Importance of intrinsic muscles for foot health

A growing body of research suggests the strength of small muscles in the foot can have big implications for a range of foot-health issues—including running-related injuries, diabetic neuropathy, obesity, flatfoot, and plantar fasciitis—and that interventions for strengthening the intrinsics may help.

By Barbara Boughton

The foot’s intrinsic muscles—once overlooked by most practitioners and researchers—have begun to attract attention. Recent studies suggest that strong intrinsic foot muscles help maintain the foot’s health, and intrinsic weakness may also contribute to painful foot conditions.

But can strengthening intrinsic foot muscles actually help prevent foot conditions, or slow symptom progression? In the past few years, research has begun to delve into this question.

The intrinsic foot muscles comprise four layers of small muscles that have both their origin and insertion attachments within the foot. They include the abductor halluces, the flexor digitorum brevis, the abductor digiti minimi, and the quadratus plantae. Most are located on the inferior part of the foot and primarily stabilize the arch.^1-3

The deterioration of intrinsic foot muscles may play an important role in foot conditions as diverse as plantar fasciitis, flatfoot, and diabetic foot deformity, according to recent research. Researchers are also studying the ways in which foot exercises may strengthen the intrinsic foot muscles in patients with flatfoot and diabetes. Scientists and clinicians are even researching the ways in which barefoot running or running in minimalist shoes may strengthen the intrinsic muscles of the foot, and help prevent running-related pain and injury.

Diabetic neuropathy

Recent studies have documented that patients with diabetic peripheral neuropathy have weak intrinsic foot muscles and intrinsic foot muscles characterized by an infiltration of adipose tissue.^4,5 This adipose tissue infiltration and intrinsic muscle weakness, along with limited ankle dorsiflexion mobility, may result in metatarso­phalangeal (MTP) joint hyperextension movement patterns during active ankle dorsiflexion in patients with diabetes. Continuous repetitions of these movement patterns may then lead to MTP joint deformity, and eventually increased risk for ulceration.^4,5

“As researchers, we are trying to look at the physiological processes upstream of amputation, including foot deformities, in order to reduce the incidence of amputation. We’ve found that weakness of intrinsic muscles paired with infiltration of adipose tissue under the fascia can lead to decreased strength and impaired foot function, helping to set the foundation for ulceration,” said Victor Cheuy, PhD, a postdoctoral research fellow in the physical therapy program at the University of Colorado Denver.

Advertisement

In a 2013 study,⁴ Cheuy and colleagues found that 23 patients with diabetic neuropathy had more deterioration and more adipose tissue in their intrinsic foot muscles on magnetic resonance imaging (MRI) scans than 12 age-matched controls. The neuropathic individuals, in fact, had severe intrinsic foot muscle deterioration with a ratio of adipose tissue to lean muscle volume that was five times higher than the controls. Those with the greatest intrinsic muscle deterioration—and the highest ratio of adipose tissue to lean muscle volume—were also more likely to have MTP joint angle deformity.

Diabetes creates a high glucose environment that not only results in peripheral neuropathy, but also causes adipose stem cells to differentiate into adipose cells, which migrate into muscle tissue, according to several studies.^6,7

In a larger 2015 study of 34 patients with diabetic neuropathy, Cheuy and colleagues also found that intrinsic foot muscle deterioration (measured in terms of total forefoot lean muscle volume) and maximum ankle dorsiflexion were correlated with severity of MTP joint deformity.⁵ Intrinsic foot muscle deterioration and limited ankle mobility may be the primary contributors to the development of MTP joint deformity, the researchers concluded in their paper.

Although exercises have been developed to build strength in intrinsic foot muscles, it’s not yet known how well patients with diabetic neuropathy will respond to such interventions, according to Cheuy and other researchers.

“Can these exercises slow, stop, or prevent muscle weakness and adipose tissue infiltration? It may depend on the severity of the diabetic neuropathy and intrinsic foot muscle weakness,” Cheuy said.

A new National Institutes of Health-funded study, however, aims to test whether foot strengthening exercises will strengthen intrinsic foot muscles and build up muscle volume in patients with diabetic neuropathy. Participants will be randomized to a foot-specific exercise intervention or a shoulder-specific exercise intervention, since many patients with diabetes also have shoulder dysfunction, said lead researcher Mary Hastings, PT, DPT, associate professor in the program in physical therapy at the Washington University School of Medicine in St. Louis, MO.

Both groups will participate in the exercise program for six months. In the foot-specific exercise group, patients will be taught how to sense the arch of their foot and perform foot shortening (or doming) exercises. Then the foot muscles will be loaded with up to half their body weight, and the patients will practice foot shortening exercises and exercises such as heel raises. Patients will be followed for three years during and after the intervention, she added.

“As well as measuring changes in muscle volume, we’ll be assessing changes in foot function as a response to the exercises,” Hastings said.

Other patient populations

Recent studies also suggest that weakness in intrinsic foot muscles negatively affects other patient populations, such as those who are overweight and obese, or have conditions such as plantar fasciitis.

In a 2015 study of 312 Australian men and women aged between 60 and 90 years, researchers found foot pain was significantly increased in overweight (body mass index [BMI] between 25 and 30 kg/m²) and obese (BMI > 30 kg/m²) patients older than 60 years, and these patients were also more likely than their normal-weight counterparts to exhibit diminished foot strength.⁸ The researchers assessed musculoskeletal foot structure using a 3D foot scanner, and recorded maximum isometric dorsiflexion strength, hallux strength, and lesser toe flexor strength using pressure platforms.

In the study, obese patients also had significantly reduced flexor strength of the hallux and lesser toes compared with overweight and normal-weight participants. The decreased muscle strength and the increased plantar pressures in obese patients were the most crucial determinants of foot pain, the researchers concluded in their paper.

“My current research is focused on exercises to strengthen the foot muscles, and I certainly advocate that individuals with weak foot muscles may benefit from doing foot strengthening exercises,” said lead researcher Karen Mickle, PhD, a postdoctoral research fellow at the Institute of Sport, Exercise & Active Living at Victoria University in Melbourne, Australia.  “However, it is difficult to say that obese people would see greater benefit [than normal-weight patients].”

Recent studies have also reported that patients with flatfoot and chronic plantar fasciitis demonstrate atrophy of the intrinsic foot muscles. A 2015 study,⁹ for instance, found that runners with chronic plantar fasciitis had smaller intrinsic rearfoot muscle volume than healthy runners, although the sample size was small. Forefoot muscle volume was also smaller in the runners with plantar fasciitis, although this difference did not reach statistical significance. The results “suggest that atrophy of intrinsic foot muscles may be associated with symptoms of plantar fasciitis in runners,” possibly by destabilizing the medial longitudinal arch, the researchers wrote. The findings may have implications for the design of clinical rehabilitation strategies to help address chronic plantar fasciitis symptoms by strengthening the intrinsic foot muscles, the scientists concluded.

Strengthening interventions

Yet researchers are only starting to test the efficacy of exercises that strengthen the foot’s intrinsic muscles for alleviating conditions such as plantar fasciitis and flatfoot. In one recent study,¹⁰ 18 patients with pronation (more than 10 mm based on the navicular drop test) were randomized to perform exercises to strengthen the abductor halluces (with toe spread exercises¹¹) or the toe spread exercises plus exercises to strengthen gluteus maximus muscles. The participants performed the exercises five times per week for four weeks. All showed significant improvement in the height of navicular drop at the end of the study, though those who performed both types of exercises showed greater improvement.

Toe spreads and squeezes are aimed at strengthening specific intrinsic foot muscles—the dorsal and plantar interrosei, according to Irene S. Davis, PhD, PT, director of the Spaulding National Running Center and a professor in the Department of Physical Medicine and Rehabilitation at Harvard Medical School in Boston. Doming or foot shortening exercises contract most of the muscles on the plantar side of the foot, and help to strengthen the abductor hallucis muscle.¹¹

In addition to exercises such as doming (foot shortening), toe spreading, and toe squeezing, some researchers recommend barefoot running or running in minimalist shoes as a way to strengthen the intrinsic muscles of the foot.

“When you run in minimalist shoes—that is, shoes with no cushioning and no arch support—you increase the demands on muscles of the foot, and these muscles strengthen. With stronger arch muscles, there is less stretch on the foot. This provides some protection from plantar fasciitis. Minimal shoes also promote landing on the ball of the foot rather than the heel, which increases the demand on the arch muscles, providing the potential for further strengthening,” Davis said.

In a study just epublished in May,¹² Davis and colleagues found a significant increase in foot muscle volume—forefoot muscles in particular—in 20 habitually shod runners who completed a six-month transition to minimalist footwear. Similar increases were not seen in a group of 18 controls who completed the same training program while continuing to wear conventional running shoes.

The findings are consistent with those of a 2016 study,¹³ in which researchers compared intrinsic foot muscle size in runners who were randomly assigned to use minimalist or traditional running shoes over 10 weeks. At the end of the study, the group who wore the minimalist shoes increased their abductor hallucis cross-sectional area by 10.6% more, compared with baseline, than the control group—a statistically significant difference. Although other muscles were examined in the study, there were no significant changes in muscle size when the intervention and control group were compared, according to the researchers.

Since barefoot and minimalist shoe running may improve intrinsic foot muscle strength, switching from conventional running shoes may help prevent the development of conditions such as plantar fasciitis and flatfoot, Davis said.

“Forefoot landings are safer, gentler landings, and the forces your body experiences are much more gradual,” she said. “As a result there may be less potential for injury. However, it is important to properly prepare the foot and ankle for the greater demands placed on the muscles by adding a strengthening program and progressing gradually.”

In a large population-based 1992 study on 2300 schoolchildren in rural India,¹⁴ researchers assessed the incidence of flatfoot among children who predominantly wore shoes and those who went barefoot. The children who wore shoes had a significantly higher prevalence of flatfoot (8.6%) than those who did not wear shoes (2.8%). Flatfoot was most common in those who wore closed-toed shoes, less common in those who wore sandals or slippers, and least common in children who went barefoot.

Transition with caution

But clinicians should be cautious when extrapolating such findings, experts say.

“The study associated absence of footwear with the development of an arch. At the same time, there have been no randomized controlled studies yet that indicate that barefoot running can heal any pre-existing foot conditions,” said Patricia Pande, MClScPT, CSCS, CPed, a physical therapist, pedorthist, and strength and conditioning specialist based in Durham, NC. She is the founder of FootCentric, an online continuing education company dedicated to foot treatment.

Still, even researchers and clinicians who extol the benefits of barefoot running say that caution should be used when advising patients with foot conditions to use barefoot running.

“Minimalist running can be done by people with foot problems, especially if they have metatarsalgia or other forefoot conditions,” Pande said. “The caveat is that one style of shoe or one style of running is not best for all. People who wish to run in minimalist shoes should have an assessment of their dynamic and static balance with and without the shoes, as well as any areas of wear and erythema while wearing these shoes.”

It’s also important to transition to barefoot running gradually to prevent injury, Pande said.

At least one study has documented that patients who transition to minimalist running shoes over 10 weeks can sustain foot bone marrow edema—a sign of potential stress injury—as well as stress fractures.¹⁵ In the 2013 study, 36 recreational runners were randomized to transition to minimalist running shoes over 10 weeks or to continue to run in conventional running shoes. MRI images were taken before and after the 10-week period. More runners in the minimalist shoe group had increases in bone marrow edema than in the control group. In the minimalist shoe group, three of 19 runners also sustained stress fractures, while no fractures were seen in the 17 runners who continued to use conventional shoes.

In the 2016 study mentioned earlier, the same group also found runners who developed bone marrow edema during the transition to minimalist shoes had significantly smaller muscle cross-sectional area at baseline than those who did not develop edema; this was the case for all of the intrinsic muscles that were assessed.¹³ This suggests runners with particularly weak intrinsic foot muscles may benefit from low-impact strengthening exercises before undertaking a transition to barefoot or minimalist running.

The risk of injury when transitioning to minimalist shoes can also be reduced by transitioning in an organized fashion over an extended time. Runners should take two to three months or more to transition to minimalist shoes, said Jean-Francois Esculier, MSc, PT, vice-president and director of research and development for the Running Clinic, an online continuing medical education site dedicated to prevention of running injuries, and a PhD candidate at Laval University in Quebec, Canada.

“The safest way to transition is to use minimalist shoes for a portion of every training,” Esculier said.

A runner may start out by wearing minimalist shoes for only a few minutes during a 60-minute training session and conventional shoes the rest of the time. The minutes spent in minimalist shoes should gradually be increased by a few minutes each session, until the runner is using minimalist shoes for the entire session, he said.

As well as wearing minimalist shoes, Esculier recommends that runners should do exercises that build the strength of the intrinsic foot muscles to improve their performance. Some of the exercises he recommends include toe flexor exercises or jumping rope in minimalist shoes, an exercise that should be incorporated gradually into a workout. The runner can then progress to strengthening the intrinsic foot muscles with foot shortening exercises performed while hopping on one foot or from foot to foot, he said.

However, the implementation of such exercises as well as the choice of whether or not to wear minimalist shoes should be individualized to each runner and/or patient, Esculier said.

“I do recommend minimalist shoes for patients with foot conditions such as those with plantar fasciitis,” he said. “But my recommendations often depend on the severity of the patient’s foot condition. And the golden rule should always to be to listen to your body—if an exercise or type of shoe is causing pain, then it’s time to pause or even stop.”

Barbara Boughton is a freelance writer based in the San Francisco Bay Area.

Advertisement