Kevin A. Kirby, DPM

11/04/2025

Biomechanics of the Stress-Strain Curve of Ligament and Tendon

Tears within the posterior tibial )PT) tendon will cause an increased magnitude of tendon tension stress in the tendon fibers which remain in the area of the PT tendon tear. This increased PT tendon stress at the site of the tendon tear may, over time, increase the risk of further tears in that torn area of tendon.

To review, tissue stress is determined by the force acting within a tissue divided by the cross-sectional area of the tissue. Tissue stress may be calculated for any of the body's structural tissues, including bone, cartilage, ligament, tendon, fascia, muscle, skin or adipose. In the biomechanical literature, stress, σ, is calculated by dividing the force applied to the object, F, by the cross-sectional area of the object, A. In equation form, σ = F/A.

In the biomechanics research laboratory, the load versus deformation characteristics of one of the body's tissue ican be measured by a device called a Materials Testing Machine. The following video is of a material testing machine doing cyclic stretching of a thread and a rat tendon.

https://www.youtube.com/watch?v=6METprZvDHI

Material testing machines can precisely measure the force applied to a specimen of tissue and the deformation of that specimen so that both the load and deformation of the specimen can be determined. Materials testing machines allows us to subject a structural tissue of a human or animal body to a strain (i.e. deformation) and compare that strain relative to the amount of stress that results from that deformation.

Strain is a dimensionless number representing the percentage elongation or compression that a material is deformed relative to its original length. If, for example, an anterior cruciate ligament is 30 mm in length at the start of an experiment, and then has tension force applied to it so that it lengthens 3 mm, the strain would be 3 mm/30 mm = 0.10.

The illustration below represents a stress-strain curve for a typical ligament or tendon. The vertical axis (Y-axis) represents the stress within the tissue and the horizontal axis (X-axis) represents the strain of the tissue. In other words, the stress-strain curve represents a type of load vs deformation curve.

It can be seen at low levels of strain and stress in a ligament or tendon, the stress-strain curve is relatively linear, with each increase in tension strain (i.e. increase in length) on the ligament of tendon producing a corresponding increase in ligament or tendon stress (i.e. in other words, an increase in force per unit cross-sectional area within the ligament or tendon). This linear region is known as the elastic region of the stress-strain curve since repeated increases and decreases in tension load will result in no permanent change in ligament or tendon length.

This elastic region within ligaments and tendons represents the tension loads that occur thousands of times a day in the bodies of animals, including humans, without injury. However, at higher levels of strain and stress, the stress-strain curve starts to flatten, indicating that the tissue is starting to increasingly deform more per given increase in tension load. In other words, the tissue is starting to permanently deform, or undergo "plastic deformation".

Plastic deformation means that the increase in ligament or tendon length at higher loads is a permanent deformation, and represents, in ligament or tendon, individual ligament or tendon fibers breaking or sliding on each other, permanently lengthening their structure. In other words, the plastic region of the stress-strain curve is where tissue deformation and tissue injury occur, whether this is a partial rupture, lengthening or complete rupture of a ligament or tendon, a fracture of a bone, a tear in a cartilage or an ulceration within the skin.

When tissue loads are increased into the plastic region of the stress-strain curve for a ligament or tendon, the ligament or tendon will permanently elongate due to tendon injury. Our jobs, as health professionals, is to keep the tissues of our patients functioning within their elastic range of their stress-strain curve since elastic deformations can occur daily with no injury. If our patients have plastic deformations of their tissues, they will likely develop pain that brings them into our offices for our expert help.

As health professionals, our advice and treatment should reduce tissue stress on these injured tissues so that plastic deformations (i.e. tissue injury) no longer occurs. This "Tissue Stress" type of treatment will eventually allow the injury to heal, which will return our patients quickly return to their daily weightbearing activities, without pain or disability.

11/04/2025

Posterior Tibial Tendon Dysfunction and Subtalar Joint Axis Location

Posterior tibial tendon dysfunction (PTTD) is a painful and disabling condition of the foot and lower extremity that most commonly affects women over the age of 50 and is one of the more common conditions that I have treated with custom foot orthoses over the past 40 years of clinical practice. In the 800+ cases of PTTD that I have treated with custom foot orthoses over the years, I have found that the combination of properly constructed foot orthoses combined with supportive shoes/boots and physical therapy reduces the pain of ambulation sufficiently in over 75% of patients so that corrective foot surgery is not required.

The main pathology of PTTD is thought to begin with some form of internal damage to the posterior tibial (PT) tendon. The tendon may develop a partial tear, may elongate, may degenerate or may even completely rupture. Weakness of the posterior tibial muscle, which occurs due to the tendon tear, will tend to increase the flatfoot deformity over time due to loss of the important internal supination effects of the PT muscle-tendon complex.

Patients with PTTD very commonly state that they had a “flatfoot” or “pronated foot” before the pain and disability of PTTD began. All of the patients with PTTD that I have examined over the past four decades have demonstrated a medially deviated STJ axis (Kirby KA: Subtalar joint axis location and rotational equilibrium theory of foot function. JAPMA, 91:465-488, 2001). Therefore, the most logical biomechanical etiology for PTTD is that the foot with a medial STJ axis has a PT muscle that is mechanically-disadvantaged. In other words, the PT muscle-tendon complex can’t generate normal magnitudes of internal STJ supination moment even before the symptoms of PTTD begin (see photo below).

The medial deviated STJ axis not only requires increased internal STJ supination moments from the PT muscle to supinate the foot but also causes a decreased supination moment arm for the PT tendon to produce these supination moments (see my illustration below).The mechanical result of this pre-existing medially deviated STJ axis in the individuals who ultimately develop PTTD is increased tension forces within the PT tendon, which greatly increases the tension stress within the PT tendon. The increase in PT tendon tension stress cause the tendon to function toward the yield point on its stress-strain curve (See my illustration below). The result is that plastic deformation can occur to the PT tendon even with seemingly trivial traumatic events (Kirby KA: Foot and Lower Extremity Biomechanics III: Precision Intricast Newsletters, 2002-2008. Precision Intricast, Inc., Payson, AZ, 2009, pp. 43-44).

The internal damage to the structure of the PT tendon structure sets up a cascade of mechanical events that eventually may cause the progressive development of a flatfoot deformity. PT tendon damage causes weakness and/or pain in the PT muscle which results in decreased STJ supination moment and leads to increased external STJ pronation moments acting from ground reaction force (GRF). The increased STJ pronation moments eventually lead to increased tension stress on the spring ligament complex which may, over time, result in its partial or complete rupture. Other "anti-pronation" plantar ligaments which help support the medial longitudinal arch of the foot may also become torn or damaged. Tearing of the spring ligament will specifically cause further longitudinal arch flattening and also will cause the forefoot to become more abducted on the rearfoot.

Progressive abduction of the forefoot on the rearfoot due to spring ligament rupture causes further increases in medial deviation of the STJ axis relative to the forefoot which further increases the magnitudes of external STJ pronation moments from GRF acting during weightbearing activities. This cascade of pathologic events will continue until the medial arch flattens completely or until some form of mechanical treatment is begun (Kirby KA: Conservative treatment of posterior tibial dysfunction. Podiatry Management, 19:73-82, 2000).

In order to stop the progression of flatfoot deformity that occurs in PTTD, the excessive external STJ pronation moments from GRF caused by the abnormally medially deviated STJ axis must be counterbalanced by STJ supination moment from some form of mechanical therapy. Custom foot orthoses offer the simplest, most effective and most reliable method to treat PTTD by increasing the external STJ supination moments by shifting the GRF more medially on the plantar foot.

My prescription for foot orthoses for patients with PTTD include a 4-5mm polypropylene shell, a 2-6 mm medial heel skive, a 16-20 mm deep heel cup, a 2-3 mm heel contact point thickness and a minimal medial expansion plaster thickness and 2-5 degree inverted balancing position of the positive cast. The medial heel skive, when combined with the increased medial arch height of the inverted balancing position and minimal medial expansion plaster, work synergistically together to increase the STJ supination moment both at the rearfoot and midfoot/forefoot portions of the custom foot orthosis (Kirby KA: The medial heel skive technique: improving pronation control in foot orthoses. JAPMA, 82: 177-188, 1992).

It is also quite important, in the earliest stages of treating the patient with PTTD, to make certain the patient is placed into a shoe with a stable sole and relatively rigid construction. Alternatively, in more advanced cases of PTTD, I recommend a high top boot or high top shoe in order to help brace the ankle and offer some resistance to STJ pronation by the bracing effect of the boot superior to the STJ axis. Even though custom foot orthoses placed in low-cut shoes with anti-pronation features may help patients with milder forms of PTTD, more severe forms of PTTD nearly always require a more supportive high top, lace-up design to achieve optimum therapeutic control of the abnormal foot motion.

In addition, I commonly use icing therapy for the first three months of PTTD treatment to reduce PT tendon edema and pain. Typical icing therapy recommendations include icing the symptomatic tendon of the medial ankle and foot for 15-20 minutes, once to twice daily. I likewise instruct the patient on PT muscle strengthening exercises once the initial inflammation of the PT tendon starts to subside with treatment. Most of the patients I have treated over the years with PTTD have been very pleased with the treatment of their painful and disabling condition with correctly-designed custom foot orthoses and greatly appreciate their ability to resume their daily activities again without pain.

[Adapted from Kirby KA: Prescribing better foot orthosis: Posterior tibial tendon dysfunction, May 2010. In Kirby KA: Foot and Lower Extremity Biomechanics IV: Precision Intricast Newsletters, 2009-2013. Precision Intricast, Inc., Payson, AZ, 2014, pp. 93-94).

11/03/2025

My friend and colleague, Seamus Kennedy, just had an excellent article published on the medial heel skive, in the O & P Edge magazine. The medial heel skive is a custom foot orthosis modification which I first developed in 1990 and is now being used worldwide for the treatment of pronation-related disorders of the foot and lower extremity with custom foot orthotics. https://opedge.com/the-kirby-medial-heel-skive/

Controlling excessive foot pronation is an important aspect of managing many lower-limb pathologies such as plantar fasciitis, posterior tibial tendon ...

10/26/2025

One of the podiatrists at the 54th Annual Spanish Podiatry Congress in Gran Canaria last weekend gave me a t-shirt, small poster and mug with the image of what is supposed to be me posed like the movie poster of "Kill Bill" by Quentin Tarantino, starring Uma Thurman. Odd but cute.

10/26/2025

I just got back from giving three lectures last weekend at the 54th Annual Spanish National Podiatry Congress held in Gran Canaria, Canary Islands. This is my 5th time lecturing in Spain and, as usual, the Spanish Podiatry Association put on a very educational and well-run two-day event with 950 podiatrists from a number of nations attending.

Other US podiatrists who were also lecturing at the conference included Drs. David Armstrong, Luke Ciccinelli and Alan Banks. Was also great to be able to catch up with my very good Spanish podiatry friends, Pascual Huerta and his wife, Lucia. Here are some photos from this excellent and fun educational event.

10/17/2025

Had a good first day of lecturing with a fine group of podiatrists at the 54th Annual Spanish National Podiatry Congress in Gran Canaria. Lecturers in my first session this morning included Yves Lecure, Emilio Merchan Ortega and Luis Enrique Roche. Always good to see Gabriel Gijon at the conference also.

09/26/2025

The Evolution of Running Shoes and What Comes Next

I was recently interviewed by the Wall Street Journal for an article on the "Evolution of Running Shoes" which discussed the barefoot/minimalist running shoe fads and now the transition toward maximalist and super shoes. Fun article.

As sneakers get taller, experts weigh in on the shoes of the future and what they mean for your running health.

09/25/2025

I was happy to welcome Mr. Frank Ding to my office yesterday who flew all the way from Shanghai, China, to visit with me and discuss his foot issues. Frank and I have been communicating by email over the past year. He is also interested in trying to establish some form of a podiatry profession in China, which at this time is nonexistent in that country. Frank has self-taught himself foot anatomy and biomechanics and is quite knowledgeable. It was my great pleasure to finally get to meet him in person yesterday at my office. Safe travels Frank!

09/14/2025

Just got back from attending and lecturing at the Balance Health Conference in the SF Bay Area the last two days and was able to catch up with my friends and colleagues including Jeffrey Christensen, Tom Chang, and Larry Huppin. Great conference!

08/18/2025

Study Demonstrates Importance of Plantar Fascia and Windlass in Maintaining Stiffness of Longitudinal Arch of Foot

Finite Element Analysis of Plantar Fascia During Walking: A Quasi-static Simulation

Yen-Nien Chen, MSc, Chih-Wei Chang, MD, MSc, Chun-Ting Li, MSc, Chih-Han Chang, PhD, and Cheng-Feng Lin, PT, PhD

Abstract: The plantar fascia is a primary arch supporting structure of the foot and is often stressed with high tension during ambulation. When the loading on the plantar fascia exceeds its capacity, the inflammatory reaction known as plantar fasciitis may occur. Mechanical overload has been identified as the primary causative factor of plantar fasciitis. However, a knowledge gap exists between how the internal mechanical responses of the plantar fascia react to simple daily activities. Therefore, this study investigated the biomechanical responses of the plantar fascia during loaded stance phase by use of the finite element (FE) modeling.

Methods: A 3-dimensional (3-D) FE foot model comprising bones, cartilage, ligaments, and a complex-shaped plantar fascia was constructed. During the stance phase, the kinematics of the foot movement was reproduced and Achilles tendon force was applied to the insertion site on the calcaneus. All the calculations were made on a single healthy subject.

Results: The results indicated that the plantar fascia underwent peak tension at preswing (83.3% of the stance phase) at approximately 493 N (0.7 body weight). Stress concentrated near the medial calcaneal tubercle. The peak von Mises stress of the fascia increased 2.3 times between the midstance and preswing. The fascia tension increased 66% because of the windlass mechanism.

Conclusion: Because of the membrane element used in the ligament tissue, this FE model was able to simulate the mechanical structure of the foot. After prescribing kinematics of the distal tibia, the proposed model indicated the internal fascia was stressed in response to the loaded stance phase.

Clinical Relevance: Based on the findings of this study, adjustment of gait pattern to reduce heel rise and Achilles tendon force may lower the fascia loading and may further reduce pain in patients with plantar fasciitis.

(Chen YN, Chang CW, Li CT, Chang CH, Lin CF. Finite element analysis of plantar fascia during walking: a quasi-static simulation. Foot & ankle international. 2015 Jan;36(1):90-97.)

https://journals.sagepub.com/.../10.1177/1071100714549189

08/04/2025

Wooden Model Demonstration of Longitudinal Arch Load-Sharing System of Foot

The plantar fascia and plantar ligaments form part of the Longitudinal Arch Load-Sharing System (LALSS) of the Foot which I first described 8 years ago (Kirby KA: Longitudinal arch load-sharing system of the foot. Revista Española de Podología, 28(2), 2017).

My article may be read here:

https://www.sciencedirect.com/.../pii/S0210123817300087

In the wooden model shown here, which I constructed for a seminar workshop on longitudinal arch function about 15 years ago, the mechanical load-sharing functions of the plantar fascia and plantar ligaments are shown. In the human foot, there are four layers of tension load-bearing elements which help maintain, stabilize and help increase the stiffness of the longitudinal arch against weightbearing forces.

Most superficially is the plantar fascia, which sends five strands distally to the bases of each of the proximal phalanges of the digits (the plantar fascia is shown in this wooden model to only connect to the metatarsal heads to make the wooden model more simple).
The remaining elements of the LALSS are, from superficial to deep, the plantar intrinsic muscles, the deep flexor muscles (i.e. posterior tibial, flexor digitorum longus and flexor hallucis longus) and peroneus longus muscles, and the plantar ligaments. Each of these tension load-bearing elements which make up the LALSS work together to maintain longitudinal arch stability during gait and other weightbearing activities.

In my demonstration in the video, the plantar fascia and plantar ligaments are shown as passive tension load-bearing elements which develop greater tension forces within their structures as the vertical load increases on the forefoot which, in turn, causes an increased tendency for the longitudinal arch to flatten. If the plantar fascia is cut or ruptured, the plantar ligaments will have increased load on them due to their load-sharing function. If the plantar ligaments are cut or ruptured, the plantar fascia will have increased load.

Combined with the active functions of the plantar intrinsic muscles and deep flexor and peroneus longus muscles that are controlled by the central nervous system, these four layers of tension load-bearing elements of the longitudinal arch of the human foot provide the longitudinal arch stability and stiffness which is necessary for the bipedal human to perform the weightbearing tasks that are necessary throughout their lifetimes.

https://www.facebook.com/watch/?v=1569619320043727

07/28/2025

International Seminars

Over the past 34 years, it has been my incredible opportunity to be invited to lecture at a total of 52 podiatry and biomechanics seminars in 13 different countries. My wife and I have traveled together on most of these international lecture trips and have become close friends with many international podiatrists and their families over this time. We have always greatly enjoyed our "lecture-vacations" since we have been able to see many beautiful and interesting sights, got to meet some amazing people and have enjoyed experiencing the different foods and cultures of our world. We are looking forward to many more "lecture-vacations" in the coming years.