The Plantar Foot from Surface to Deep

 The soles of our feet are under a lot of stress — mechanical stress, that is. Consider how much we rely on our plantar foot to quietly manage some pretty impactful tasks, from bearing our body weight as we stand at the massage table to absorbing impact as we walk, run, or jump. Our feet are able to meet the challenges of compression, shock absorption, shearing, and tensile loading with specialized tissues that are tough, resilient, and surprisingly sensitive. 

Anatomy education sometimes focuses on isolated structures, but when it comes to understanding function, we have to widen our view — it takes a village of different tissues working together to get the job done. 

Let’s take a tour of the plantar foot from surface to deep to get better acquainted with three of these stress management specialists. 

SURFACE: the SKIN

Shoes first appeared on the scene in the human story only about 10,000 years ago (with running shoes not even making their debut until 1852). Before that, we had our skin. Fortunately, plantar skin is highly specialized and well-suited for the job of repetitive weight-bearing and negotiating abrasive terrains during locomotion. It needs to be tough enough to protect itself from injury, as well as be able to “take one for the team” by offloading external stresses before they impact the rest of the foot.

The scientific term for specialized plantar skin is simply thick skin or glabrous. It’s characterized by a protective epidermis that is far thicker than skin found elsewhere on the body with the ability to thicken further in response to strain. We recognize this familiar response as calluses. After a few weeks of walking barefoot in the summer, the epidermis on the bottom of your feet will have toughened and thickened by adding more keratinized layers. These new layers create a reinforced protection to handle the extra friction and shearing forces that come with increased barefoot time. 

Even with all that epidermis thickening, the plantar skin is exquisitely sensitive, equipped with sensory nerve endings, as well as proprioceptive mechanoreceptors. Sensitivity is so essential to the skin’s protective role that it’s preserved even in thickly callused skin.

GOING DEEPER: FAT PADS

The toughness of the plantar foot doesn’t stop at the skin. Immediately beneath the dermis, we encounter fibrous fatty tissue: the plantar fat pads. While the word “fat” often has a connotation of soft or weak, make no mistake, this fibro-adipose tissue is designed for work. The fat pads are thickest under the heel and metatarsal joints and work with the skin as a shock-absorbing stabilizer.  

The fat pads are made of micro sized fat lobules that look like tightly packed, tiny, yellow beads. Measuring just 1-2 millimeters in diameter, each micro lobule is nearly incompressible, hardly deforming at all when under load due to its encasement in a dense, three-dimensional collagen network of specialized septa (skin ligaments). The septa are continuous with the dermis and strongly adhered to the plantar fascia forming a tight tethering between the two that prevents gliding and helps the foot grip the ground. Without the supportive architecture of the fat pad and its stabilizing relationship to the skin and plantar fascia, walking would feel like slipping on banana peels with a constant risk of losing balance and falling.

Looking specifically at the heel’s fat pad, we find an extra layer of cushioning between the calcaneus and the skin. Deep to the micro lobules, we encounter larger, macro fat lobules. Approximately the size of mini marshmallows, the macro lobules are not as tightly constrained as their micro lobule counterparts allowing them to deform slightly under loading and to provide a dampening effect for impact at the heel.

EVEN DEEPER: the Plantar Fascia

Deep to the fibrous fat pads, we find the plantar fascia (PF). Tough in its own right, this thick, fibrous band of tendon-like connective tissue has a different composition than the fat pads with a different job to do. The PF extends from the calcaneal tuberosity along the length of the sole of the foot. As it approaches the metatarsal bones, it spreads out into five distinct slips that merge into the fascia surrounding each toe. 

Observing the PF up close, you can clearly see perfectly aligned silvery-white collagen fibers running longitudinally from heel to toes. The flat, dense construction of the PF is not optimized for cushioning, but for tensile loading. Under tension, the PF helps propel us forward while walking. Increasingly recognized for its fascial continuities with the Achilles tendon and the posterior chain of the body, the PF also has many important connections locally. Deeply rooted into its surrounding tissues of skin, muscle, and bone, the PF is richly supplied with proprioceptive mechanoreceptors indicating its guiding role in the control and stability of the entire foot. 

WHY WE CARE 

Foot pain is often due to the inability of the foot’s tissues to adapt to the loads placed on them. Understanding their function and relationships to each other, helps us refine our touch and more comprehensively support our clients. From surface to deep, each tissue in the plantar foot is pretty amazing. Combined as a team, however, they allow our feet to do “shockingly” good things. 

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Further Reading:

Benjamin, Mike. “The Fascia of the Limbs and Back – a Review.” Journal of anatomy 214, no. 1 (2009): 1–18.

Chanda, Arnab, and Stephen McClain. “Mechanical Modeling of Healthy and Diseased Calcaneal Fat Pad Surrogates.” Biomimetics (Basel, Switzerland) 4, no. 1 (2019): 1–.

Earls, James. Understanding the Human Foot: An Illustrated Guide to Form and Function for Practitioners. Chichester, England: Lotus Publishing, 2021. 

Jahss, M. H., Michelson, J. D., Desai, P., Kaye, R., Kummer, F., Buschman, W., Watkins, F., & Reich, S. (1992). Investigations into the fat pads of the sole of the foot: anatomy and histology. Foot & ankle, 13(5), 233–242. 

Natali, Arturo N., Piero G. Pavan, and Carla Stecco. “A Constitutive Model for the Mechanical Characterization of the Plantar Fascia.” Connective tissue research 51, no. 5 (2010): 337–346.

 Pavan, P. G., C. Stecco, S. Darwish, A. N. Natali, and R. De Caro. “Investigation of the Mechanical Properties of the Plantar Aponeurosis.” Surgical and radiologic anatomy (English ed.) 33, no. 10 (2011): 905–911.

Ross, M. H., & Pawlina, W. (2010). Histology: A text and atlas: With correlated cell and molecular biology (6th ed.). Philadelphia: Lippincott Williams & Wilkins.

Stecco, Carla, et al. Functional Atlas of the Human Fascial System. Churchill Livingstone Elsevier, 2015. 

Swenson, O. “Specialized Keratin Expression Pattern in Human Ridged Skin as an Adaptation to High Physical Stress.” British journal of dermatology (1951) 139, no. 5 (1998): 767–775.

Welte, Lauren, Luke A Kelly, Sarah E Kessler, Daniel E Lieberman, Susan E D’Andrea, Glen A Lichtwark, and Michael J Rainbow. “The Extensibility of the Plantar Fascia Influences the Windlass Mechanism During Human Running.” Proceedings of the Royal Society. B, Biological sciences 288, no. 1943 (2021): 20202095–20202095.

Wu, Chueh-Hung, Che-Yu Lin, Ming-Yen Hsiao, Yu-Hsuan Cheng, Wen-Shiang Chen, and Tyng-Guey Wang. “Altered Stiffness of Microchamber and Macrochamber Layers in the Aged Heel Pad: Shear Wave Ultrasound Elastography Evaluation.” Journal of the Formosan Medical Association 117, no. 5 (2017): 434–439.

Wynands, Bert, Claudio Zippenfennig, Nicholas B. Holowka, Daniel E. Lieberman, and Thomas L. Milani. “Does Plantar Skin Abrasion Affect Cutaneous Mechanosensation?” Physiological reports 10, no. 20 (2022): e15479–n/a.