Barefoot locomotion has recently gained significant interest and consideration for sports and health purposes, leading to increased numbers of barefoot runners or the use of minimalist footwear (31). Concomitantly barefoot locomotion has gained scientific attention by researchers and clinicians with numerous scientific papers as well as narrative and systematic reviews published over the last decade (16–18,22,32). Until now, there is an ongoing controversial discussion about the possible benefits as well as associated risks of barefoot locomotion (41,47). Short-term effects of barefoot versus shod walking and running have been investigated in adults as well as in children (16,17,22,29), and it has been illustrated that the transitioning from shod running to barefoot running may lead to a higher risk of certain running-associated injuries (19,49).
Subsequently, most authors conclude that the long-term effects of barefoot running or walking need to be addressed through further research (2,22,41). However, to our knowledge, there are just few and heterogeneous studies, and no systematic approaches to review the literature on the effects of long-term habitual barefoot (HB) locomotion. Most recent reviews are narrative in character (33,36), and one can only speculate on long-term effects. Therefore, the possibility of drawing conclusions for clinical guidance may be limited. Thus, there is a need for a comprehensive systematic synthesis of the current evidence base to provide direction for further research and guidance for the clinical debate. Evaluation of populations in which daily activities and/or exercises are performed regularly barefoot can address the problem of missing longitudinal studies with randomly assigned barefoot and shod cohorts.
In the context of current research, the aim of this systematic review was therefore to summarize the current evidence on the influence of HB locomotion on biomechanics, foot morphology, motor performance, and health problems (i.e., injuries and diseases) and guide future research in this area.
This review was conducted according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines (40). A review protocol was registered at the University of York, Centre for Reviews and Dissemination PROSPERO database: Registration number CRD42015024541 (http://www.crd.york.ac.uk/prospero/).
For this study, inclusion and exclusion criteria were determined a priori. Inclusion criteria were randomized controlled trials, case control, cohort, and cross-sectional studies from peer-reviewed journals in which HB walking or running individuals were compared with habitual shod controls. Studies investigating biomechanics, foot morphology, motor performance and injuries of children, adolescents, adults, and elderly were considered. Exclusion criteria were studies reporting on individuals with preexisting medical pathologies, such as diabetes, neuromuscular or cardiovascular diseases. The full eligibility criteria are listed in Table S1 (see Table, Supplemental Digital Content 1, Eligibility criteria, http://links.lww.com/MSS/A799).
Ovid MEDLINE, EMBASE, Cochrane Library and Web of Science (Thomson Reuters, New York, NY) were searched from inception until August 2015 (repeated in January 2016) using the following keywords: running OR walking AND habitual walking OR habitual running OR regular walking OR regular running OR habitual barefoot OR habitual shod OR regular barefoot OR regular shod AND barefoot OR shod OR footwear OR shoe OR foot OR arch OR plantar pressure OR ground reaction force OR biomechanic OR kinetic OR dynamic OR kinematic OR motor performance OR injury OR health OR physical activity. To limit the search, the Boolean operation NOT was used for: neuropath OR diabetes OR cerebral palsy OR stroke. Primarily, the databases were searched to prepare a list of applicable studies based on article title and abstract. Two independent reviewers (K.H. and C.H.) identified relevant studies, with a third reviewer (A.Z.) available for consensus. The identified studies were then tested against the inclusion criteria first by title, then abstract, and finally the full text. The bibliographical information of included articles were examined for further relevant references. Citation tracking was performed using Web of Science (Thomson Reuters).
The search was restricted to articles from peer-reviewed journals published in English, German, Dutch, or Spanish languages. Randomized controlled trials, case control, cohort, and cross sectional studies were included, whereas reviews, systematic reviews, commentaries, case studies, and case series were excluded from the review.
Data extraction and quality analysis
The data collection was performed independently by two reviewers (K.H. and C.H.). The following data were extracted: study type, characteristics (number, age, sex, and geographical distribution) of the participants, main outcome measures and, if applicable, the definition of “habitual barefootness.”
The risk of bias was assessed independently by three reviewers (K.H., B.vd.Z., and C.H.) using a modified Downs and Black quality index that was found to be valid and reliable for randomized and nonrandomized studies (13). In addition to the modified quality assessment used by Hall et al. (22), the question concerning randomized intervention order (number 24) was excluded due to the inability to randomize habitual use of footwear. Question 4 (intervention of interested) was rated “yes” if a definition of HB was given. Thus, a total of 20 points could be achieved in the rating. A fourth reviewer (A.Z.) was asked to additionally assess the quality if consensus could not be reached. The assessment score was used to determine a low (score, <6), moderate (score, 7–13), or high quality (score ≥ 14) of the studies investigated.
A meta-analysis was performed with Review Manager 5.3 (The Nordic Cochrane Centre, The Cochrane Collaboration, Copenhagen, Denmark) if corresponding data were found for more than one compared population. Authors were contacted for additional data when insufficient information was provided in the article. To enable comparability, the sagittal ankle angle kinematics were converted so that positive values indicate dorsiflexion. Pooling was performed if studies examined corresponding outcomes with comparable methodology. Thus, we included mean and standard deviation values for biomechanical, foot morphological, and foot arch–related outcome measures. Random effects model was used to calculate standard mean differences for all metrical values (P < 0.05).
I2 statistics and χ2 test were used to test for statistical heterogeneity (P < 0.05). Pooled effect (PE) sizes were determined as small (≤0.59), medium (0.60–1.19), or large (≥1.20) according to Barton et al. (8).
Level of evidence
Levels of evidence were defined using the adapted van Tulder criteria (61) depicted in the biomechanical systematic review by Hall et al. (22) (see Table, Supplemental Digital Content 2, Definitions of levels of evidence, http://links.lww.com/MSS/A800).
The initial search resulted in 2535 studies, of which 476 duplicates were excluded. The deduplication was performed manually for articles by the same authors using the same title. Additionally, 15 studies were identified through other sources. A total of 15 studies met the inclusion criteria and were included in the qualitative analysis. Of these, six investigated biomechanics (12,20,21,37–39), five investigated foot morphology (5,7,12,34,52) and three examined the foot arch (14,48,53). One study assessed the injury incidence (3), and another single study reported on diseases (4). No study investigated motor performance. The full selection process is displayed in Figure 1.
Characteristics of Included Studies
Study characteristics of all included studies are summarized in Table 1 (3–5,7,12,14,20,21,34,37–39,48,52,53). Most studies on HB subjects were performed in Asia and Africa (Fig. 2). Seven studies reported on recreational and competitive runners (3,4,20,37–39,52), whereas eight studies did not specify the sporting activities of the participants. These studies reported on healthy children (5,14,48), adults (5,7,12,21,34), and barefoot fishermen (53).
Risk of Bias
There are no randomized controlled trials comparing HB and shod subjects. Most included studies are cross-sectional or observational cohort study. There are two survey studies, and only one had a prospective study design (3). The sample size in the included studies varied between 38 and 2300 participants with a total of 8399 participants considered in this systematic review.
The risk of bias score of included studies can be found in Table 1 (also see Table, Supplemental Digital Content 3, Quality assessment, Modified Downs and Black Score, http://links.lww.com/MSS/A801). None of the studies was rated with a score of 11 or more points out of 20. Accordingly, none of the studies was of low risk of bias, 12 studies had a moderate risk of bias, and three studies a high risk of bias.
The pooled analysis of three studies showed limited evidence indicating that HB running is associated with reduced ankle dorsiflexion at footstrike compared with habitual shod running (I2 = 93%, P < 0.0001) with a large PE size (−3.47; 95% CI, −5.18 to −1.76) (Fig. 3.1) (37–39). There is conflicting evidence for knee kinematics with a PE size of −0.25 (95% CI, −1.34 to 0.85) for the knee angle at footstrike (I2 = 86%, P < 0.0001) (Fig. 3.2) (37,38). Very limited evidence by one study revealed reduced hip flexion, higher peak ankle eversion angles, and higher maximal hip internal rotation angles for HB runners (39).
Very limited evidence exists from one study, respectively, for a reduced measured (38) and self-reported (20) rate of rearfoot strikes in HB runners. Furthermore, HB individuals with a reduced dorsal excursion of the metatarsophalangeal joint were found (21).
Three studies examined spatial–temporal variables, and very limited evidence was found for a reduced contact time, stride length and stride time, and an increased cadence while running in HB runners (37,39) and nonrunners (21). For HB walking, very limited evidence exists for longer contact times (21).
There is very limited evidence for reduced vertical loading rates (39) and reduced or nonpresent impact peaks in HB runners (37). Ground reaction force–time curves normally reveal one peak for HB runners, whereas two peaks (impact and active peak) are found for habitual shod runners (39). One study found lower peak pressures in the heel, and second and third metatarsal region, whereas higher peak pressures appeared in the lateral midfoot and toe region for HB individuals (12).
A total of five studies noted foot morphology characteristics (5,7,12,34,52). Foot length, width, and hallux angle were assessed with a meta-analysis (Figs. 4 and 5). HB groups show wider (small PE, 0.55; 95% CI, 0.06–1.05) but no shorter (PE, −0.22; 95% CI, −0.51 to 0.08)) feet compared with habitual shod populations. When relativized to body length, HB groups show larger feet than habitual shod groups (5,12).
In HB populations, two studies found a reduced hallux angle (medium PE, −1.16; 95% CI, −1.64 to −0.68), with significant differences for adults (large PE, −1.35; 95% CI, −1.73 to −0.97) and nonsignificant findings for those under 18 yr of age (Fig. 5) (7,52).
Very limited evidence exists for a higher pliability of the foot of HB individuals (34).
Two studies compared the rate of flat feet in children (14,48) indicating limited evidence for a reduced incidence of flat feet in HB children compared with their shod counterparts. Conflicting evidence was found for the foot arch of adults with one study (53), showing a higher incidence of flat feet in a HB population and another study (12), showing no statistically significant differences for navicular height.
No study reported on motor performance outcomes. Only three of the included studies mentioned personal bests and training paces for HB runners (3,20,37). Thus, there is currently no evidence from studies for the effects of HB locomotion on motor performance.
Injuries and Diseases
One study compared injury rates between habitual shod and HB runners (3). Very limited evidence suggests that there are differences in injury patterns and that injuries to the plantar surface occur more often in the HB group. No differences in injury rates were found when injury incidence was related to mileage. One study found less foot defects and deformities and more foot diseases in HB individuals (4).
This systematic review is the first to synthesize the current findings on the effects of HB compared with habitual shod locomotion. Given the absence of high-quality studies in this field, it is difficult to provide conclusive evidence for the implications of HB locomotion. Nevertheless, an exploratory meta-analysis for biomechanical and foot metrical outcomes was possible to conduct. The main findings revealed effects of HB locomotion on biomechanics as well as foot and injury outcomes. All of these findings show only limited or very limited evidence.
Limitations and Methodological Considerations of Current Research
No randomized controlled trials were found comparing HB and habitual shod participants. Only one article used a prospective study design (3). This negates the ability to draw conclusions on causality of found relations and provide conclusive recommendations.
Using the modified Downs and Black Quality Index (13,22), none of the articles was assessed as “high quality.” The included articles mainly show good values in reporting and internal validity (bias), but there is a lack of blinding in all studies. Even though blinding might not be very feasible in HB research, it does reduce the risk of bias. Furthermore, the articles had a low external validity and are susceptible to confounding. Selection bias was not addressed or discussed by most of the included articles. Almost no power analysis was performed a priori.
The high heterogeneity provided by the included studies might be due to difficulties in comparing different ethnic groups. As seen inter alia by the geographical distribution (Fig. 2), HB cohorts were mainly found in Africa and Asia. If the investigated population did not provide an adequate habitual shod cohort, data were compared with cohorts from America (38) or Europe (7,12,21,34). Several other studies reporting on HB population (15,24,27,42) did not compare them with habitual shod counterparts and, consequently, could not be included in the systematic review. A major challenge remains to clearly define study cohorts and standardize measures which form the bases of high-quality prospective studies.
Another limitation of the current research is the inconsistency in using a common definition for an HB individual. Although Altman and Davis (3) define HB runners as running at least 50% of their yearly mileage barefoot, others use 66% (38) or 80% (37) of the time barefoot or in minimalist footwear. Some authors define the HB cohort as living barefoot all life (5,39,52) or simply state that it is common to walk barefoot in the area were the participants live (7,14). This classification was conducted by self-reporting or observation (38), only one study (12) used a short questionnaire about footwear habits but did not report on the findings, nor their definition.
Limited evidence was found for sagittal ankle kinematics at footstrike with larger plantarflexion in HB runners. This finding is analogous with other cross-sectional studies looking at acute effects of barefoot running conditions (1,28,29). The more plantarflexed position while landing is in agreement with the increased rate of forefoot or midfoot strikes reported in the included studies (20,38). The findings for sagittal knee kinematics are still contradictory, studies showing higher knee flexion angles at initial contact (9,17) could not always be confirmed in other studies (28,56). For HB running, the evidence for knee kinematics is also conflicting. Furthermore, although running velocity (11) and previous injuries (23) have an influence on biomechanics and therefore should be evaluated for confounding, this was rarely considered.
For HB running, very limited evidence indicates a reduced stride length, stride time, and an increased cadence in shod and unshod testing conditions. These variables all relate to one another if (running) velocity is comparable in an HB or shod situation. Different running velocities during barefoot and shod conditions may therefore have influenced the results and should be kept in mind as a potential limitation factor. Only one study mentioned the average speed during testing (37), whereas another study (39) controlled the running speed during the tests.
These spatial–temporal findings are in agreement with investigations on short-term effects of barefoot running (22,28) and is discussed to be beneficial for injury prevention (25,26). Very limited evidence indicates a longer contact time in HB walking, which is discussed to be due to the confounding factor walking speed (12,21).
Very limited evidence indicates reduced vertical loading rates and peak vertical forces in HB populations. This is consistent with the effects of acute barefoot running in children (29) and adults (22). The findings of higher cadence with reduced stride length might be associated with lower ground reaction forces (26).
As seen in other studies on short-term effects of barefoot running (28,38), ground reaction force–time curves differ between barefoot and shod running. Barefoot running reveal one peak more often than shod running, where more frequently two peaks in the ground reaction force–time curve are observed (39). The reasons for the extra peak is thought to be due to the rear-foot strike during landing while shod (10,44). However, consequences of this additional peak are currently unknown, especially for injury rates (43).
During walking, lower-peak pressures in heel and mid-metatarsal region and higher peak pressures in the lateral midfoot and toe region for HB individuals are found (12). The plantar skin of habitually barefoot individuals is described as “thick and tough” (27,53). Possibly, this accounts for the more equally distributed plantar pressure found when compared with the thinner plantar skin of the habitually shod (58). Furthermore, walking velocity was not controlled for in all studies, and therefore, slight differences could have led to differences in plantar pressure (45). Thus, for better comparing plantar pressure distribution, future research should use similar velocities.
Limited evidence indicates that HB populations tend to have wider feet, with a reduced hallux angle. However, when relativized to body length, the foot length was not wider but longer (5,12). Other research does not support the finding that an HB life leads to increased forefoot width (60). Having in mind that the foot morphology is influenced by several factors, such as being overweight or ligament laxity (51), it seems difficult to draw conclusions from these data because neither factor was evaluated for confounding. The included studies determine the hallux angle by pedobarographic measurements using the angle between two medial tangential lines. Those two lines are, respectively, drawn from the most medial part of the first metatarsophalangeal joint to the most medial part of the heel and to the most medial aspect of the hallux (7,52). Although in the clinical assessment, a radiographic determination of the hallux angle is preferred and defined by the angle between the first metatarsal and the proximal phalanx, the pedobarographic determination is feasible to be used in the field (7,30). Additionally, evidence for a reduced hallux angle in HB population is consistent with lower prevalence of hallux valgus deformities (4,53). The habitual footwear use has been discussed to be an etiological factor for hallux valgus deformities (55). However, a more recent review shows that it is just one of several factors leading to hallux valgus (46). The decreased foot pliability for habitually shod individuals found by Kadambande et al. (34) might be an adaptation to a decreased range of motion in the first metatarsophalangeal joint whilst wearing shoes, as found in children (63,64). Thus, even though it has been of scientific interest for a long time (6,27), more longitudinal or prospective research is still needed to better understand the influence of habitual footwear use on foot morphology.
There is limited evidence for a reduced incidence of flat feet in HB children. This is in accordance with the finding that flat foot incidence is higher the earlier footwear is used (51). In comparison to this finding, conflicting evidence was found for the foot arch of adults (12,53). Other research comparing the arch between African and European adults, also report a statistically significant higher arch index (= lower arches) in the African group (58).
There might difficulties when comparing different ethnic groups using footprints as a measure for the arch height. HB populations or even populations with HB and habitual shod individuals are rarely distributed within the same countries (Fig. 2). The clinical importance of understanding the influence of footwear on foot arch pathologies is very important and should be addressed in future longitudinal research as well as the assessment methodology.
Successful long distance runners competing at Olympic Games or world major marathons have a high medial impact and evoke the idea that HB running is prone to be more efficient. Young male runners who consider themselves as “elite” have the highest interest in barefoot running (50). Scientifically, there are conflicting findings when looking at motor performance and running economy (47,62), and this systematic review could not elucidate any evidence for improved motor performances in HB runners. It is discussed that the arch of the foot serves as an important energy storage (35) that is reduced or disabled by using footwear. However, this relation still needs to be confirmed with high-quality investigations.
The primary motivation of runners to try barefoot or minimalist shoe running is to prevent injuries (50). Only one study investigated injury rates between habitual shod and HB runners (3). Thus, this systematic review found only very limited evidence for differences in injury patterns and no evidence for different injury rates when related to mileage. When looking at the wide range of literature discussing possible benefits and risks of barefoot and shod running (19,33,43), this finding shows a huge research gap in the current literature. First prospective studies are trying to elucidate the injury risk of barefoot running (59), but more research is needed before conclusive recommendations can be given.
Clinical and Research Implications
Positive effects of barefoot locomotion have been widely speculated, but there is an ongoing debate on potential advantages and hazards of barefoot locomotion and adapting a barefoot running technique. At the moment, there is a dilemma for advice in the clinical setting. It has been shown that acute barefoot running cause changes in gait biomechanics (17,22), but limited evidence exists that habitual footwear use leads to long-term biomechanical adaptations (ankle kinematics, cadence, stride length, vertical ground reaction forces). Nevertheless, for an optimal foot development and partially preventative for foot pathologies, barefoot locomotion seems beneficial and when the use of footwear is unavoidable, it should be flexible, flat, light weight, and not constrictive (57).
In the injury debate, not enough evidence is provided to draw clinically relevant conclusions on injury prevention or treatment of injuries. It seems reasonable that certain injury patterns (iliotibial band syndrome, patellofermoral pain, gluteal/hamstring strain and tendinitis, plantar fasciitis) may benefit from barefoot running, while others (Achilles tendinitis and peroneal/gastrocnemius/soleos/tibialis posterior strain and tendinitis) may benefit from shod running (3,54). Appropriate conclusions for training regarding motor performance cannot be drawn from the current literature.
Finally, for all relevant outcomes investigated in this systematic review, there is a need for more appropriate designed research to enable the possibility to better estimate clinical implications and to take the relevant confounding factors (such as ethnicity, lifestyle, culture, region, and diet) into account. To ensure the external validity of future studies, researchers should either utilize prospective study designs or perform large epidemiological studies with a thoughtful consideration of the setting, population, and ethnic background investigated. Because time is a threat to external validity, it should be kept in mind that some of the results displayed in this article are rather ancient (7,53) and might need an updated investigation.
A definition of “habitual barefootness” is missing and the current literature does only offer inconsistent explanations. Only two thirds of the included studies used a HB definition. Most studies applied self-reported identification and only one study identified HB runners by observation and interviews (38).
Despite a large amount of research on short-term effects of barefoot locomotion, only little evidence exists for long-term effects. Thus, conclusions are difficult to be drawn from current data and remain mostly speculative. Little evidence was found for biomechanical and foot metrical outcomes, with almost no evidence for clinically relevant outcomes, such as injury rates or foot pathologies. Furthermore, the effect of HB locomotion on motor performance remains unclear. Analyzing HB populations or comparing HB and habitual shod subjects within one population does help to identify some effects of long-term effects of barefoot locomotion. However, it does not replace well-designed prospective randomized controlled studies.
Most current studies have a low study quality and lack external validity, blinding and might be prone to selection bias. Furthermore, the analyzed research is of high inconsistency concerning a HB definition. Besides different definitions, most studies used self-reported identification. Consequently, we conclude that a consensus for a HB definition is needed, and that future research needs to address these deficits and use a clear definition of a HB individual.
This work was funded by the Ministry for Science and Research in Hamburg (grant number LFF-FV13). There are no conflicts of interest, financially or otherwise, among any of the authors of this article.
The results of the study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation.
The authors would like to thank Elke Rieger for support with the graphics.
All authors contributed to the formulation of the idea, K. H. and C. H. led on the analysis, whereas A. Z. and B. C. v. d. Z. also scored the included articles. All authors contributed significantly to the writing of, and also approved, the final article. The results of the present study do not constitute endorsement by the ACSM.
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