The human foot is uniquely adapted to bipedal locomotion and has a deformable arch of variable stiffness. Intrinsic foot muscles regulate arch deformation, making them important for foot function. In this study we explore the hypothesis that normal daily activity in minimal footwear, which provides little or no support, increases foot muscle strength. Western adults wore minimal footwear for a six-month period (the “intervention” group). Foot strength, i.e., maximum isometric plantarflexion strength at the metatarsophalangeal joints, and foot biometrics were measured before and after the intervention. An additional group was investigated to add further insight on the long-term effects of footwear, consisting of Western adults with an average 2.5 years of experience in minimal footwear (the “experienced” group). This study shows that foot strength increases by, on average, 57.4% (p < 0.001) after six months of daily activity in minimal footwear. The experienced group had similar foot strength as the post intervention group, suggesting that six months of regular minimal footwear use is sufficient to gain full strength, which may aid healthy balance and gait.

In addition to strength and muscle size, minimal footwear may also influence foot morphology because they tend to have a wider toe box than conventional shoes. Habitually minimally shod participants have been found to have significantly higher longitudinal arches than conventionally western shod participants 38 . This is in agreement with a study by Hollander et al. 39 who found significantly higher static arch heights in habitually barefoot children between the ages of six and 18 years when compared to their conventionally shod children. A study by D' Août et al. 40 found no differences between static or dynamic longitudinal arch heights of habitually barefoot Indians, habitually shod Indians and conventionally western shod Europeans but, based on pressure recordings, the Indian groups had slightly more (albeit less variable) midfoot contact. In terms of morphology, most studies agree that habitually barefoot and/or minimally shod populations have wider feet [41][42][43] . This study will investigate minimal footwear influence on morphology as well as foot strength on habitually conventionally Western shod healthy adults.This study has two central aims. The first is to quantify the influence of six months of regular minimal footwear use on foot strength and morphology, for adults that were previously habitually conventionally western shod. The second is to quantify the longer-term influence of regular minimal footwear use on foot strength and biometrics. We hypothesise the following:1. Foot width increases in conventionally western shod populations after using minimal footwear for daily activity over a six-month period. 2. Foot strength increases in conventionally western shod populations after using minimal footwear for daily activity over a six-month period. 3. Foot strength is further increased in conventionally western shod populations if regular use of minimal footwear is longer than a six-month period. MethodsThe present paper combines a prospective study and a cross-population study. The prospective study investigates the influence of six months of daily activity in minimal footwear on foot strength and biometrics, for adults that were habitually using conventional Western shoes prior to the study, and a control group. This group will be further referred to as the "intervention" study group. We further report on a group that has been studied crosssectionally. This group consists of formerly conventionally shod western adults that have switched to minimal shoes for a longer period (2.5 ± 2.4 years). This group will be further referred to as the "experienced" study group.

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