, P., Higgins, E., Kaminski, J., Decker, T., Preble, J., Lyons, D., McIntyre, K., & Normile, A. (2011). Foot strike patterns of recreational and sub-elite runners in a long-distance road race. J Sports Sci, 29, 1665-1673. Lieberman, D. E., Venkadesan, M., Werbel, W. A., Daoud, A. I., D'Andrea, S., Davis, I. S., Mang'eni, R. O., & Pitsiladis, Y. (2010). Foot strike patterns and collision forces in habitually barefoot versus shod runners. Nature, 463, 531-535. McCarthy, C., Fleming, N., Donne, B., & Blanksby, B. (2013). 12 Weeks of Simulated Barefoot Running Changes Foot-Strike Patterns in Female Runners. Int J Sports Med, 35, 443-450. Nigg, B. M. (2001). The role of impact forces and foot pronation: a new paradigm. Clin J Sport Med, 11, 2-9. Nunns, M., House, C., Fallowfield, J., Allsopp, A., & Dixon, S. (2013). Biomechanical characteristics of barefoot footstrike modalities. J Biomech, 46, 2603-2610. Pontzer, H., Suchman, K., Raichlen, D. A., Wood, B. M., Mabulla, A. Z. P., & Marlowe, F. W. (2014). Foot strike patterns and hind limb joint angles during running in Hadza huntergatherers JSHS, 3, 95-101. Ridge, S. T., Johnson, A. W., Mitchell, U. H., Hunter, I., Robinson, E., Rich, B. S., & Brown, S. D. (2013). Foot bone marrow edema after a 10-wk transition to minimalist running shoes. Med Sci Sports Exerc, 45, 1363-1368. Ryan, M., Elashi, M., Newsham-West, R., & Taunton, J. (2014). Examining injury risk and pain perception in runners using minimalist footwear. Br J Sports Med, 48, 1257-1262. Shih, Y., Lin, K. L., & Shiang, T. Y. (2013). Is the foot striking pattern more important than barefoot or shod conditions in running? Gait Posture, 28, 490-494. Squadrone, R., & Gallozzi, C. (2009). Biomechanical and physiological comparison of barefoot and two shod conditions in experienced barefoot runners. J Sports Med Phys Fitness, 49, 6-13. van Gent, R. N., Siem, D., van Middelkoop, M., van Os, A. G., Bierma-Zeinstra, S. M., & Koes, B. W. (2007). Incidence and determinants of lower extremity running injuries in long distance runners: a systematic review. Br J Sports Med, 41, 469-480; discussion 480. Warne, J. P., Kilduff, S. M., Gregan, B. C., Nevill, A. M., Moran, K. A., & Warrington, G. D. (2013). A 4-week instructed minimalist running transition and gait-retraining changes plantar pressure and force. Scand J Med Sci Sports, 24, 964-973. Warne, J. P., & Warrington, G. D. (2012). Four-week habituation to simulated barefoot running improves running economy when compared with shod running. Scand J Med Sci Sports, 24, 563-568. Williams, A., & Hoggart, B. (2004). Pain: a review of three commonly used pain rating scales. Issues in Clinical Nursing, 14, 798-804. Williams, D. S., 3rd, Green, D. H., & Wurzinger, B. (2012). Changes in lower extremity movement and power absorption during forefoot striking and barefoot running. Int J Sports Phys Ther, 7, 525-532. Willy, R., Pohl, M. B., & Davis, I. S. (2008). Calculation of vertical load rates in the absence of vertical impact peaks. In 32nd American Society of Biomechanics. Ann Arbor, MI. University of Michigan Willy, R. W., & Davis, I. S. (2014). Kinematic and kinetic comparison of running in standard and minimalist shoes. Med Sci Sports Exerc, 46, 318-323.
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Acknowledgements
The authors would like to thank Kush Gaur for his assistance in data collection. This study was
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partially funded by a Southern California CTSI pilot grant (12-2176-4124).
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Table 1.
19 20 21 22 23 24 25 26
n/a
RFS
n/a
MFS MFS MFS MFS MFS MFS MFS MFS
10 8 10 10 10 8 9 9
MFS
n/a
MFS
n/a
MFS
n/a
MFS
n/a
FFS FFS FFS
9 10 8
FFS FFS
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RFS
Reason for Attrition
n/a
n/a
o
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Adherence: Did not adhere to transition protocol 2 to school conflicts. o Adherence: Did not adhere to transition protocol 2 to distaste for barefoot running.
o
Adherence: Did not adhere to transition protocol 2 to marathon training. Schedule Conflict: Unable to come in for follow-up o testing within 10 weeks 2 to work conflicts. Schedule Conflict: Unable to come in for follow-up o testing within 10 weeks 2 to school conflicts. Schedule Conflict: Unable to come in for follow-up o testing within 10 weeks 2 to school conflicts.
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10 11 12 13 14 15 16 17 18
Transition Duration (Weeks) 9 10 10 9 8 9 9
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Strike-Pattern RFS RFS RFS RFS RFS RFS RFS
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Participant 1 2 3 4 5 6 7 8
o
Adherence: Did not adhere to transition protocol 2 to work conflicts. o Adherence: Did not adhere to transition protocol 2 to distaste for barefoot running.
Table 1. Transition duration and reason for attrition (when applicable), for the 26 participants initially enrolled in the transition (i.e. completed baseline testing). Eight of the 26 participants did not complete the transition and were excluded from analysis.
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StrikePattern RFS
Week 5
Barefoot Volume (% of total) 60%
NRS Score 4/10
7
80%
4/10
Pain Description Soreness of dominant plantar fascia Soreness of dominant soleus Blistering of soft tissue along bilateral metatarsal heads
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Participant 3
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Table 2.
Clinical Response Attributed to delayed onset muscle soreness. Continued transition. Attributed to delayed onset muscle soreness. Continued transition. Following 5-day suspension from running, participant progressed to 50% the following week. Reported of pain 3/10 on dominant foot and 4.5/10 on nondominant foot. Pain resolved (to 2/10, bilaterally) following 3 days of rest. Attributed to delayed onset muscle soreness. Continued transition.
RFS
4
40%
8/10
5
RFS
6
70%
4/10
Soreness of non-dominant soleus
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MFS
5
53%
4/10
Soreness of solei
Attributed to delayed onset muscle soreness. Continued transition.
7
77%
7/10
Soreness of bilateral plantar fascia
Following 3-day period of limited barefoot running volume (25% of total). Reported of soreness, 3/10, bilaterally. Participant progressed to 100% the following week.
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Table 2. Adverse events for those who reported musculoskeletal-related NRS scores greater than or equal to 4/10 during the transition period.
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RFS (n=7)
. -1
Loading Rate (BW s )
Habituated
Novice
Habituated
Novice
Habituated
3.70±0.47
3.75±0.45
3.85±0.47
3.78±0.51
3.93±0.38
3.89±0.45
6.7±3.4
6.7±3.1
-4.7±3.5
-3.0±6.2
-8.9±1.3
-2.8±3.1
s,t,fr,mr
210.0±57.4
182.3±47.5
125.1±88.2
128.7±62.9
62.6±25.9
85.9±34.7
s, fr
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Ankle IC Angle ( )
FFS (n=3)
Novice
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. -1
Velocity (m s )
MFS (n=8)
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Table 3.
Table 3. Running velocities, ankle initial contact angles, loading rates and arch indices, of barefoot runners, during novice and habituated barefoot running. Statistics were performed based off the novice barefoot foot-strike pattern. Following the transition, there were 10 RFS runners, 8 MFS runners, and 0 FFS runners.
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main effect of strike-pattern. main effect of transition. difference between FFS and RFS runners. difference between MFS and RFS runners. within group difference.
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Indicates a significant Indicates a significant Indicates a significant Indicates a significant Indicates a significant
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(s) (t) (fr) (mr) (*)
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Fig. 1 A.
Energy Absorption Ankle
Knee
30
20 J/kg
*
*
Novice
Experienced RFS
Novice
a
B.
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a
Experienced FFS
Energy Generation Ankle
*
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Knee
#
#
*
*
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J/kg
20
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30
Novice
Experienced
RFS
Novice a
Experienced MFS
Novice a
Experienced FFS
Figure 1. Joint energy (A.) absorption and (B.) generation for barefoot runners during novice and experienced barefoot running. (*) Indicates a significant within group difference for ankle energy absorption and generation, respectively. (#) Indicates a significant within group difference for total energy absorption and generation, respectively.
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Fig. 2
Vertical Ground Reaction Force 3
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2.5
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2
1.5
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Body Weights
RFS MFS FFS
1
0
0.04
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Figure 2. Representative vertical ground reaction force profiles for a novice barefoot RFS (dashed line), MFS (dotted line), and FFS (solid line) runner. There was a significant main effect of strike-pattern for vertical loading rate (p=0.19) and a significant difference between FFS and RFS runners (166%; p=0.022). Vertical loading rate was quantified as the peak rate of the vertical ground reaction force profile from initial contact to 13% of stance.
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Highlights
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Performing an 8-10 week transition to barefoot running may result in transient pain
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• •
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Runners do not innately adopt a forefoot strike following an 8-10 week barefoot transition Runners demonstrate reductions in ankle energetics following an 8-10 week barefoot transition Runners do no demonstrate changes in knee absorption following an 8-10 week barefoot transition
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Ethical Statement
Informed consent for this study was obtained from all participants on a format approved by the
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Institutional Review Board for the University of Southern California (USC) Health Sciences
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Campus.
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Funding Statement:
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This study was partially funded by a CTSI pilot grant.
S1466-853X(16)30209-7
DOI:
10.1016/j.ptsp.2016.12.003
Reference:
YPTSP 792
To appear in:
Physical Therapy in Sport
Received Date: 12 August 2015 Revised Date:
17 November 2016
Accepted Date: 7 December 2016
Please cite this article as: Hashish, R., Samarawickrame, S.D., Sigward, S., Azen, S.P., Salem, G.J., Lower-limb dynamics and clinical outcomes for habitually shod runners who transition to barefoot running, Physical Therapy in Sports (2017), doi: 10.1016/j.ptsp.2016.12.003. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Lower-limb dynamics and clinical outcomes for habitually shod runners who transition to barefoot running Rami Hashish1, Sachithra D Samarawickrame1, Susan Sigward1, Stanley P Azen2, George J Salem1
1
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Author Affiliations: Division of Biokinesiology and Physical Therapy University of Southern California Los Angeles, CA Division of Preventive Medicine
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University of Southern California
Corresponding Author: Rami Hashish, PhD, DPT
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Los Angeles, CA
Division of Biokinesiology and Physical Therapy University of Southern California
1540 Alcazar Street, Los Angeles, CA 90033
Phone: 206.226.4402
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Email: [email protected]
Running Title: Transition to Barefoot Running
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Key Words: Footwear, Injuries, Energetics, Loading Rate, Ankle Acknowledgements: The authors would like to thank Kush Gaur for his assistance in data
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collection. This study was partially funded by a Southern California CTSI pilot grant (122176-4124).
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Abstract Recent investigations have revealed lower vertical loading rates and knee energy absorption amongst experienced barefoot runners relative to those who rear-foot strike (RFS). Although this has led to an adoption of barefoot running amongst many recreational shoe runners, recent
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investigations indicate that the experienced barefoot pattern is not immediately realized. Accordingly, we examined lower-extremity dynamics and clinical outcomes for 26 RFS shod runners who performed an 8-10 week transition to barefoot running. Foot-strike patterns, vertical load rates, and joint energetics were evaluated before and after the transition using inverse dynamics.
Clinical assessments were conducted throughout the transition by two licensed
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clinicians. Eighteen of the 26 runners successfully completed the transition: 7 maintained a RFS, 8 adopted a mid-foot strike (MFS), and 3 adopted a forefoot strike (FFS) during novice
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barefoot running. Following the transition, novice MFS/FFS runners often demonstrated reversions in strike-patterns and associated reductions in ankle energetics. We report no change in loading rates and knee energy absorption across transition time points. Importantly, there were no adverse events other than transient pain and soreness. These findings indicate that runners do not innately adopt the biomechanical characteristics thought to lower injury risk
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in-response to an uninstructed barefoot running transition.
ACCEPTED MANUSCRIPT 2|Page Introduction Upwards of 75% of habitual shoe rear-foot strike (RFS) runners sustain an annual overuse injury (Hespanohl, et al., 2013; van Gent, et al., 2007). Largely attributed to evolutionary adaptations in the human leg and foot, some researchers contend that Homo – the genus of
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hominids that includes the modern human (Homo sapiens) – may actually be predisposed to contact the ground with a mid-foot strike (MFS) or forefoot strike (FFS) (Bramble & Lieberman, 2004; Lieberman, et al., 2010). This is corroborated by recent investigations that indicate to a propensity to employ such a strike-pattern amongst experienced barefoot runners (Lieberman,
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et al., 2010; Squadrone & Gallozzi, 2009).
Nearly 90% of habitual shoe runners employ a RFS (Larson, et al., 2011). Similar findings have
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been reported for inexperienced runners from habitually barefoot populations (Hatala, et al., 2013; Pontzer, et al., 2014). Employing a RFS when barefoot exposes the calcaneus to the ground without a mechanism to soften the collision force. In contrast, employing a MFS or FFS allows for the posterior ankle muscles and tendons to slowly lower the body over the compliant ankle-foot complex. Possibly as a result of the altered impact pattern, barefoot FFS running is associated with lower vertical loading rates (Lieberman, et al., 2010; Shih, Lin, & Shiang, 2013), knee joint mechanical demand (Williams, et al., 2012), and knee contact forces (Kulmala, et al.,
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2013), relative to when rearefoot striking.
Recent investigations have demonstrated variability in foot-strike patterns amongst novice barefoot runners (Hamill, et al., 2011; Hashish, et al., 2015; Nunns, et al., 2013; Willy & Davis, Reports of higher vertical loading rates amongst novice RFS minimalist footwear
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2014).
runners indicates a heightened risk of lower-extremity injury (Willy & Davis, 2014). Corroborating this premise, a recent study conducted by Ryan and colleagues (2014) reported a
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higher incidence of injury amongst runners following a 12-week transition from traditionally shod to minimalist footwear running (Ryan, et al., 2014). This suggests habitual shoe runners do not innately employ a FFS and the biomechanical characteristics consistent with lower injury risk when acutely transitioning to barefoot or minimalist footwear running.
Researchers have attempted to examine the effects of transitioning to minimalist footwear on running dynamics. Three separate research groups have examined the influence of 4- (Warne, et al., 2013; Warne & Warrington, 2012), 6- (Lieberman, et al., 2010), and 12-week (McCarthy et al., 2013) transitions to Vibram FiveFingers on lower-extremity dynamics. Following the
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The aforementioned findings notwithstanding, these investigations examined changes between
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shod and minimally shod running, as opposed to pure barefoot running. However, recent examinations have demonstrated differences between running in minimalist footwear and purely barefoot (Bonacci, et al., 2013; Divert, et al., 2008; Squadrone & Gallozzi, 2009). Accordingly, the use of minimalist footwear as a surrogate for bare feet presents a potential source of confusion. In light of this, the current understanding of barefoot dynamics in response to the
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transition is limited. Furthermore, investigations that have examined clinical outcomes in response to the transition to minimalist footwear have either warned against the effects of
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rearfoot striking (McCarthy, et al., 2013), or did not examine lower-limb dynamics (Ridge, et al., 2013; Ryan, et al., 2014).
A previous investigation conducted by the authors examined changes in lower-extremity dynamics between shod and novice barefoot running (Hashish, et al., 2015). The purpose of the present investigation is to expand upon those findings and explore changes in lower-limb dynamics and pain scores for habitually shod runners who undergo an 8-10-week transition to
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pure barefoot running without instruction regarding foot-strike patterns. We hypothesized that in response to the transition, shod RFS runners would adopt a consistent FFS contact pattern, resulting in a shift in mechanical demand from the knee to the ankle and an attenuation of the vertical loading rate. Attributed to the change in loading dynamics, we expected favorable
Methods
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reports regarding pain from participants involved in the transition.
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An a priori power analysis was conducted using data from pilot work for this study. The pilot study consisted of three habitually shod RFS runners, all of whom adopted a FFS following an 8-week transition. Using the variable with the highest standard deviation (vertical loading rate), it was revealed that 7 subjects were required to adequately power the study (effect size = 1.08, α =0.05, β=0.20). To account for potential variability in foot-strike patterns, twenty-six (11 male, 15 female) recreational runners, who ran between 15-40 km.wk-1, participated in this investigation. Their mean age, height, weight and running distance were 26.3±3.6 years, 1.69±0.13 m, 64.9±12.7 kg, 22.4±6.2 km.wk-1, respectively. To qualify, each runner was required to be a habitually shod RFS runner (Cavanagh & Lafortune, 1980); between the age of 19 and
ACCEPTED MANUSCRIPT 4|Page 40 years; free of injury for the past 6 months; and have no previous experience in barefoot, or minimalist shoe running (including participation in barefoot sports). Minimalist footwear was operationally defined as shoes with a heel-to-toe drop less than or equal to 8mm. Participants were recruited from the University Health Sciences campus via posted paper fliers, and Informed consent was obtained from all participants on a format
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classroom presentations.
approved by the Institutional Review Board for the University Health Sciences Campus.
The protocol consisted of two visits to the University’s Research Laboratory. Prior to and following the transition, three-dimensional motion analysis data were collected using an 11Ground reaction forces were
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camera system (Qualisys, Gothenburg, Sweden) at 250 Hz.
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collected at 3000 Hz using a ground-embedded force platform (AMTI, Newton, MA). Design
During the initial laboratory visit, skin-mounted markers and tracking clusters were affixed to the pelvic, thigh, shank, and foot segments bilaterally. The foot was modeled with a dorsal tracking plate (Hashish, et al., 2014). Following instrumentation, a standing calibration trial was recorded. Each subject then completed 4-6 successful over-ground shod, and novice barefoot running trials in the laboratory at their self-selected speed.
The laboratory path was
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approximately 9 meters in length. The surface of the ground-embedded force platform was consistent with that of the running path. Timing gates were used to determine running velocity along this path. All successful running trial velocities were within a range of 5% of one another. No other instruction (e.g. foot-strike pattern, cadence, etc.) was provided. A successful trial was
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defined as a running trial in which the foot of the dominant limb was entirely on the force plate and running speed was within the stated range. After 8-10 weeks, the participants underwent
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the same protocol with the exception that shod running was not evaluated. Transition Protocol
Participants incrementally increased their weekly barefoot running percentage over a period of 8-10 weeks, until all running was performed barefoot. Participants were asked to maintain typical weekly running volume throughout the transition period and to perform all running on either a paved track or sidewalk. Participants began by performing 10% of weekly running volume barefoot during the first week. They incrementally increased weekly barefoot running percentage by 10-20% per week every week thereafter until all running was performed barefoot. Instruction regarding preferred foot-strike pattern was not provided. A successful transition was
ACCEPTED MANUSCRIPT 5|Page defined as the ability to complete the pre-transition weekly running volume, entirely barefoot, free of injury, in no more than 10 weeks. Compliance was monitored via a web-based tracker, in which the participants would log their daily running volume, and the percentage of which was
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performed barefoot.
Throughout the transition period, the PI and co-PI – a Doctor of Physical Therapy and Medical Doctor, respectively – contacted the participants on a bi-weekly basis. If participants reported musculoskeletal pain greater than or equal to 4/10 according to a numeric rating scale (indicating moderate pain), standard physical therapy assessments were conducted to deduce
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the cause and severity (Williams & Hoggart, 2004). The progressive nature of the transition was altered according to the runner’s presentation. Upon successful completion of the transition,
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participants completed the post-transition biomechanics testing.
Data Analysis
Three-dimensional marker coordinates during the running trials were reconstructed using Qualisys Track Manager Software (Qualisys, Gothenburg, Sweden). Visual 3D (C-motion, Rockville, MD) was used to process the raw coordinate data and compute segmental kinematics and kinetics for the dominant lower limb. Trajectory data were filtered with a fourth-order zero
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lag Butterworth 12 Hz low-pass filter. The local coordinate systems of the pelvis, thigh, shank, and foot were derived from the standing calibration trial. Joint kinematics were calculated using the joint coordinate system approach with a six-degree of freedom model. Net joint moments were calculated using inverse dynamics equations and were normalized to body mass. Joint
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powers were the product of the angular velocity and moment for each joint (Heiderscheit, et al., 2011). Initial contact was defined as the first instance when the vertical ground reaction force (GRF) exceeded 20 Newtons. The loading phase of running was operationally defined as initial
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contact to 13% of stance (Willy, et al., 2008). The absorption phase was operationally defined as initial contact to peak knee flexion (Heiderscheit, et al., 2011). All outcome variables were averaged over 4-6 successful trials.
Outcome Measures
Foot-Strike Patterns and Ankle Contact Angles: The length of the foot was determined from the standing calibration and was tracked during movement using a dorsal triad (Hashish, et al., 2014). Foot-strike patterns were categorized into a RFS, MFS, or FFS, according to the strike index method (Cavanagh & Lafortune, 1980). As
ACCEPTED MANUSCRIPT 6|Page indicated by a center of pressure analysis, initial contact made with the rear one-third of the foot was categorized as a RFS, initial contact made with the middle third was categorized as a MFS, and initial contact made with the anterior third was categorized as a FFS. This methodology is consistent with previous investigations that have examined foot-strike patterns in shod and
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barefoot runners (Hamill, et al., 2011; Hashish, et al., 2015). Ankle angle was also determined at initial contact.
Joint Energetics: Ankle and knee energy absorptions were determined by integrating the negative net joint power for that respective joint during the absorption phase. Ankle and knee
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energy generations were determined by integrating the positive net joint power for that
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respective joint during the stance phase (Derrick, et al., 1998; Heiderscheit, et al., 2011).
Vertical Loading Rate: The peak vertical loading rate was determined by taking the maximum, instantaneous derivative of the vertical GRF during the loading phase (Cheung & Rainbow, 2014). Statistical Analysis
Runners were grouped according to their novice barefoot foot-strike pattern (i.e. RFS, MFS, or
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FFS). A 3x2 Fixed-Effects Mixed Model ANOVA (strike-pattern [3] x transition [2]) was used to compare the change in outcome measures across strike-patterns and transition period (p ≤ 0.05). In the event of a significant F ratio, Bonferonni post hoc tests were used to analyze pairwise comparisons (p ≤ 0.05). When there was a significant interaction, or main effect of
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transition, post hoc comparisons using paired t-tests were conducted between pre- and posttransition conditions (p ≤ 0.05). When data violated Mauchley’s test of sphericity, a GreenhouseGeisser correction was used. P values between 0.05 and 0.10 were considered evidence of
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a statistical trend. Cohen’s d effect size (ES) are reported with the associated p value for the post hoc comparisons. All statistical calculations were conducted using PASW Version 18.0 (IBM Corporation; New York, USA).
Results
Eight of the 26 participants who completed baseline testing (i.e. novice barefoot running) did not complete the intervention. Three participants dropped-out due to scheduling conflicts and 5 were unable to adhere to the program (Table 1). No participants left the study because of injury. The mean age, height, weight, and running distance for the 18 participants (6 male, 12 female)
ACCEPTED MANUSCRIPT 7|Page included in the analysis were 25.2±2.9 years, 1.69±0.14 m, 62.4±12.3 kg, 23.2 ± 6.2 km.wk-1, respectively. There were 6 reported ‘adverse events,’ all of which were attributed to transient pain or soreness.
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Upon clinical assessment, 4 of the adverse events were attributed to delayed onset muscle soreness and thus no changes were made to the transition protocol. Delayed onset muscle soreness was defined according to the criteria put forth by the American College of Sports Medicine (Braun & Sforzo, 2011). All reported instances of delayed onset muscle soreness
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reduced to 3/10 pain or less within 3 days.
Two participants reported adverse events that warranted modifications. One participant
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presented with pain 8/10 due to blistering of the soft tissue along their metatarsal phalangeal joints, bilaterally. Following a 5-day suspension from running, this participant reported pain 3/10 on their dominant foot and 4.5/10 on their non-dominant foot. Following an additional 3-day suspension from running, pain resolved to 2/10 bilaterally; the participant was then progressed such that all running was performed barefoot within 10 weeks. Another participant reported pain 7/10 to their plantar fascia, bilaterally. Following a 3-day period of limited barefoot running volume (from 80% to 25%), pain resolved to 3/10 bilaterally. This participant was progressed
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and fully habituated within 8 weeks (Table 2). Weekly follow-ups with this participant for the remainder of the transition revealed pain less than 2/10 and a one-month follow-up revealed
Running Velocity
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that this participant was pain free.
There were no differences in running velocity between groups or running conditions (Table 3).
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Foot-Strike Patterns and Ankle Initial Contact Angles Of the 18 RFS shod runners, 3 adopted a FFS, 8 adopted a MFS, and 7 maintained a RFS during novice barefoot running. Following the transition, the 3 FFS runners reverted to a MFS. Of the 8 MFS runners, 6 maintained a MFS, and 2 reverted to a RFS following the transition. Of the 7 RFS runners, 6 maintained a RFS and 1 adopted a MFS. Results regarding ankle initial contact angles are presented in Table 3.
Joint Energetics
ACCEPTED MANUSCRIPT 8|Page There was a significant strike-pattern x transition interaction for ankle energy absorption (p=.012) and significant main effects of strike-pattern (p<.001) and transition (p=.001). There were also significant differences between FFS and RFS runners (132%; p<.001) and MFS and RFS runners (88%; p=.001). FFS and MFS runners demonstrated significant reductions in ankle | MFS: ∆= -1.9 ± 2.2 J.kg-1; ES=0.92; 1-β=1.0; p=0.044).
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energy absorption following the transition (FFS: ∆= -3.8 ± 1.3 J.kg-1; ES=3.07; 1-β=1.00; p=.034
There was a significant main effect of strike-pattern for knee energy absorption (p=0.003). RFS
p=.021) and MFS runners (40%; p=.005).
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runners presented with significantly higher knee energy absorption than FFS runners (44%;
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There was a significant strike-pattern x transition interaction for ankle energy generation (p=0.045) and a significant main effect of transition (p<0.001). There was also a non-significant trend towards an effect of strike-pattern (p=0.064). All groups of runners demonstrated significant reductions in ankle energy generation following the transition (FFS: ∆= -5.6 ± 1.4 J.kg; ES=3.95, 1-β=1.0; p=0.021 | MFS: ∆= -3.8 ± 1.7 J.kg-1; ES=2.26; 1-β=1.0; p<0.001 | RFS: ∆= -
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2.2 ± 2.1 J.kg-1; ES=1.04; 1-β=1.0; p=0.034). There were no significant differences in knee
Vertical Loading Rate
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energy generation (Fig. 1).
There was a significant main effect of strike-pattern for loading rate (p=0.019). RFS runners
2; Table 3).
Discussion
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presented with significantly higher vertical loading rates than FFS runners (166%; p=0.026) (Fig
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This is the first prospective study to examine the transition to pure barefoot running in habitually shod RFS runners. Importantly, no runner in this investigation presented with injury other than transient soft-tissue pain/soreness to the plantar fascia and/or soleus. Furthermore, no participant presented with pain (other than general calf muscle soreness) following the 8-10 week transition. The consistent anatomical location of soreness across participants indicates training effects – and plausibly, morphological adaptations – to the muscles and connective tissues that control and support the ankle and foot. Similarly, Hryviak et al (2014) reported that in a survey of 509 runners who tried barefoot running, most respondents either did not sustain new injuries, or reported diminished symptoms of pre-existing injuries (Hryvniak, et al., 2014). In
ACCEPTED MANUSCRIPT 9|Page contrast, Ridge et al (2013) reported an increase in foot bone marrow edema amongst recreational runners who transitioned to minimalist footwear running (Ridge, et al., 2013). In light of these cumulative findings, in order to prevent any possible adverse events, a slower progression, and/or introductory exercises that would prepare the ankle and foot for the altered
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mechanical demands is proposed.
An important finding from this investigation was that no runner demonstrated a FFS following the transition period. These findings are in contrast with studies that have examined transitions to minimalist footwear running. Lieberman et al (2010) reported significantly more plantar-flexion
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at initial contact and substantially more forefoot strikes in response to a 6-week transition (Lieberman, et al., 2010). Warne et al (2012) reported similar findings in response to a 4-week
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transition (Warne, et al., 2013). In contrast, McCarthy et al (2013) reported non-significant changes in ankle contact angles for female runners who underwent a 12-week transition (McCarthy, et al., 2013). The studies by Warne et al (2012) and McCarthy et al (2013) are fundamentally different from the present investigation as they provided guidance regarding technique. These findings nonetheless highlight the variability in how habitually shod runners respond to a transition to barefoot or minimalist footwear running. Furthermore, it appears that if
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a FFS is desired, explicit instruction may be necessary.
The reversion in foot-strike patterns following the 8-10 week transition period likely reduced the demand to the plantar-flexors, plantar aponeurosis, and Achilles tendon. This is consistent with our findings of reduced ankle, but increased knee absorption following the transition period We also found significant reductions in ankle energy generation
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amongst these runners.
amongst all strike-pattern groups following the transition. It is likely that runners reverted their strike-patterns, or continued to RFS, due to their preferred motor pattern (Nigg, 2001), or as a
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mechanism to offload the posterior compartment of their calf and foot. These findings have important clinical implications. Barefoot running is often promoted based on its associative reduction in knee mechanical demand (Bonacci, et al., 2014).
Yet, we demonstrate that
following a transition period, runners may shift mechanical demand from their ankle to their knee, potentially increasing the likelihood of injury at this joint. This reversion may be the result of soreness in the plantar fascia and soleus reported by participants in this study. It is thus plausible that continuing to barefoot run could result in significant injury to the lower-extremity. This indeed is supported by the findings of Ryan et (2014) who reported a higher incidence of
ACCEPTED MANUSCRIPT 10 | P a g e injury amongst minimalist shoe runners, relative to those who ran in standard running shoes over a 12-week transition (Ryan, et al., 2014).
Contrary to our hypotheses, there was no significant difference in vertical loading rates between
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transition time points. We also clearly demonstrate that runners do not inherently (i.e. without instruction) adopt a FFS and a more plantar-flexed ankle at initial contact. These findings oppose the premise that running unshod will result in the adoption of a more forefoot contact pattern and a reduction in loading rate (Lieberman, et al., 2010). Furthermore, there does not appear to be a pattern between clinical outcome measures and calculated vertical load rates.
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Accordingly, we encourage further research to investigate the contributing patterns to high
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loading rates, as well as the influence of such patterns on actual injury risk.
We recognize limitations of this study. The 8-10 week transition may have been insufficient in duration to alter movement patterns, explaining the variability in lower-extremity dynamics amongst participants. Similarly, the running path was short in length (approximately 9 meters), likely hindering the ability to achieve steady-state running mechanics. This may in part explain the lack of change in strike patterns across transition time points. Furthermore, without a control group, we cannot definitively conclude whether the stated changes were attributed to the
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transition. The FFS group was also quite small, limiting the generalizability of these findings. It is also important to note that 8 of the 26 runners who completed the baseline testing did not complete the transition. Reasons for dropout included discomfort when managing street surfaces, and a general dislike for barefoot running (due to it being “inconvenient” or “different”).
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Whilst runners who presented with pain scores ≤ 3 (out of 10) were allowed to participate during the transition period, it is plausible that injuries could have developed if they continued to run
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long-term. The aforementioned findings should be interpreted accordingly.
Conclusion
The present study indicated variability in foot-strike patterns and stance phase dynamics amongst habitually shod runners who perform an 8-10 week uninstructed transition to barefoot running. The transition resulted in a reduced reliance upon the ankle regardless of preferred foot-strike pattern. Furthermore, runners demonstrated an inability to reduce vertical load rates and knee energy absorption following the transition period. These findings indicate that performing a barefoot transition without explicit instruction by a clinician and/or trainer may be deleterious. This may explain the high attrition rate amongst participants in this study, and
ACCEPTED MANUSCRIPT 11 | P a g e contributed to the fact that 4 of the 18 runners presented with moderate pain at some point during the transition. Long-term prospective studies are required in order to determine whether a slower, guided progression, and/or introductory exercises that prepare the ankle and foot for
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Disclosure statement No potential conflicts of interest are reported by the authors.
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the altered mechanical demands help to alleviate injury potential.
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References
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Bonacci, J., Saunders, P. U., Hicks, A., Rantalainen, T., Vicenzino, B. G., & Spratford, W. (2013). Running in a minimalist and lightweight shoe is not the same as running barefoot: a biomechanical study. Br J Sports Med, 47, 387-392. Bonacci, J., Vicenzino, B., Spratford, W., & Collins, P. (2014). Take your shoes off to reduce patellofemoral joint stress during running. British Journal of Sports Medicine, 48, 425428. Bramble, D. M., & Lieberman, D. E. (2004). Endurance running and the evolution of Homo. Nature, 432, 345-352. Braun, W., & Sforzo, G. (2011). ACSM Information on: Delayed Onset Muscle Soreness (DOMS). In ACSM. Cavanagh, P. R., & Lafortune, M. A. (1980). Ground reaction forces in distance running. J Biomech, 13, 397-406. Cheung, R. T., & Rainbow, M. J. (2014). Landing pattern and vertical loading rates during first attempt of barefoot running in habitual shod runners. Hum Mov Sci, 34, 120-127. Derrick, T. R., Hamill, J., & Caldwell, G. E. (1998). Energy absorption of impacts during running at various stride lengths. Med Sci Sports Exerc, 30, 128-135. Divert, C., Mornieux, G., Freychat, P., Baly, L., Mayer, F., & Belli, A. (2008). Barefoot-shod running differences: shoe or mass effect? Int J Sports Med, 29, 512-518. Hamill, J., Russell, E. M., Gruber, A. H., & Miller, R. (2011). Impact Characteristics in Shod and Barefoot Running. Footwear Science, 3, 33-40. Hashish, R., Samarawickrame, S. D., Powers, C. M., & Salem, G. J. (2015). Lower limb dynamics vary in shod runners who acutely transition to barefoot running. Journal of Biomechanics, 49, 284-288. Hashish, R., Samarawickrame, S. D., & Salem, G. J. (2014). A comparison of dorsal and heel plate foot tracking methods on lower extremity dynamics. J Biomech, 47, 1211-1214. Hatala, K. G., Dingwall, H. L., Wunderlich, R. E., & Richmond, B. G. (2013). Variation in foot strike patterns during running among habitually barefoot populations. PLoS One, 8, e52548. Heiderscheit, B. C., Chumanov, E. S., Michalski, M. P., Wille, C. M., & Ryan, M. B. (2011). Effects of step rate manipulation on joint mechanics during running. Med Sci Sports Exerc, 43, 296-302. Hespanohl Junior, L., Pena Costa, L., & Lopes, A. (2013). Previous injuries and some training characteristics predict running-related injuries in recreational runners: a prospective cohort study. J Physiother, 59, 263-269. Hryvniak, D., Dicharry, J., & Wilder, R. (2014). Barefoot running survey: Evidence from the field JSHS, 3, 131-136. Kulmala, J. P., Avela, J., Pasanen, K., & Parkkari, J. (2013). Forefoot strikers exhibit lower running-induced knee loading than rearfoot strikers. Med Sci Sports Exerc, 45, 23062313.
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Larson, P., Higgins, E., Kaminski, J., Decker, T., Preble, J., Lyons, D., McIntyre, K., & Normile, A. (2011). Foot strike patterns of recreational and sub-elite runners in a long-distance road race. J Sports Sci, 29, 1665-1673. Lieberman, D. E., Venkadesan, M., Werbel, W. A., Daoud, A. I., D'Andrea, S., Davis, I. S., Mang'eni, R. O., & Pitsiladis, Y. (2010). Foot strike patterns and collision forces in habitually barefoot versus shod runners. Nature, 463, 531-535. McCarthy, C., Fleming, N., Donne, B., & Blanksby, B. (2013). 12 Weeks of Simulated Barefoot Running Changes Foot-Strike Patterns in Female Runners. Int J Sports Med, 35, 443-450. Nigg, B. M. (2001). The role of impact forces and foot pronation: a new paradigm. Clin J Sport Med, 11, 2-9. Nunns, M., House, C., Fallowfield, J., Allsopp, A., & Dixon, S. (2013). Biomechanical characteristics of barefoot footstrike modalities. J Biomech, 46, 2603-2610. Pontzer, H., Suchman, K., Raichlen, D. A., Wood, B. M., Mabulla, A. Z. P., & Marlowe, F. W. (2014). Foot strike patterns and hind limb joint angles during running in Hadza huntergatherers JSHS, 3, 95-101. Ridge, S. T., Johnson, A. W., Mitchell, U. H., Hunter, I., Robinson, E., Rich, B. S., & Brown, S. D. (2013). Foot bone marrow edema after a 10-wk transition to minimalist running shoes. Med Sci Sports Exerc, 45, 1363-1368. Ryan, M., Elashi, M., Newsham-West, R., & Taunton, J. (2014). Examining injury risk and pain perception in runners using minimalist footwear. Br J Sports Med, 48, 1257-1262. Shih, Y., Lin, K. L., & Shiang, T. Y. (2013). Is the foot striking pattern more important than barefoot or shod conditions in running? Gait Posture, 28, 490-494. Squadrone, R., & Gallozzi, C. (2009). Biomechanical and physiological comparison of barefoot and two shod conditions in experienced barefoot runners. J Sports Med Phys Fitness, 49, 6-13. van Gent, R. N., Siem, D., van Middelkoop, M., van Os, A. G., Bierma-Zeinstra, S. M., & Koes, B. W. (2007). Incidence and determinants of lower extremity running injuries in long distance runners: a systematic review. Br J Sports Med, 41, 469-480; discussion 480. Warne, J. P., Kilduff, S. M., Gregan, B. C., Nevill, A. M., Moran, K. A., & Warrington, G. D. (2013). A 4-week instructed minimalist running transition and gait-retraining changes plantar pressure and force. Scand J Med Sci Sports, 24, 964-973. Warne, J. P., & Warrington, G. D. (2012). Four-week habituation to simulated barefoot running improves running economy when compared with shod running. Scand J Med Sci Sports, 24, 563-568. Williams, A., & Hoggart, B. (2004). Pain: a review of three commonly used pain rating scales. Issues in Clinical Nursing, 14, 798-804. Williams, D. S., 3rd, Green, D. H., & Wurzinger, B. (2012). Changes in lower extremity movement and power absorption during forefoot striking and barefoot running. Int J Sports Phys Ther, 7, 525-532. Willy, R., Pohl, M. B., & Davis, I. S. (2008). Calculation of vertical load rates in the absence of vertical impact peaks. In 32nd American Society of Biomechanics. Ann Arbor, MI. University of Michigan Willy, R. W., & Davis, I. S. (2014). Kinematic and kinetic comparison of running in standard and minimalist shoes. Med Sci Sports Exerc, 46, 318-323.
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Acknowledgements
The authors would like to thank Kush Gaur for his assistance in data collection. This study was
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partially funded by a Southern California CTSI pilot grant (12-2176-4124).
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Table 1.
19 20 21 22 23 24 25 26
n/a
RFS
n/a
MFS MFS MFS MFS MFS MFS MFS MFS
10 8 10 10 10 8 9 9
MFS
n/a
MFS
n/a
MFS
n/a
MFS
n/a
FFS FFS FFS
9 10 8
FFS FFS
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RFS
Reason for Attrition
n/a
n/a
o
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Adherence: Did not adhere to transition protocol 2 to school conflicts. o Adherence: Did not adhere to transition protocol 2 to distaste for barefoot running.
o
Adherence: Did not adhere to transition protocol 2 to marathon training. Schedule Conflict: Unable to come in for follow-up o testing within 10 weeks 2 to work conflicts. Schedule Conflict: Unable to come in for follow-up o testing within 10 weeks 2 to school conflicts. Schedule Conflict: Unable to come in for follow-up o testing within 10 weeks 2 to school conflicts.
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10 11 12 13 14 15 16 17 18
Transition Duration (Weeks) 9 10 10 9 8 9 9
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9
Strike-Pattern RFS RFS RFS RFS RFS RFS RFS
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Participant 1 2 3 4 5 6 7 8
o
Adherence: Did not adhere to transition protocol 2 to work conflicts. o Adherence: Did not adhere to transition protocol 2 to distaste for barefoot running.
Table 1. Transition duration and reason for attrition (when applicable), for the 26 participants initially enrolled in the transition (i.e. completed baseline testing). Eight of the 26 participants did not complete the transition and were excluded from analysis.
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StrikePattern RFS
Week 5
Barefoot Volume (% of total) 60%
NRS Score 4/10
7
80%
4/10
Pain Description Soreness of dominant plantar fascia Soreness of dominant soleus Blistering of soft tissue along bilateral metatarsal heads
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Participant 3
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Table 2.
Clinical Response Attributed to delayed onset muscle soreness. Continued transition. Attributed to delayed onset muscle soreness. Continued transition. Following 5-day suspension from running, participant progressed to 50% the following week. Reported of pain 3/10 on dominant foot and 4.5/10 on nondominant foot. Pain resolved (to 2/10, bilaterally) following 3 days of rest. Attributed to delayed onset muscle soreness. Continued transition.
RFS
4
40%
8/10
5
RFS
6
70%
4/10
Soreness of non-dominant soleus
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MFS
5
53%
4/10
Soreness of solei
Attributed to delayed onset muscle soreness. Continued transition.
7
77%
7/10
Soreness of bilateral plantar fascia
Following 3-day period of limited barefoot running volume (25% of total). Reported of soreness, 3/10, bilaterally. Participant progressed to 100% the following week.
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Table 2. Adverse events for those who reported musculoskeletal-related NRS scores greater than or equal to 4/10 during the transition period.
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RFS (n=7)
. -1
Loading Rate (BW s )
Habituated
Novice
Habituated
Novice
Habituated
3.70±0.47
3.75±0.45
3.85±0.47
3.78±0.51
3.93±0.38
3.89±0.45
6.7±3.4
6.7±3.1
-4.7±3.5
-3.0±6.2
-8.9±1.3
-2.8±3.1
s,t,fr,mr
210.0±57.4
182.3±47.5
125.1±88.2
128.7±62.9
62.6±25.9
85.9±34.7
s, fr
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Ankle IC Angle ( )
FFS (n=3)
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Velocity (m s )
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Table 3.
Table 3. Running velocities, ankle initial contact angles, loading rates and arch indices, of barefoot runners, during novice and habituated barefoot running. Statistics were performed based off the novice barefoot foot-strike pattern. Following the transition, there were 10 RFS runners, 8 MFS runners, and 0 FFS runners.
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main effect of strike-pattern. main effect of transition. difference between FFS and RFS runners. difference between MFS and RFS runners. within group difference.
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Indicates a significant Indicates a significant Indicates a significant Indicates a significant Indicates a significant
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(s) (t) (fr) (mr) (*)
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Fig. 1 A.
Energy Absorption Ankle
Knee
30
20 J/kg
*
*
Novice
Experienced RFS
Novice
a
B.
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Experienced
MFS
Novice
a
Experienced FFS
Energy Generation Ankle
*
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0
Knee
#
#
*
*
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J/kg
20
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30
Novice
Experienced
RFS
Novice a
Experienced MFS
Novice a
Experienced FFS
Figure 1. Joint energy (A.) absorption and (B.) generation for barefoot runners during novice and experienced barefoot running. (*) Indicates a significant within group difference for ankle energy absorption and generation, respectively. (#) Indicates a significant within group difference for total energy absorption and generation, respectively.
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Fig. 2
Vertical Ground Reaction Force 3
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2
1.5
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Body Weights
RFS MFS FFS
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0
0.04
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0.12
0.16
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Time (s)
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Figure 2. Representative vertical ground reaction force profiles for a novice barefoot RFS (dashed line), MFS (dotted line), and FFS (solid line) runner. There was a significant main effect of strike-pattern for vertical loading rate (p=0.19) and a significant difference between FFS and RFS runners (166%; p=0.022). Vertical loading rate was quantified as the peak rate of the vertical ground reaction force profile from initial contact to 13% of stance.
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Highlights
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Performing an 8-10 week transition to barefoot running may result in transient pain
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Runners do not innately adopt a forefoot strike following an 8-10 week barefoot transition Runners demonstrate reductions in ankle energetics following an 8-10 week barefoot transition Runners do no demonstrate changes in knee absorption following an 8-10 week barefoot transition
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Ethical Statement
Informed consent for this study was obtained from all participants on a format approved by the
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Institutional Review Board for the University of Southern California (USC) Health Sciences
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Campus.
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Funding Statement:
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This study was partially funded by a CTSI pilot grant.