Author: Kevin B. Rosenbloom, C.Ped, Sports Biomechanist

It is well known that the specialists at Kevin Orthopedic enjoy keeping up to date with topics regarding biomechanical function of the lower extremities. When the latest research released discussed the effectiveness of plantar intrinsic muscles (PIMs) support on the longitudinal arch, we felt it was necessary to read and provide a comment on the subject.

Firstly, the teams at the University of Exeter (UK) and University of Queensland (AU) should be commended for finally opening the door to the discussion of what others have only hinted. The teams’ experiment involved using a nerve block to prevent muscle contraction in the foot in different situations and comparing the results to the control group.

Farris et al. included that past research and cited studies have implied that large and strong PIMs are related to stiffer arches and are helpful with load management (4). Empirically, this study stated that there is minimal contribution of the PIMs to maintaining a stiff long arch, “probably because of their small size” (Farris et al. 1). It concluded that the plantar aponeurosis deserved the credit for maintaining most of the arch support. An explanation the authors suggested for what assisted the plantar aponeurosis:

It is conceivable that some missing active force contribution of the PIMs to LA support could have been compensated for by increased activation of extrinsic foot muscles, or by passive tension developed via lengthening of the PIMs. (Farris et al. 5)

Even the teams that produced these findings admit that there needs to be more exploration into the ideas because “passive tension was unlikely” and it was “doubtful that extrinsic foot muscles could compensate” (Farris et al. 5). However, the importance of the PIMs are apparent. The researchers wrote that the contribution of the plantar intrinsic muscles do appear significant:

Removing the ability to activate the PIMs reduces the positive mechanical work done (…) during push-off because of an earlier decline in power output (…). Activation of the PIMs in late stance is required to generate sufficient impedance about the MTP joint for an effective push-off through propulsive impulses and ankle joint work. (Farris et al. 4)

Although the study’s hypothesis, experiments and statistical analysis appear sound, there are some apparent faults regarding the diversity and backgrounds of the obviously small sample size. Farris et al. stated that, “Experiment 1 included 12 participants (eight men, four women; ± SD age = 28 ± 5 y; mass = 72 ± 11 kg), as did experiment 2 (nine men, three women; mean ± SD age + 30± 6 y; mass = 77  ± 12 kg)” (5). This is a common trope in most cases regarding vertebrate-based experiments, but should not be ignored.

Kevin Orthopedic contends that the life history of the individual patient is significant and needs to be evaluated along with the experiment. Individuals from various backgrounds and those who also began running early in life on non-paved roads and pathways could potentially display different results. With these evaluations, one could determine whether factors besides small size play a role, such as density, strength or endurance.

A question worth proposing: “Has modern life changed the function of PIMs?” One such factor could be the effects of shoe wear. Researchers and practitioners have alluded to the idea that shoe wear can inhibit or even atrophy PIMs. Another example would be the effects of smooth, modern surfaces on people’s feet. In combination with the effects of modern shoe wear, walking on a modern flat surfaces may contribute to the atrophy of PIMs by limiting the muscles’ tension. The shoes themselves could also be assisting with stabilization, thus relieving any stresses placed upon the PIMs. This is important because it could imply that as humanity has adapted to modern life, our adaptations could be at a detriment.

The article teases foot evolution, and these questions could expand that notion. The specialists at Kevin Orthopedic would like to see further discussion around the comparison of PIMs of modern, healthy humans and ancient ones. This idea could be achieved by an incremental, lifelong comparison study between individuals who grew up without modern roads and individuals who grew up in cemented city environments.

These are obviously grander ideas that need elaboration with more study, and this research paper is only a preliminary report. Kevin Orthopedic encourages more expansion of study into the topics mentioned in the article. Again, these researchers have opened the doors for examination and given the world of podiatry a progressive step forward.

Farris, D. J., Kelly, L. A., Cresswell, A. G., Lichtwark, G. A. 2019. “The functional importance of human foot muscles for bipedal locomotion.” Proceedings of the National Academy of Sciences, 1-6.  https://doi.org/10.1073/pnas.1812820116.

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Kevin B. Rosenbloom, C.Ped, Sports Biomechanist

Kevin B. Rosenbloom, founder, and president of KevinRoot Medical, is a renowned certified pedorthist and sports biomechanist practicing in Santa Monica, CA. With his continuing research on the historical development of foot and ankle pathologies, comparative evolution of lower extremities and the modern environmental impacts on ambulation, he provides advanced biomechanical solutions for his patients and clients.