Abstract and Introduction
Abstract
Introduction: Weakness of foot muscles may contribute to a variety of loading-related injuries. Supportive footwear may contribute to intrinsic foot muscle weakness by reducing the muscles' role in locomotion (e.g., absorbing forces and controlling motion). Increased stimulus to the foot muscles can be provided through a variety of mechanisms, including minimalist footwear and directed exercise.
Purpose: To determine the effect of walking in minimalist footwear or performing foot strengthening exercises on foot muscle size and strength.
Methods: Fifty-seven runners were randomly assigned to one of three groups—minimalist shoe walking (MSW), foot strengthening (FS) exercise, or control (C). All groups maintained their prestudy running mileage throughout the study. The MSW group walked in provided footwear, increasing weekly the number of steps per day taken in the shoes. The FS group performed a set of progressive resistance exercises at least 5 d·wk−1. Foot muscle strength (via custom dynamometers) and size (via ultrasound) were measured at the beginning (week 0), middle (week 4), and end (week 8) of the study. Mixed model ANOVA were run to determine if the interventions had differing effects on the groups.
Results: There were significant group–time interactions for all muscle size and strength measurements. All muscle sizes and strength increased significantly from weeks 0 to 8 in the FS and MSW groups, whereas there were no changes in the C group. Some muscles increased in size by week 4 in the FS and MSW groups.
Conclusions: Minimalist shoe walking is as effective as foot strengthening exercises in increasing foot muscle size and strength. The convenience of changing footwear rather than performing specific exercises may result in greater compliance.
Introduction
The foot is a complex structure with its 26 bones, 33 articulations (each with 6 degrees of freedom of motion) and its 20+ muscles including four layers of arch muscles. This structure allows the foot to serve many different functions, such as providing a base of support, serving as a shock attenuator, being able to adapt to uneven terrain, and serving as a rigid lever for push off.[1] The intrinsic muscles of the arch, with their small cross-sectional areas (CSA) and short moment arms are primarily stabilizers, in contrast to the larger extrinsic foot muscles. These intrinsic arch muscles have been referred to as the foot core,[1] because they are analogous to the small muscles of the lumbopelvic core that provide stability to the hip and pelvis regions.
Weakness of the intrinsic foot muscles has been associated with loading-related injuries.[2–6] These muscles contract with every footstep to control the magnitude and velocity of the downward deflection of the arch.[7] The plantar fascia also prevents this deflection[8] and undergoes strain as the arch deflects downward. Weakness of the intrinsic muscles of the arch can lead to a greater deflection and/or velocity of deflection of the arch, placing increased strain on the plantar fascia.[3,9] With the repetitive nature of walking (over 2000 steps per mile), this can easily accumulate into an overuse injury and may explain, in part, the high lifetime incidence (10% of population) of plantar fasciitis reported today.[10]
Muscles also provide a balance of forces around bones resulting in a healthy strain environment. For example, Milgrom et al. reported increased tibial bone strain upon fatigue of the lower leg muscles, which may be a major factor in the development of stress fractures.[11] Although their study was focused on the tibia, the results are also likely applicable to the bones of the feet. In particular, the metatarsals are at risk as they undergo high bending moments and inherently have low resistance to bending due to their geometry.[12,13] Therefore, strong foot muscles may help to resist this bending and reduce the risk of bone stress injuries.
Foot muscle strengthening is often a component of injury prevention and treatment programs in healthy and pathological populations.[6,14–17] Although strengthening of the long toe flexors is often a component of these programs, targeted contraction of the arch musculature to maintain an arch (i.e., foot doming/short foot exercise) is not as well used in strengthening programs. There is increasing evidence to suggest that training the foot muscles via doming exercises reduces excessive flattening of the arch.[18,19] One study reported 4 wk of foot doming training in healthy individuals reduced navicular drop and increased the arch height index during weight bearing.[18] In healthy young adults with pes planus, there were significant increases in great toe (GT) flexion strength and the CSA of the abductor hallucis (ABDH) muscle after 4 wk of doming exercises and foot orthotic intervention compared with foot orthotic intervention alone.[19]
Minimalist footwear use during running has also been associated with strengthening the intrinsic foot muscles. Several recent studies have assessed the intrinsic foot muscles in runners following transition programs to minimalist footwear compared with runners in conventional footwear.[20–22] These studies have reported increased CSA and/or muscle volumes of some of the intrinsic foot muscles in as few as 10 wk.[22] Miller et al.[20] also noted an increase in the arch stiffness (i.e., less deformation) during gait. However, there are bone stress injury risks associated with transitioning to running in minimalist footwear too rapidly.[23] Walking in minimalist footwear may provide sufficient load to strengthen foot muscles resulting in lower injury risk, but this has not yet been examined.
Strengthening the foot muscles either through walking in minimalist footwear or targeted exercise may improve foot function and decrease risk of injury. Therefore, the purpose of this study was to measure the changes in intrinsic muscle strength and size in either a footwear or a strengthening intervention. It was hypothesized that minimalist footwear use and foot strengthening would both result in greater strength and size of the foot muscles by 8 wk of intervention.
Med Sci Sports Exerc. 2019;51(1):104-113. © 2019 American College of Sports Medicine
