Plantar foot pain is a broad term used to cover a wide range of diagnoses or issue. Plantar fasciitis, flat feet, hallux valgus, and Charco Marie Tooth just to name a few. While the exact derivative of each of these conditions is different, one thing they share is a lack of intrinsic foot muscle strength. Research has demonstrated a link between foot pain and decreased toe flexor strength and intrinsic foot muscle size. Performing exercises to help maintain foot flexor and foot intrinsic muscle strength may improve foot pain and function. However, foot intrinsic exercises such as “toe yoga,” “arch lifts,” and “foot shortening,” are inherently confusing and yield poor patient adherence. Luckily, biofeedback has been used previously to help improve patient compliance with exercise programs and has been shown to improve form, speed, and accuracy in patients performing foot intrinsic exercises.
What are the foot intrinsic muscles?
The foot intrinsic muscles have their origin and insertion within the foot. They largely contribute to the support of the medial longitudinal arch of the foot. The foot extrinsic muscles on the other hand, originate in the leg and are connected to the foot via long tendons. However, they also help support the medial longitudinal arch.
The plantar intrinsic muscles are divided into 4 layers, from superficial to deep:
- Layer 1: abductor hallucis, flexor digitorum brevis, and abductor digiti minimi
- Layer 2: quadratus plantae and the lumbricals
- Layer 3: adductor hallucis, adductor hallucis oblique, flexor hallucis brevis, and flexor digiti minimi brevis
- Layer 4: all three plantar interossei
The dorsal intrinsic muscles are divided into 2 layers, from superficial to deep:
- Layer 1: extensor hallucis brevis and extensor digitorum brevis
- Layer 2: dorsal interossei
What is the role of the foot intrinsics?
The foot intrinsic muscles (especially the abductor hallucis, flexor digitorum brevis, and the quadratus plantae) actively resist bending (or collapsing) of the arch during walking and more demanding activities. In sort, they offer active support to the arch of the foot. This is seen especially during the propulsion phase of walking, running, and jumping. When these muscles become weak and in cases of pes planus (or flat feet) the arch flattens or lowers, and medial longitudinal arch is no longer sufficiently supported.
This is where the use of mTrigger biofeedback can be extremely helpful. In patients with flat feet, the activity of the abductor hallucis muscle is significantly lower and learning how to correctly train this muscle is challenging.
What are foot intrinsic exercises?
Common exercise examples that target the medial longitudinal arch and foot intrinsic muscles include towel scrunches, picking up marbles, toe spreading, toe yoga, D1/D2 patterning, and arch lifts/foot shortening.[3,4] Lets go over some specific examples.
In this video of the ‘towel scrunch’ we see the foot intrinsics working together to shorten the plantar surface of the foot and accomplish the task. Since the arch of the foot is so small, when using sEMG to detect muscle activation, you are really looking at a combination of the activity of all the muscles under the sensing electrodes.
The foot shortening, or “arch lift” exercise is a bit more challenging. Having the mTrigger biofeedback was a great way to help reinforce proper movement and teach this patient the proper way to activate her arch.
Now let’s make things a little more complicated. PNF patterns of the foot are an excellent way to emphasize the foot intrinsic muscles. For the abductor hallucis in particular, the D1 and D2 pattern exercises have high activation levels.
Notice how in this video, the patient struggles to reach the proper levels of muscle activation consistently. You can see her adjust several times to achieve the correct muscle activation. This is exactly why biofeedback is so important!
Finally, in this last example, we will see how the hip controls so much of what happens at the foot. Weak hip abductors can cause increase pronation at the foot and flattening of the arch. When an isometric hip abduction is added to the toe shortening or arch lift exercise, there is an increase in activation levels of the foot intrinsics.
Check this out:
mTrigger can be an excellent tool for patients struggling with flat feet, foot pain, and plantar fasciitis. Learning to control the small muscle of the foot is very challenging and sometimes a little biofeedback goes a long ways in helping patients understand how to control such small muscles.
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 Latey PJ, Eisenhuth J, McKay MJ, et al. Feasibility of the Archercise biofeedback device to strengthen foot musculature. J Foot Ankle Res [Internet]. 2020 [cited 2023 Feb 21];13. Available from: https://pubmed.ncbi.nlm.nih.gov/32660591/.
 Soysa A, Hiller C, Refshauge K, et al. Importance and challenges of measuring intrinsic foot muscle strength. J Foot Ankle Res [Internet]. 2012 [cited 2023 Feb 21];5. Available from: https://pubmed.ncbi.nlm.nih.gov/23181771/.
 Choi JH, Cynn HS, Yi CH, et al. Effect of Isometric Hip Abduction on Foot and Ankle Muscle Activity and Medial Longitudinal Arch During Short-Foot Exercise in Individuals With Pes Planus. J Sport Rehabil [Internet]. 2020 [cited 2023 Feb 21];30:368–374. Available from: https://pubmed.ncbi.nlm.nih.gov/32717719/.
 Park DJ, Hwang YI. Comparison of the Intrinsic Foot Muscle Activities between Therapeutic and Three-Dimensional Foot-Ankle Exercises in Healthy Adults: An Explanatory Study. Int J Environ Res Public Health [Internet]. 2020 [cited 2023 Feb 21];17:1–10. Available from: https://pubmed.ncbi.nlm.nih.gov/33019530/.