Minimal Footwear and the Musculoskeletal System
The human musculoskeletal system normally adapts to the mechanical loads it experiences. As proposed by Frost's[71,72] mechanostat model, tissues responds to the mechanical demands placed on them by altering their mechanical properties to better meet the new demands. Although criteria governing this response are not well understood, there is emerging evidence that footwear may influence the adaptation of the musculoskeletal system including fibroadipose and dense connective tissue structures. The majority of this research has been conducted on adults.
Influence on Bone
There is a dearth of articles examining the effect of minimal footwear on bone health, and these articles focus on running. One study measured bone mineral density in runners before and after a structured, 26-week transition to running in minimal footwear. The authors reported no significant changes in the apparent density of the measured bones, including the tibia, calcaneus, and metatarsals. However, other more detailed measures of bone quality, such as cortical thickness and trabecular bone density, may be more indicative of strength. These measures are acquired using high-resolution, peripheral, quantitative computed tomography, which is not currently widely available. As its use becomes more prevalent, we will be able to better study the effect of progressive load on bone strength. The effect of minimal footwear on bone injury has been addressed in a few more studies. Bone marrow edema (BME) is used as an indication of both bone turnover and bone injury. Ridge et al. reported on the Bone Marrow Edema Score after a largely unstructured, 10-wk transition to running their habitual mileage in minimal footwear. Increased edema was noted in 10 of the 19 runners. However, not all had pain that would indicate an injury. It was noted that those with the greatest amount of edema were the ones who reported pain. It is possible that some with lower levels of edema were cases of bone remodeling that would be expected with an increased load, as opposed to an injury. However, there have been other case reports of individuals with bony (primarily metatarsal) injuries associated with minimal footwear. In these cases, it was pain that sent them to seek medical attention,[56,75] and definitive stress fractures were diagnosed. In both these reports, runners transitioned rapidly to their full mileage rather than progressing slowly. These studies indicate the need for engaging in a slow transition to minimal footwear for running to provide the time for adaptation. This gradual addition of loading may not only reduce injury risk but may also potentially result in some bone strengthening, according to Wolf's law.
Influence on Muscle
There have been a number of investigations of the effect of minimal footwear on muscle size and strength. A study by Holowka et al. reported that habitual daily users of minimal footwear had larger abductor hallucis and abductor digiti minimi (ADM) muscles compared with a supportive shod population. This likely is due to the greater demand placed on these muscles when walking in unsupportive footwear. Other studies have shown foot muscles hypertrophy when transitioning to minimal footwear for walking. A recent study reported that an 8-wk, progressive walking program in minimal shoes increased intrinsic and extrinsic muscle size and strength. In fact, walking in these shoes produced similar increases in the size and strength of foot muscles as the strengthening program completed by the foot strengthening group (Figure 7). As the loads of running are higher than those of walking, the potential for strengthening in minimal footwear is greater. Studies of minimal footwear use during running and athletic activities[76–79] have shown increases in the size and strength of a number of intrinsic and extrinsic foot muscles (EFM). In fact, every study that has examined the effect of minimal shoes on foot intrinsic and extrinsic foot muscle size or strength has reported increases (Table). The benefits of stronger intrinsic foot muscles include improved propulsion during walking and running[80,81] and control of midfoot deformation.[81–84] In addition, simulated contraction of cadaveric foot intrinsic muscles during loading has been shown to reduce the bending strain in the metatarsals. Finally, a recent study has demonstrated that runners who completed an 8-wk foot exercise program were 2.4 times less likely than the control group to develop a running-related injury. Therefore, we postulate that a gradual transition to minimal shoes, which promote foot strengthening, may also reduce the risk for injury in runners.
Comparison of muscle size changes between the control (C), foot strengthening (FS), and minimal shoe wear (MSW) groups for the flexor digitorum brevis (FDB), the flexor hallicus brevis (FHB), the abductor hallicus (ABDH), and the quadratus plantae (QP). Note the similar increases between the FS and MSW groups for three of the four muscles. [Adapted with permission from (74). Copyright © 2019 American College of Sports Medicine. All permission requests for this image should be made to the copyright holder.]
Although habituating to minimal footwear results in foot muscle strengthening, muscle injuries can occur if transitioning is done too quickly. In 1 study, 7 of 14 runners reported pain in the gastrocnemius/soleus/Achilles tendon complex during a 12-wk transition to running in minimal footwear. Similar to bone injuries, it is possible that many of the muscle strains or soreness injuries could be prevented by a slow increase in activity in minimal footwear. A foot core program has been shown to significantly increase the size and strength of the intrinsic and extrinsic foot muscles.[74,88] The addition of such a program can also help prepare the foot for the transition and reduce injury risk during this period.
Foot orthotic devices provide support to the foot. They are often prescribed for long-term use, which may negatively affect the foot. As these devices support the arch, the demand on the foot intrinsic muscles is reduced. In fact, an article by Protopapas and Perry reported a 10%–17% reduction in the foot intrinsic muscle sizes as a result of 12 wk of orthotic use. Therefore, just as minimal footwear that removes support from the foot has been shown to strengthen muscles, adding chronic support to the arch likely will weaken them. Therefore, if a foot injury requires additional temporary support of a foot orthosis, it should be gradually removed once the injury has healed to help strengthen the foot once it has recovered.
Influence on Tendon and Aponeurosis
The Achilles tendon is an important component of the stretch-shorten cycle of the triceps (TS) surae muscle-tendon unit. The tendon's mechanical properties, and particularly its material stiffness, affect force production and the performance of complex movement. There is some controversy regarding the capacity of mature tendon to adapt to loading. However, animal studies suggest that, like bone, the material properties of tendons can be dramatically increased with loading during growth and development. High peak loads have been shown to be most beneficial for homeostasis and adaptation of human tendon properties. An FFS strike pattern during running results in greater activation of the triceps surae and a higher rate and magnitude (8%–24%) of Achilles tendon loading than heel strike running.[92–94] Hence, minimalist footwear is associated with a loading stimulus that is more likely to induce Achilles tendon adaptation. Indeed, runners who wear minimalist footwear have been shown to have greater stiffness and cross-sectional area of the Achilles tendon than traditionally shod runners.[95,96] Moreover, the Achilles tendon of habitual FFS runners has been shown to be functionally stiffer during both walking and running, thereby aiding its "spring-like" function. Therefore, the additional loading of the Achilles tendon associated with minimalist footwear likely results in a stiffer tendon. This may be beneficial for activities requiring rapid force development and protective against strain-induced injury for a given load.
The plantar fascia effectively connects the expanse of the medial longitudinal arch and has been regarded as the primary structure stabilizing the arch during weight-bearing. Along with the ligaments of the medial longitudinal arch, the plantar fascia also may contribute to the elastic behavior of the foot and to improved locomotor efficiency. However, the medial longitudinal arch also is traversed by the intrinsic muscles of the foot and the long tendons of extrinsic foot muscles. These muscles are well positioned to reduce the load borne by the plantar fascia.[99–102] Running in minimal footwear is associated with intrinsic muscle hypertrophy,[68–71] a thinner plantar fascia, and a less compliant medial longitudinal arch. These findings are consistent with a shared load bearing role between these structures. With stronger intrinsic muscles, the plantar fascia can be thinner and more compliant. When the muscles are weaker, the plantar fascia adapts to become thicker to control the arch deformation during loading. Indeed, a thickened and stiff investing muscle fascia has been implicated in the development of other pathologies, such as chronic compartment syndromes of the lower leg.[104,105]
The calcaneal heel pad is a specialized fibroadipose tissue that is thought to play a number of mechanical roles during gait.[106,107] The first is shock reduction. During walking, the heel pad undergoes approximately 9–11 mm of vertical deformation, which is thought to lower the peak impact force. However, the heel pad offers minimal resistance to the rapid deformation induced by initial heel strike, suggesting it has only a minor role in shock reduction during walking and running. The second role of the heel pad is energy dissipation. However, only about 1.0 J of the strain energy stored in the heel pad during walking is dissipated with unloading. This only equates to about 20% of the impact energy of the foot and approximately 1% of the total energy exchanged during a single gait cycle (~100 J for a 70-kg human). This is less than that of the Achilles tendon (~2.5 J) and ligaments of the medial longitudinal arch (~3.1–4.5 J).[112–114] These structures reportedly behave as "springs" and are key structures associated with energy storage rather than energy dissipation. The energy dissipating properties of the heel pad are relatively insensitive to strain rate. Thus, the relatively low level of energy dissipation provided by the heel pad is unlikely to change substantially with increases in gait speed. This results in the heel pad being a less than ideal structure for dissipating the impacts associated with running. The third role of the heel pad is the protection against excessive plantar pressure. The mechanoreceptive and nociceptive nerve endings of fibroadipose tissues are localized between fat cells, and their sensitivity is related to the degree and rate of deformation of the tissue.[116,117] This endows the fat pad with a proprioceptive role for monitoring mechanical vibrations associated with heel strike, as well as for detecting pain.[118–120] Deformation of the heel pad during barefoot walking (approximately 60% or 10 mm) approaches that associated with the limits of pain tolerance. Hence, the FFS pattern adopted during barefoot running likely reflects a pain-avoidance strategy.
In conventional shoes, the heel pad is constrained and deforms only about 35% during walking and running.[106,122] Therefore, conventional footwear likely lowers the potential for strain-related injury of the heel pad. However, by inducing a slower loading rate and a lower final strain in the tissue, it also has the potential to lower the sensitivity of the heel pad to detect pain and potentially harmful vibrations. Minimalist shoes, in contrast, tend to promote a forefoot foot strike gait pattern during running. Cadaveric studies have shown that the fibroadipose tissues of the forefoot have both a higher material stiffness and greater capacity to dissipate energy than the heel pad. These tissues also have a higher density of vibration-sensitive mechanoreceptors.[124,125] Hence, fibroadipose tissues of the forefoot may better damp the impact vibrations associated with running than those of the heel and tend to be preferentially loaded in minimalist footwear.
Exerc Sport Sci Rev. 2021;49(4):228-243. © 2021 American College of Sports Medicine