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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BIOMECHANICS; FOOT MORPHOLOGY; MOTOR PERFORMANCE; INJURIES; FOOTWEAR; LOCOMOTION
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