Kevin A. Kirby, DPM

12/17/2025

Classic Experiment on Plantar Fascia Tension Force During Walking

One of my favorite studies on the biomechanics of the plantar fascia came from Ahmet Erdemir, PhD, and his colleagues at the Penn State Biomechanics Lab from over 20 years ago. In their study, they used their "Dead-Man Walking" apparatus to measure plantar fascia tension in cadaver specimens that were "walked" over a platform while the extrinsic foot muscle tendons of the cadaver specimens were all connected to servomotors to simulate normal walking biomechanics (Erdemir A, Hamel AJ, Fauth AR, Piazza SJ, Sharkey NA: Dynamic loading of the plantar aponeurosis in walking. JBJS, 86A:546-552, 2004).

Erdimir et al’s cadaver walking study measured plantar fascia tension in seven fresh-frozen cadaver foot-leg specimens by threading a 0.5 mm fiberoptic cable into the plantar fascia, therefore embedded it within the plantar fascia during their walking experiments. The tension within the plantar fascia was then calculated later by measuring the change in light transmitted through the fiberoptic cable using a materials testing machine.

The researchers found that the tension within the plantar fascia was nearly zero at heel strike and that plantar fascia tension gradually increased during midstance to peak at heel-off at 0.96 times body weight. In addition, these researchers found that the tension force within the Achilles tendon was an effective predictor of plantar fascial tension (r = 0.76).

This research clearly shows the important effect of Achilles tendon tension on plantar fascial tension and points to the clinical importance of Achilles tendon stretching exercises in treating plantar fasciitis. In addition, this research also suggests how important obesity may be in the production of plantar fasciitis symptoms since for every extra pound of weight on a patient's body, another pound of tension force will be present within the plantar fascia during walking activities.

https://journals.lww.com/jbjsjournal/abstract/2004/03000/dynamic_loading_of_the_plantar_aponeurosis_in.13.aspx

12/17/2025

ProLab Monthly Newsletter

Ever since January 2023, I have been writing a monthly newsletter for ProLab Orthotics, an orthotic lab founded by the late Dr. Paul Scherer which is located in Napa, California. A few of my colleagues have recently mentioned how much they enjoy the newsletters, which are now available by signing up for them on the ProLab website. Happy reading.

ProLab Orthotics is a manufacturer of evidence-based, custom foot orthoses. Learn more on our "Newsletter" page.

12/15/2025

Wright and Rennels: The Elastic Properties of the Plantar Fascia

One of the most important research papers to come out of the University of California Biomechanics Laboratory in Berkeley and San Francisco, California in the 1960's, was a study on the biomechanical characteristics of the plantar fascia. In this classic foot biomechanics research, Wright and Rennels became the first researchers to ever perform actual load-deformation testing of the plantar aponeurosis in both living and cadaver feet to determine its mechanical characteristics (Wright DG, Rennels DC: A study of the elastic properties of plantar fascia. JBJS, 46 (A):482-492, 1964).

For the testing of the load-deformation characteristics of the plantar fascia, Wright and Rennels dissected the plantar fascia from four fresh-frozen cadaver specimens and clamped each end of the dissected plantar fascia into an experimental mechanical apparatus, (in modern times now done by a "materials testing machine") that could apply and measure varying magnitudes of tension load across each specimen (see my illustration which is slightly modified from the original paper). The cross-sectional area of each plantar fascia specimen was also measured so that the stress within the plantar fascia could be determined. The change in length of the plantar fascia that occurred with variations in tension load on the plantar fascia specimen was measured by a strain gauge attached to the plantar fascia during the course of the experiment (Wright, Rennels, 1964).

Once firmly clamped into the experimental apparatus, the tension on each specimen was increased incrementally to 125 to 150 pounds. Next the load was decreased to 0 pounds and then increased again until failure of the specimen occurred. The change in length of the plantar fascia was noted after each 25-pound increase in tension. Total testing time of each specimen was less than 1.5 minutes. All the specimens returned to nearly their original length when unloaded, indicating a near elastic recovery of the plantar fascia. All four specimens failed at 200-225 pounds of tension, with all specimen failures occurring at the edges of the serrated jaws of the testing apparatus. Therefore, the authors concluded that the true failure point of each plantar fascia specimen was “probably somewhat higher” than what was found during their experiments (Wright, Rennels, 1964).

For all four specimens, a load-deformation curve was graphed (see original graph from the paper below), where the plantar fascia tension force was plotted against the length of the plantar fascia. The upward slope of the load-deformation curve is, by definition, the stiffness of the plantar fascia. A stress-strain curve of each specimen was also graphed where the stress (i.e. force/cross-sectional area) was plotted against the strain (i.e. change in length of the specimen/initial length of specimen) of the specimen. By definition, the slope of the stress-strain curve of a material is a measure of the modulus of elasticity (i.e. Young’s modulus) of that material and also is representative of its stiffness. The modulus of elasticity increased from an initial value of 50,000 lb/in2 to 120,000 lb/in2 as the tension loads increased (Wright, Rennels, 1964). The modulus of elasticity of the plantar fascia in the 1964 experiment of Wright and Rennels, in SI units, would be 0.34 gigapascals (GPa) to 0.83 GPa.

In the second part of their experiment to determine the load-deformation characteristics of the plantar fascia, the authors performed an in-vivo study of the elongation of the medial longitudinal arch (MLA) of one subject under increasing loading forces. The subject was seated in a chair while multiple 50-pound weights were placed on top of the knee up to 200 pounds. The elongation of the plantar fascia was then measured by taking lateral foot radiographs at 0, 50, 100, 150 and 200 pounds of vertical load and then using the distance from the plantar aspect of the medial calcaneal tubercle to the distal first metatarsal head as a measure of the length of the plantar fascia.

From their modelling of the plantar fascia as being part one of the tension load-bearing structures of the MLA, the authors estimated that 94 pounds of tension load produced a 1.68% increase in length of the plantar fascia. They also noted that it was likely that the plantar intrinsic muscles contributed to the higher stiffness calculated for the plantar fascia in the live subject versus the plantar fascia specimens tested within the experimental apparatus (Wright, Rennels, 1964).

In their discussion, the authors noted that both their in-vitro and in-vivo studies of the plantar fascia were static in nature and could not be considered to be representative of the elongation of the plantar fascia during the dynamics of gait. They also noted that when the specimens were maintained at higher loads, the plantar fascia would slowly elongate or “creep” over time (Wright, Rennels, 1964).

This "creep response" is now a well-known, time-dependent load-deformation characteristic of all ligament and tendon structures where the ligament or tendon will gradually elongate over time while being placed under a constant load. The 'creep-response" of the plantar fascia seen in this experiment from over six decades ago that is due to theviscoelastic nature of the plantar fascia (Nordin M, Frankel VH: Basic Biomechanics of the Musculoskeletal System, 3rd Edition. Lippincott Williams and Wilkins, Philadelphia, 2001, pp. 111-112).

Finally, the authors noted that the 200 pound load on the foot of the seated subject that produced a 1.68% elongation of the plantar fascia was well within the range of the physiologic strains within the plantar fascia during most weightbearing activities since, during the cadaver experiments, the elongations of the specimens were 3.5-4.5% before failure occurred. The point was likewise made that the increasing modulus of elasticity of the specimens with increasing loads indicated that plantar fascia elongated easily at first and became progressively stiffer with higher tension loads. The authors believed this made the plantar fascia, and all ligaments, to be “well suited for the task of providing postural support to the skeletal system while allowing the small amount of play that is characteristic of bone articulations” (Wright, Rennels, 1964).

From their experiments on cadaver feet and on a live subject, Wright and Rennels became the first scientific researchers to study the load-deformation characteristics of one of the structural components of the foot, the plantar fascia. Their important research, even though using relatively crude instrumentation by today’s standards, provided important insight into the biomechanical function of plantar fascia. Podiatrists who wrongly believe that the plantar fascia does not stretch or does not exhibit spring-like behavior during weightbearing activities should pay close attention to this classic 1960s foot biomechanics research.

[Reprinted with permission from Kirby KA: Foot and Lower Extremity Biomechanics V: Precision Intricast Newsletters, 2014-2018. Precision Intricast, Inc., Payson, AZ, 2018, pp. 17-18.]

12/15/2025

Is Hallux Limitus Caused by the Plantar Fascia?

Six years ago, a very interesting research study was published which shed considerable light on the biomechanical etiology of hallux limitus (Viehöfer AF, Vich M, Wirth SH, Espinosa N, Camenzind RS. The role of plantar fascia tightness in hallux limitus: a biomechanical analysis. The Journal of Foot and Ankle Surgery. 2019 May 1;58(3):465-9.

Arnd Viehofer and coworkers from Zurich, Switzerland, used seven cadaver limbs that were each placed into a materials testing machine with each foot being loaded to 350 N (78.6 pounds). The tendons of the specimens were loaded to simulate weightbearing tension loads within the extrinsic muscles of the foot (see illustration from their paper below). The plantar fascia was then fixed to a clamping system that could be tensioned so that four different tension forces were applied (100 N, 200 N, 300 N, and 350 N) to see their effects on hallux dorsiflexion.

These researchers found a positive correlation between the decrease in hallux dorsiflexion and the magnitude of tension applied to the plantar fascia. One limitation of the study was that the researchers did not apply plantar hallux force in their experiment to measure hallux dorsiflexion at each tension level of the plantar fascia, but rather measured the position of a K-wire driven through the hallux interphalangeal joint relative to the ground at different plantar fascia tension levels. They found the angle of the hallux decreased 4.2 degrees (p

12/13/2025

Dorsal Midfoot Interosseous Compression Syndrome (DMICS)

I first described Dorsal Midfoot Interosseous Compression Syndrome (DMICS) over 28 years ago in my February 1997 Precision Intricast Newsletter.

Patients with DMICS complain of pain at the dorsal aspect of their midfoot joints during weightbearing activities. Upon taking the history, patients with DMICS point to the area of the metatarsal-cuneiform joints, navicular-cuneiform joints, and sometimes to the area of the metatarsal-cuboid joint as the source of most of their pain. Much less frequently, the pain is noted more proximally, in the dorsal aspects of either the talo-navicular or calcaneo-cuboid joints.

The pain from DMICS generally worsens with increased weightbearing activities. Patients report the pain from DMICS will occur during walking gait just before heel off and/or during propulsion. Walking in low-heeled shoes usually exacerbates the pain, while walking in shoes with an increased heel height usually eases the pain. There is usually no history of trauma even though patients who have suffered blunt trauma injuries to the dorsal midfoot may complain of very similar symptoms.

On physical examination of the patient with DMICS, there is discrete tenderness along the dorsal joint lines of the affected midfoot joints but no tenderness along the dorsal aspects of the extensor tendons with dorsiflexion resistance applied at the digits. Minimal edema is only detected at the dorsal midfoot in the worst cases of DMICS and plantar edema never occurs.

Upon clinical examination, there is no pain with forceful manual dorsiflexion of the forefoot on the rearfoot. However, there is very significant pain with plantarflexion of the forefoot on the rearfoot. This test, the Forefoot Plantarflexion Test, is the hallmark in the physical examination of patients with DMICS is that they all have very significant pain with plantarflexion of the forefoot on the rearfoot (see previous post with video). The Forefoot Plantarflexion Test is a remarkably sensitive indicator of the level of severity of DMICS.

https://www.facebook.com/kevinakirbydpm/posts/1451668971597008

The most likely reason that manual plantarflexion of the forefoot on the rearfoot during the clinical examination causes such significant and consistent pain in patients with DMICS is that the dorsal margins of the midfoot joints have, over time, developed microfractures and/or bone edema due to the excessive compression forces within their dorsal midfoot joints (see my illustration). The dorsal capsular ligaments which attach to these damaged area of dorsal midfoot joint bone will pull on these areas of damaged bone which causes pain. Again, the cause of the bone damage within the dorsal articular margins of the midfoot joints is the chronic excessive interosseous compression force (ICF) in these joints during weightbearing activities.

The combination of three main forces act together on and within the foot during late midstance to cause an increase in the ICF across the dorsal margins of the midfoot joints (see my illustration). First, the weight of the body exerts a plantarly directed force through the tibia onto the talar dome at the ankle joint. This ankle joint compression force is increased by any tension forces within the Achilles tendon, tendons of the deep posterior compartment muscles and peroneal muscle tendons.

Second, due to the requirements of the gastrocnemius and soleus muscles to be active during late midstance, the Achilles tendon has large tension forces which cause a rearfoot plantarflexion moment which, in turn, has a tendency to flatten the longitudinal arch of the foot. Lastly, since the center of mass of the body is over the metatarsal heads during late midstance, ground reaction force (GRF) is at its peak on the metatarsal heads which causes a dorsiflexion moment on the forefoot.

The net result of these three forces acting together is a very strong flattening force, or arch-flattening moment, on both the medial and lateral longitudinal arches of the foot. The stronger the flattening moments on the medial and lateral longitudinal arches, the greater is the ICF across the dorsal joint surfaces of the midfoot. The flattening moments on both the medial and lateral longitudinal arches are increased by such factors as increased body weight, low heeled shoes and limited ankle joint dorsiflexion. Weak plantar ligaments and weak plantar intrinsic and plantar extrinsic muscles also increase the dorsal ICF at the midfoot since these ligaments and muscles help prevent medial and lateral longitudinal arch collapse.

Repetitive trauma at the dorsal margins of these midfoot joints with each step is the most likely cause of the pain from DMICS. Treatment revolves around both reducing the inflammation to the dorsal midfoot joints and trying to eliminate the mechanical factors causing the increased flattening moments on the medial and lateral longitudinal arches.

Local treatment to reduce inflammation may include relacing shoes or choosing shoes that do not cross dorsally over the affected area of the dorsal midfoot. In addition, icing and non-steroidal anti-inflammatory drugs and even cortisone injections may be necessary in resistant cases. The worst cases are treated initially with cam-walker brace boot walkers for 3-6 weeks.

Mechanical treatment involves, first of all, having the patient stretch their Achilles tendons and either adding a heel lift to their shoes or getting them into a slightly higher heeled shoe. Most helpful is to prevent the medial and lateral longitudinal arches from collapsing during gait as much as possible with either padding, strapping or prescription foot orthoses. The foot orthoses must be stiff enough to support the medial and lateral longitudinal arches and should be well contoured to the foot. I find that if the initial treatment of the patient with temporary insoles or padding is helpful, the patient is very happy to proceed further with the more corrective and much more beneficial prescription foot orthoses since DMICS can be quite a painful and debilitating condition. Proper conservative treatment, outlined above, is routinely very effective.

[With permission from: Kirby KA.: Foot and Lower Extremity Biomechanics: A Ten Year Collection of Precision Intricast Newsletters. Precision Intricast, Inc., Payson, AZ, 1997, pp. 165-166.]

12/13/2025

Forefoot Plantarflexion Test for Diagnosing Midfoot Pathologies

The Forefoot Plantarflexion Test is a very useful clinical test which I invented over 25 years ago to diagnose causes of dorsal midfoot pain, including Dorsal Midfoot Interosseous Syndrome (DMICS). I first described the pathological condition, DMICS, in my February 1997 Precision Intricast Newsletter (Kirby KA.: Foot and Lower Extremity Biomechanics: A Ten Year Collection of Precision Intricast Newsletters. Precision Intricast, Inc., Payson, Arizona, 1997, pp. 165-168).

The Forefoot Plantarflexion Test opens up the dorsal midfoot joint surfaces which passively increases the tension force within the dorsal midfoot ligaments. If these dorsal midfoot ligaments are inflamed or damaged, or, more commonly, if the dorsal cortical margins of these midfoot joints have developed stress reactions due to abnormal magnitudes of interosseous compression forces within them, then the pulling force on the dorsal midfoot ligaments, which attach to these injured dorsal margins of the midfoot joints, will be painful for the patient when performing the Forefoot Plantarflexion Test.

To perform the Forefoot Plantarflexion Test, have the the patient sit on the exam table or chair. Then the examiner should cup the posterior calcaneus and pull the calcaneus towards them to stabilize the foot on the tibia. The other hand of the examiner is then used to next grasp the medial or lateral dorsal forefoot, with the thumb dorsally and fingers plantarly in the medial arch. The forefoot is then plantarflexed on the rearfoot to place a tension force on the dorsal joint ligaments of the midfoot joints.

The Forefoot Plantarflexion Test is normally negative when there is no midfoot pathology in the patient. However, the Forefoot Plantarflexion Test is considered to be positive if the patient reacts with pain with plantarflexion of the forefoot on the rearfoot. A positive Forefoot Plantarflexion Test indicates either dorsal capsular ligamentous injury in the midfoot joints and/or subperiosteal bone injury at the dorsal midfoot joint surfaces where the capsular ligaments are attached.

The Forefoot Plantarflexion Test is always positive in Dorsal Midfoot Interosseous Compression Syndrome (DMICS) and is always positive with midfoot crush injuries and with Lisfranc’s joint injuries. The Forefoot Plantarflexion Test also is helpful at objectively determining healing from midfoot joint injuries. The test can be modified by using variations in locations in manual pressure on the dorsal forefoot so that each individual metatarsal ray or a group of metatarsal rays may be tested simultaneously.

The Forefoot Plantarflexion Test was first described in: Kirby KA: “Prescribing Better Foot Orthoses: Lateral-Dorsal Midfoot Pain”, June 2010 Precision Intricast Newsletter, Payson, AZ, 2002.

12/10/2025

Anatomy, Range of Motion and Neutral Position of the Subtalar Joint

The subtalar joint (STJ) allows triplanar motion of the foot relative to the leg. STJ pronation involves dorsiflexion, abduction and eversion of the foot relative to the leg. STJ supination involves plantarflexion, adduction and inversion of the foot to the leg. This triplanar motion of the STJ is important during human bipedal locomotion to allow the frontal plane motions of the foot to occur along with transverse plane motions of the tibia when the foot is bearing weight. While the foot is on the ground, STJ pronation causes tibial internal rotation and STJ supination causes tibial external rotation.

The talus and calcaneus make up the two bones of the subtalar joint (STJ), otherwise known as the talo-calcaneal joint. The talo-calcaneal joint consists of three articular facets, the posterior, middle and anterior facets.

The two physiologic motions of the STJ are supination and pronation. During STJ supination, the talus glides posterior-superiorly upon the posterior facet of the calcaneus. During STJ pronation, the talus glides anterior-inferiorly upon the posterior facet of the calcaneus. The maximally pronated position of the STJ occurs when the lateral process of the talus slides down the posterior facet of the calcaneus until it comes to an abrupt end of pronation motion when it hits the floor of the sinus tarsi of the calcaneus.

STJ supination involves posterior translation, abduction and dorsiflexion rotation of the talus relative to the calcaneus. STJ pronation involves anterior translation, adduction and plantarflexion rotation of the talus relative to the calcaneus.

The neutral position of the STJ likely occurs when the posterior articulating facets of the STJ become the most congruent to each other. With maximal pronation of the STJ, the anterior half of the talar posterior articulating facet separates from the posterior facet of the calcaneus. With supination of the STJ, the posterior half of the talar posterior articulating facet separates from the posterior facet of the calcaneus. This can be appreciated on lateral radiographs of the foot in different subtalar joint rotational positions.

Since the subtalar joint is the most important joint in the foot and lower extremity allowing inversion and eversion range of motion of the foot relative to the leg, understanding the three-dimensional rotations and translations of the talus relative to the calcaneus is of great biomechanical importance. Having an excellent grasp of STJ biomechanics, including the functional significance of the STJ maximally pronated position, neutral position and STJ rotational stability during weightbearing activities, will allow the podiatrist and foot-health specialist to better treat their patients with mechanically-related pathologies of the foot and lower extremity.

12/09/2025

Subtalar Joint Arthroereisis Procedure Reduces Pronated Position of Foot by Resetting Maximally Pronated Position of Subtalar Joint

The subtalar joint (STJ) arthroereisis procedure has been used many years by foot surgeons to treat flatfoot deformity. In this surgical procedure, an implant is placed into the sinus tarsi which acts as a compression-resistant spacer between the anterior edge of the lateral process of the talus and the floor of the sinus tarsi of the calcaneus. In essence, a STJ arthroereisis reduces the pronation range of motion of a foot by resetting the maximally pronated position of the STJ to a less pronated rotational position.

Vogler proposed that there were three types of STJ arthroereisis implants including the "stable self-locking wedge implants", "axis-altering implants" and "direct impact implants". Unfortunately, this classification scheme for STJ arthroereisis implants is not only innaccurate, but also ambiguous.

First of all, Vogler's classification scheme for STJ arthroereisis implants is fatally flawed since all subtalar joint arthroereisis implants are "axis-altering" implants and "direct impact" implants. Any time the maximally pronated position of the STJ is altered, as all STJ arthroeisis implants do, the STJ axis spatial location will also be altered.

In addition, all STJ arthroeisis implants work by exerting a compression force or, "directly impacting", against both the lateral process of the talus and the floor of the sinus tarsi, in essence working as a compression-resistant spacer. Unfortunatley, Vogler's classification system for STJ arthroeisis implants is still being taught to this day in podiatry schools and podiatry seminars (Vogler HM. Subtalar joint blocking operations for pathological pronation syndromes. In: McGlamry ED (ed): Comprehensive Textbook of Foot Surgery, Williams & Wilkins, Baltimore, 1987, pp. 447-465).

Due to the relatively constant exit point of the STJ axis from the dorsal talar neck anteriorly and from the posterior-lateral calcaneus posteriorly, the STJ axis will rotate and translate in space relative to the plantar foot and ground during closed kinetic chain STJ pronation and supination. When the STJ supinates, the STJ axis abducts, dorsiflexes and translates laterally relative to the plantar foot and ground. When the STJ pronates, the STJ axis adducts, plantarflexes and translates medially relative to the plantar foot and ground.

When the podiatric surgeon places any arthroereisis implant into the sinus tarsi and/or tarsal canal, this resets the maximally pronated position of the STJ into a more supinated (i.e. less pronated) rotational position postoperatively. Subsequently, during weightbearing activities, the STJ spatial location at the maximally pronated position will also be altered postoperatively.

Since the STJ axis exits anteriorly at the dorsal neck of the talus, as the STJ is supinated away from its maximally pronated position by the STJ arthroereisis implant, the external rotation and dorsiflexion of the talar head and neck that occurs will also cause an abduction and dorsiflexion of the STJ axis. As a result, any arthroereisis implant will make the talus and the STJ axis become more abducted and dorsiflexed in the maximally pronated STJ position postoperatively.

Biomechanically speaking, all subtalar implants work by filling the sinus tarsi with a spacer material which resists compression forces and resets the maximally pronated position of the STJ to a new maximally pronated position.. By altering the STJ axis spatial location in the maximally pronated STJ position to a new, more normal location, the subtalar arthroereisis procedure is able to effectively limit calcaneal eversion and talar adduction and talar plantarflexion during weightbearing activities.

14 years ago, I published an article where a more thorough review where the biomechanical function of the STJ arthroereisis procedure was discussed (Kirby KA: Understanding the biomechanics of subtalar joint arthroereisis. Podiatry Today. 24:(4)36-45, 2011).

https://www.hmpgloballearningnetwork.com/site/podiatry/understanding-biomechanics-subtalar-joint-arthroereisis