With new found enthusiasm about barefoot mobility has come interest in intrinsic foot musculature. The term intrinsic foot muscles was probably first used along with intrinsic foot shock absorption with the report published in 1987, entitled RUNNING RELATED INJURY PREVENTION THROUGH BAREFOOT ADAPTATIONS. In this report, subjects who performed barefoot weight-bearing and mobility experienced rise in their medial longitudinal arch. It could only be explained by amplified intrinsic muscle tone. It was found to result from plantar tactile sensibility when barefoot. It was related to plantar fasciitis incidence in barefoot and shod populations:
Barefoot activity induces an adaptation that appears capable of transferring this impact to the yielding musculature thus sparing the fascia. Support for this hypothesis could come from studying the incidence of this condition in traditionally barefoot populations. The reports that the authors have received indicate a low frequency of plantar fasciitis in barefoot populations.
The intrinsic foot muscles most capable of raising the medial longitudinal arch are the flexor digitorum brevis and the flexor hallucis brevis. In a recent report, intrinsic foot muscle mass as measured by MRI was found to be significantly greater in subjects with healthy feet as compared to those with plantar fasciitis.(1) Either intrinsic foot musculature atrophy followed the onset of plantar fasciitis or weak intrinsic foot musculature cause plantar fasciitis, or both. Since this was not a prospective study, it could not distinguish cause from effect of plantar fasciitis, as noted by the authors. These authors remain in the self-imposed solitude whereby plantar tactile control processes are ignored.
In another report, it was found that a transition to minimalist shoes was associated with increase mass of intrinsic foot muscles.(2) Barefoot humans are indicted to use intrinsic foot muscles to avoid contact with skin near the metatarsal-phalangeal joints which has a low threshold to plantar-shear combined with vertical deformations - the adequate stimulus for plantar SA II mechanoreceptors. All shoes (including minimalist ones) substantially attenuate plantar shear, leaving SA II mechanoreceptor with no adequate stimulus from tactile sources, therefore the sensations that induce metatarsal-phalangeal avoidance through use of intrinsic foot musculature in wearers of minimalist shoes has a different source. Since minimalist shoes cause damage to metatarsal-phalangeal joints in a high percentage of users, as well as fractures of metatarsals near these joints, increase use of intrinsic foot muscles found in this report must have resulted from either an ongoing or earlier attempt to unload painful damaged metatarsal-phalangeal joints or metatarsals.(3) Because of this the use of minimalist shoes can hardly be recommended to improve intrinsic foot muscle tone thereby preventing plantar fasciitis because it does so at a considerable health cost. Only barefoot mobility seems capable of safely improving intrinsic muscle tone sufficiently to prevent plantar fasciitis while sparing metatarsal-phalangeal joints. The authors of this report remained in "the biomechanists' solitude" by ignoring reports regarding plantar tactile processes.
(1) Cheung, R., Sze, L., Mok, N., Ng, G. Intrinsic foot muscle volume in experienced runners with and without chronic plantar fasciitis. J Sci Med Sport 2015. [Epub ahead of print]
(2) Johnson, A., Myrer, J., Mitchell, U., Hunter, I., Ridge, S. The Effects of a Transition to Minimalist Shoe Running on Intrinsic Foot Muscle Size. Int J Sports Med. 37: 154-8 (2016).
(3) Ridge, S., et al. Foot bone marrow edema after a 10-wk transition to minimalist running shoes. Med Sci Sports Exerc.