Foot biomechanics and metatarsal structure in runners with minimal and cushioned shoes

Project Summary Bone stress injuries (BSI) are overuse injuries that result from the accumulation of microdamage in bones. They are among the most common injuries in running athletes and military professionals, accounting for 20% of sports medicine outpatient visits. The metatarsals bones in the feet are a common location, accounting for 38% of BSI in collegiate athletes and up to 58% in the military. Running is a highly repetitive activity involving approximately 1000 footstrikes per mile. Bone adapts to mechanical strain to become stronger. However, when strains are too high, microdamage accumulates faster than it can be repaired and a BSI can result. During the stance phase of running, the metatarsals, which are long and thin, experience high bending moments as the arch of the foot deflects downward and ground reaction forces increase. These bending moments cause high strains on the dorsal surface of the metatarsals. The four layers of plantar intrinsic muscles (PIMs) help to mitigate the bending moments. Wearing minimal footwear, which is flexible and offers no external support of the arch, has been consistently shown to increase the size and strength of the PIMs. In contrast, chronic support of the foot (e.g. from orthotics) results in significant foot weakening. Among the many differences, runners typically use a forefoot strike in minimal shoes and a rearfoot strike in supportive shoes. Because both bone and muscle adapt to habitual mechanical loads, we hypothesize that long-term running in minimal footwear will be associated with improved PIMs and metatarsal strength compared to running in traditional cushioned footwear. We further hypothesize that the biomechanics associated with running in minimal footwear stimulates these positive adaptations to the muscles and bones of the feet. This cross-sectional observational study will evaluate associations between foot biomechanics, foot structure, and foot strength in a population of male and female recreational runners who are habituated to either minimal or supportive shoes. To uncover the mechanisms that explain the biomechanical influence of footwear on risk factors for metatarsal BSI, we will use a combination of quantitative imaging, biomechanics, and modeling. All three Aims will use data from the same participants and experimentally collected data. Aim 1 will evaluate the role of habitual footwear on foot structure and strength in the two groups. Aim 2 will evaluate foot biomechanics, and Aim 3 will quantify the associations between foot muscle strength, foot biomechanics, and metatarsal strength. By investigating differences between runners who are well habituated to either minimal or cushioned shoes, we will clarify the potential role that footwear has on strengthening the muscles and bones within the foot. This will directly inform interventions in the future to prevent injury. Beyond the immediate impact, the unique data will form a foundation for more sophisticated analyses to better understand load transmission within the feet. This has broad applications in preventing and treating other common foot injuries and progressive deformities that have biomechanical pathologies, such as bunion, collapsing foot deformity, and plantar fasciitis.