Effests of Minimalist Footwear on Healthy Subjects.

Background and current situation The concept of foot core defines the set of structures responsible for providing dynamic control to the foot; for it to function correctly, the integrity of the three subsystems that comprise it is required, which are: * Passive subsystem: comprises the bones, joint capsules and ligaments, including the plantar fascia, which form the plantar arch; this is defined by the medial and lateral longitudinal arches and the anterior and posterior transverse arches. * Active subsystem: includes the intrinsic foot muscles (IFM) and the extrinsic muscles, whose functions are to stabilise and mobilise the foot, respectively. * Neural subsystem: formed by all the capsuloligamentous and musculotendinous receptors of the active and passive subsystems, in addition to the plantar cutaneous receptors. Of all the elements that make up the foot core, the importance of the IFM stands out. Some of the most relevant functions of the IFM when acting together are: adapting the position of the foot in the different phases of the gait cycle, stabilising the midfoot and forefoot, improving load absorption and force transmission capacity and controlling foot pronation. The literature has shown that morphological or functional deficits of the IFM are involved in the development of pathologies such as plantar fasciitis, hallux valgus, hammer toes, Achilles tendinopathy and chronic ankle instability. For the foot core to function correctly, the integrity of the neural subsystem is also essential. Plantar skin sensitivity is a key element in static and dynamic body stability. It is interesting to bear this in mind when treating various pathologies that involve stability deficits and even for athletic performance. The use of regular footwear causes a series of morphological and functional alterations in the different elements of the foot core. Regular footwear restricts the mobility of the toes, which is essential, especially in the propulsion phase of walking, to maintain stability and promote the forward movement of the body's centre of pressure. Furthermore, according to Johnson et al., traditional footwear also promotes IFM atrophy, limits sensory and proprioceptive stimulation of the foot, and reduces the load stimulus necessary to achieve adequate bone mineral density (since it is the footwear itself that absorbs part of the load). Two studies comparing various foot-related variables between indigenous populations who walk barefoot and population groups who walk in regular footwear describe that people who walk barefoot have a wider foot and a more homogeneous distribution of plantar pressures (compared to the heterogeneous distribution of plantar pressures, with load peaks in the calcaneus, the heads of the 2nd-4th metatarsals and the hallux, characteristic of subjects who walk with footwear), as well as greater height and rigidity of the medial longitudinal arch (31% of subjects with footwear have flat feet compared to only 1% of subjects who walk barefoot). Along the same lines, the study by Korchi et al. describes how a three-month physical training programme (based mainly on strength and endurance exercises) produces significantly greater improvement in elderly people who perform the exercises barefoot than in those who do so wearing shoes. The Chinese bandaged foot syndrome has even been defined, describing other changes in the foot caused by regular footwear use, such as calluses on the back of the toes, thickening of the nails, decreased interdigital space, or lateral deviations of the toes with subluxation of the metatarsophalangeal joints. Clearly, footwear is necessary in contemporary society to protect the foot from the environment and provide comfort, especially in certain situations such as sports and in some pathologies (for example, to reduce the risk of injury in diabetic patients). However, evidence shows that everyday footwear does not respect the natural morphology and biomechanics of the foot. In addition, fashion and aesthetics usually play a more important role in footwear design than comfort and respect for the morphology and function of the foot, which causes further damage. Therefore, taking these factors into account, the use of minimalist footwear as a replacement for regular footwear could be justified. Minimalist footwear has a number of characteristics that differentiate it from regular footwear, such as: a) it has no stabilising elements in the midfoot, which makes both the sole and the last completely flexible, so these elements do not restrict the normal mobility of the foot; b) the forefoot is very wide, allowing the toes to spread out sideways; c) the difference in height between the heel and the forefoot (drop) is zero, facilitating an even distribution of plantar pressure and preventing shortening of the triceps surae; d) uniform sole 3-4 mm thick, which promotes stimulation of the plantar skin receptors; thin, rigid soles allow for better stability and balance compared to footwear with thicker, less rigid soles. There is evidence showing the benefits of wearing minimalist footwear over regular footwear. For example, the systematic review by Franklin et al. concludes that there are differences in the kinetics and kinematics of walking between subjects who walk in minimalist footwear and those who walk in regular footwear, highlighting the difference in the distribution of plantar pressures as the most relevant finding. Other studies compare the use of minimalist footwear with regular running shoes; these studies show some advantages of using minimalist footwear, such as an increase in IFM size and toe flexion strength or improved plantar arch function. However, the use of minimalist running shoes can be harmful, as it has been reported that up to 50% of subjects who run in minimalist shoes develop bone oedema; it is advisable to allow for a period of adaptation to this type of footwear when using it for running. In summary, the existing scientific evidence shows the harmful effects that the use of regular footwear can have on the morphology and function of the foot. On the other hand, minimalist footwear also appears to be a good alternative to regular footwear, as it allows for a more natural footfall but avoids the problems associated with walking barefoot. There is research on the use of minimalist footwear for running, but there is little research on the use of this type of footwear in everyday life, assessing the long-term effects it may have. Therefore, the investigators consider that this study, which assesses the medium- and long-term effects of wearing minimalist footwear in everyday life, is justified. Hypothesis The use of minimalist footwear promotes adequate mobility of the toes, which can contribute to morphological and functional improvement of IFM. In addition, minimalist footwear allows for increased plantar skin stimulation compared to traditional footwear. These changes in the foot, both at the motor and somatosensory levels, may be associated with a series of general improvements, such as increased static and dynamic stability or modification of the type of footfall. Primary and secondary objectives: The primary objective of the study is to evaluate the medium- and long-term changes produced by the use of minimalist footwear, compared to traditional footwear, on the morphology and function of the IFM. Another objective established in the study is to determine the medium- and long-term effect of wearing minimalist footwear on other related variables such as foot strike type, static and dynamic stability, morphology and compressibility of the talar fat pad, alignment of the hallux MTF joint, and plantar skin sensitivity. Type of study: This is a single-blind, randomised controlled clinical trial that aims to assess the medium- and long-term effects of continued use of minimalist footwear in healthy subjects, analysing different variables such as the morphology and function of the IFM, the study of plantar pressures during walking, static and dynamic stability, the morphology and compressibility of the talar fat pad, the alignment of the MTF joint of the hallux, and plantar skin sensitivity. Materials and techniques: The following materials are needed to carry out this project: * Ultrasound scanner and linear probe: used to assess the morphology of the IFM and the morphology and compressibility of the talar fat pad. * Pressure platform: used to analyse plantar pressures under static load, measure the function of the flexor muscles of the toes, and assess static stability by recording the displacement of the centre of pressure. * Semmes-Weinstein monofilaments of different resistances to assess plantar skin sensitivity. In addition to this material used to perform the assessments, each subject in the experimental group will be given minimalist footwear to wear during the study. The chosen footwear must meet the following characteristics to be considered minimalist: zero drop, wide forefoot, total flexibility, and sole thickness less than 0.5 cm. Sample: The study sample will consist of 80 healthy adults, who will be randomly divided into an experimental group (use of minimalist footwear for at least 4 hours a day during the 12-month study period) and a control group (use of traditional footwear and no intervention) with 40 subjects in each. For statistical analysis, the sample will be stratified according to age. Subjects in the sample must meet the following inclusion criteria: * Adults over 18 years of age. * No pain in the feet, ankles or legs in the previous 3 months * Healthy subjects, with no musculoskeletal pathology in the lower extremities in the three months prior to the study (fractures, sprains, tendinopathies). * Signature of informed consent. Likewise, subjects who meet any of the following exclusion criteria will not be included in the study: * Previous use of minimalist footwear. * Significant musculoskeletal deformities in the lower extremities. * Cognitive, neurological, visual or vestibular disorders that may affect the outcome of any of the variables included in the study. Outcomes: The following data will be used as descriptive variables for the sample: age (continuous quantitative variable, measured in years), sex (dichotomous qualitative variable, male or female), body mass index (continuous quantitative variable, measured in kilogrames/square meters) and weekly sports practice (continuous quantitative variable, measured in hours). The independent variable of the study will be the group to which each subject belongs (experimental or control). The variables considered as dependent are the following: * IFM morphology: the thickness and cross-sectional area (CSA) at rest and under load of the abductor muscles are assessed by musculoskeletal ultrasound of the abductor hallucis, abductor digiti minimi, flexor digitorum brevis, and flexor hallucis brevis muscles. The ultrasound assessment follows the protocol defined by Crofts et al., which has excellent inter-rater reliability (ICC 0.90-0.98). * IFM function: this is assessed by analysing the flexion strength of the toes, differentiating between the hallux on one side and the other toes on the other. This variable is evaluated using the pressure platform, following the protocol described by Mickle et al. This method is suitable and reliable (ICC=0.92) for measuring the flexor strength of the toes. * Plantar pressures under static load: the pressure platform allows the distribution of plantar pressures to be assessed in a static standing position. This provides a morphological and functional assessment of the plantar footprint. * Static stability: the static stabilometry test is used, employing a pressure platform, which is described as a valid method with very good to excellent reliability (CCI 0.79-0.91) for assessing this variable. The displacement of the centre of pressure experienced by the subject when asked to maintain a certain position is recorded. A greater shift in the centre of gravity indicates a deficit in static stability. * Dynamic stability: this is measured using the Y balance test. This is a widely used tool as it has very good validity and reliability levels, as well as being a predictive factor for certain sports injuries. Dynamic stability is assessed by measuring the range of motion of each lower limb in three different directions. Greater ranges of motion are associated with better dynamic stability. * Morphology and compressibility of the talar fat pad: these are assessed using musculoskeletal ultrasound, which has an excellent level of reliability (ICC=0.99). The thickness of the talar fat pad is assessed in two different situations: at rest and under compression with the probe. The morphology corresponds to the thickness of the fat pad at rest, while the compressibility index is obtained by dividing the thickness under compression by the thickness at rest. A value of this index close to 1 indicates that the fat pad is excessively rigid, while a value close to 0 indicates that the structure is excessively deformable. * Frontal plane alignment of the MTF joint of the hallux: measured using the Manchester scale. This is a visual scale that assesses the severity of the hallux valgus deformity presented by the subject. It ranges from 'no deformity' (grade 1) to 'severe deformity' (grade 4). The Manchester scale has very good to excellent test-retest reliability (Kappa=0.78-0.90). * Plantar skin sensitivity: the pressure sensitivity threshold is assessed at three points on the sole of the foot (heel, base of the fifth metatarsal, and head of the first metatarsal); Semmes-Weinstein monofilaments of varying degrees of rigidity are used for this purpose, based on the protocol described in the study by Patel et al. A total of four assessments of these dependent variables will be carried out on all subjects in the sample: the first assessment will be made prior to the start of the intervention (assessment 1). The second and third assessments will be carried out during the follow-up period (assessment 2, three months after the intervention; and assessment 3, six months after the intervention). The final assessment will be carried out at the end of the follow-up period (assessment 4, post-intervention). All these measurements will be carried out by two members of the research team at the facilities of the FisioSport Salamanca physiotherapy centre, which is registered with the health authorities. To ensure the blindness of the evaluators, the rest of the research team members will be responsible for selecting the sample and monitoring the intervention in the subjects of the experimental group. The intervention simply consists of wearing minimalist footwear for 4 hours a day for 12 months in the experimental group. To reduce the risk of any adverse effects from switching from traditional footwear to minimalist footwear, there will be an adaptation period, so that the use of minimalist footwear is introduced gradually (1 hour a day during the first week, 2 hours a day during the second week, 3 hours a day during the third week, and 4 or more hours a day from the fourth week onwards). Subjects are recommended to use the footwear in daily activities or in non-impact sports; that is, the use of minimalist footwear should be avoided for activities involving running and jumping. The control group does not have to undergo any intervention and will continue to use their usual footwear. Work plan The estimated time frame for completing the study is 14 months, beginning in January 2025. First, approval for the study will be sought from the Ethics Committee of the University of Salamanca, and the study will then be included in the Clinical Trials registry. During January and February 2025, the sample will be recruited and minimalist footwear will be purchased for the subjects in the experimental group. During this initial period, the participants' informed consent will also be obtained. During March, a pre-intervention assessment will be carried out on all subjects in the sample. After this initial assessment, the subjects in the experimental group will be informed of the considerations to be taken into account during the intervention period (use of minimalist footwear for 4 hours a day and reporting of discomfort or any type of adverse effects). Analysis plan First, basic descriptive methods will be used to perform the statistical analysis of the descriptive variables. For qualitative variables, the number of cases in each group and the percentage participants represent will be calculated; meanwhile, for quantitative variables, the following values will be calculated: minimum, maximum, mean, and standard deviation. To compare the dependent variables between the experimental group and the control group in each of the four assessments that will be carried out, the Student's t-test for related samples will be used, once the assumptions of normality have been checked with the Kolmogorov-Smirnov test. Cohen's d test will be used to measure the effect size of the intervention. The Statistical Package for the Social Sciences (SPSS) version 25.0 for Windows (Armonk, NY: IBM Corporation) will be used to perform all statistical analyses. A p-value \<0.05 will be considered statistically significant. Limitations and potential biases of the study: The main limitation of the study is the inability to monitor compliance with the proper use of minimalist footwear by the subjects in the experimental group. A sample loss of 10% is anticipated.