Plantar Intrinsics: Important Or Useless Muscles Of The Foot?

We often think the plantar intrinsic muscles of the foot are relatively unimportant muscles of the foot and lower extremity. Researchers have largely ignored these muscles over the past century.

With that in mind, the plantar intrinsics have just recently been getting much more attention within the international running community. Advocates of barefoot running and minimalist running shoes have, for the last five years, been claiming that wearing traditional shoes “weaken the foot,” suggesting that the plantar intrinsic muscles are strengthened by being barefoot or by wearing minimalist running shoes. Unfortunately, without good scientific research that specifically studies plantar intrinsic muscle function, one can only guess the degree of importance of these muscles.

Luckily, multiple well-designed scientific research studies have emerged in recent years on the function of the plantar intrinsic muscles and the studies have shed more light on the importance of these muscles. Luke Kelly, PhD, a podiatric researcher from the University of Queensland, Australia, has published four important papers since 2012 on the function of the plantar intrinsic muscles that all podiatrists should be aware of.^1-4

Dr. Kelly and his coworkers published their first important plantar intrinsic muscle research in 2012.¹ In this study, they measured the electromyographic (EMG) activity of the abductor hallux, flexor digitorum brevis and quadratus plantae muscles using bipolar fine-wire electrodes in 10 male patients. Researchers also measured the center of pressure of the patients during sitting with the foot unloaded and during double-leg and single-leg standing. The researchers demonstrated that the patients activated their plantar intrinsic muscles during both single-leg and double-leg standing to help stiffen their longitudinal arches and optimize balance.

In their second study on the plantar intrinsic muscles, Kelly and colleagues loaded the distal femur in nine seated people with weights ranging from 0 to 150 percent body mass in 25 percent increments.² Researchers measured the EMG activity of the abductor hallux, flexor digitorum brevis and quadratus plantae muscles under varying loads. As the load increased, the longitudinal arch decreased in height and this resulted in increased EMG activation of the plantar intrinsics. Researchers also found that as the plantar intrinsics had electrical stimulation, the longitudinal arch height increased. With abductor hallucis and flexor digitorum brevis stimulation, the calcaneus inverted and abducted. With stimulation of the abductor hallucis, flexor digitorum brevis and quadratus plantae muscles, the forefoot plantarflexed and adducted. These experiments showed that plantar intrinsics can actively supinate the subtalar joint and raise the longitudinal arch in response to increased foot loading.

In another 2014 paper by Kelly and coworkers, the plantar intrinsic EMG activity increased during the stance phase of both walking and running with the greatest EMG activity occurring during faster running and the least EMG activity occurring during walking.³ The peaks and troughs in EMG activity during walking and running corresponded closely with the peaks and troughs of ground reaction force acting on the plantar foot. Finally, in their latest research published this year, Kelly and colleagues found that the peak flexor digitorum brevis activation and total stance activation of the flexor digitorum brevis and abductor hallux muscles increased when patients ran in shoes versus running barefoot.⁴ This suggests that the plantar intrinsic muscles are more active in patients with shoes in comparison to when they are barefoot.

With these four excellent research papers, Kelly and colleagues have demonstrated that the plantar intrinsic muscles are, indeed, important muscles that help control longitudinal arch deformation with the magnitude of their EMG activity being dependent on the type and intensity of the weightbearing activities patients perform. In addition, their latest research directly refutes the claims from barefoot/minimalist shoe advocates that being barefoot strengthens the foot and that wearing shoes “weaken the foot.”

It is time for the podiatric profession to take notice of this important recent foot muscle research in our podiatric medical schools, podiatric residency programs and podiatric seminars so we can stay current in our foot biomechanics knowledge, and continue to be considered the foot function experts within our medical communities.

Dr. Kirby is an Adjunct Associate Professor within the Department of Applied Biomechanics at the California School of Podiatric Medicine at Samuel Merritt University in Oakland, Calif. He is in private practice in Sacramento, Calif.

References

1. Kelly LA, Kuitunen S, Racinais S, et al. Recruitment of the plantar intrinsic foot muscles with increasing postural demand. Clin Biomech. 2012; 27(1):46-51.  
2. Kelly LA, Cresswell AG, Racinais S, et al. Intrinsic foot muscles have the capacity to control deformation of the longitudinal arch. J R Soc Interface. 2014; 11(93):20131188.
3. Kelly LA, Lichtwark G, Cresswell AG. Active regulation of longitudinal arch compression and recoil during walking and running. J R Soc Interface. 2015; 12(102):20141076.
4. Kelly LA, Lichtwark GA, Farris DJ, Cresswell A. Shoes alter the spring-like function of the human foot during running. J R Soc Interface. 2016; 13(109):20160174.