Kevin A. Kirby, DPM

Effects of Medial Arch Height of Foot Orthoses on "Pronation Control" There was a previous common misconception within the podiatric profession that foot orthoses act to limit excessive subtalar joint (STJ) pronation by methods which are either unexplainable or explainable only by vague, unscientific explanations. For example, as a podiatry student at the California College of Podiatric Medicine, I was taught that one of the ways that foot orthoses exert their control on pro...Continue reading

Understanding Modern Biomechanics Terminology: Viscoelasticity When either a biologic or non-biologic material is subjected to a loading force, it will deform in response to that load. For example, if a material is placed under tension load, it will elongate and if a material is placed under compression load, it will shorten. As the loading force is applied to the material, the material will deform according to its particular load-deformation characteristics. During its ...Continue reading

Midfoot Compression Test The Midfoot Compression Test is a clinical test I developed in 2010 to help diagnose and follow the progress in patients with midfoot pain. This clinical test is yet another way to assess whether there are any "hidden" pathologies within the midfoot complex of the patient's foot. To perform the Midfoot Compression Test, have the patient sitting on the exam table. Then the examiner grasps the midfoot of the affected foot at the level of the first c...uneiform medially and 5th metatarsal base laterally with one hand to apply a medial-to-lateral compression force on the midfoot. Basically, the examiner squeezes the midfoot both medially and laterally in an to increase the dorsal convexity in the midfoot. During the Midfoot Compression Test, the medial-lateral compression forces are increased in the midfoot joints, the dorsal midfoot capsular ligaments are placed under greater tension forces and the dorsal joint surfaces (i.e. where the capsular ligaments are attached) are placed under greater tension forces (from the capsular ligaments). A positive Midfoot Compression Test occurs when the patient indicates pain with midfoot compression. Normally, the Midfoot Compression Test produces no pain in the patient's foot. A positive Midfoot Compression Test often occurs with Dorsal Midfoot Interosseous Compression Syndrome (DMICS), crush injuries of the midfoot and Lisfranc’s joint injuries. Even thought this clinical test, the Midfoot Compression Test, has not been described previously within the medical literature, I use it in my clinic on a weekly basis to help diagnose midfoot pain and follow the progress of patients with midfoot symptoms.

Lecture in Amsterdam, The Netherlands, on subtalar joint axis location in September 2017.

Dorsal Midfoot Interosseous Compression Syndrome (DMICS) I first described Dorsal Midfoot Interosseous Compression Syndrome (DMICS) 24 years ago in my February 1997 Precision Intricast Newsletter. At that time, after 12 years of careful clinical observation, I had observed a number of patients that complained of dorsal midfoot pain but had no radiographic evidence of midfoot pathologies. What all these patients did have, however, with manual manipulation of their feet, was ...Continue reading

Grinding Plantar Fascial Accommodations into Foot Orthoses The plantar fascia (i.e. central component of the plantar aponeurosis) is under significant tension loading forces during weightbearing activities, with the tension loading forces within the plantar fascia being estimated at approximately 1x body weight during walking. As a result of this "bowstringing" that occurs within the plantar fascia, and especially within the more medial fibers of the plantar fascia, plantar... fascial accommodations have been used within custom foot orthoses for at least 40 years. If a patient is experiencing a sensation of abnormal pressure or pain in the medial longitudinal arch with foot orthoses, and it has been determined that the medial band of the plantar fascia is the anatomical structure being irritated, then adding in a plantar fascial accommodation to the orthosis is an excellent way to make the orthosis more comfortable. One of the best ways to add a plantar fascial accommodation into a foot orthosis is to grind it into the dorsal surface of the plate, as shown in my video below. Instead of marking the plantar fascia onto the orthosis plate before grinding as many podiatrists recommend, for the past 30 years I have instead been grinding in plantar fascial accommodations into the foot orthoses of my patients by using my anatomical knowledge that the medial band of the plantar fascia originates from the medial calcaneal tubercle and inserts into the medial and lateral sesamoids of the plantar first metatarsal head. In other words, I never mark the medial band of the plantar fascia on the patient's foot or on the orthosis before grinding since, anatomically, the plantar fascia has a constant anatomical location. As a result, I will always grind the plantar fascial accommodation into the orthosis plate starting from the area of the medial calcaneal tubercle to the first intermetatarsal space. The reason the plantar fascial accommodation is ground toward the first intermetatarsal space and not directly toward the first metatarsal head is that the forefoot generally slides about a half centimeter laterally on the orthosis plate when the foot is inside the shoe when compared to where the orthosis should match to the plantar contours of the foot. Using these anatomical landmarks to grind in a plantar fascial accommodation into the dorsal shell of a foot orthosis, rather than marking the plantar fascia location onto the orthosis plate, produces a very consistently located and comfortable plantar fascial accommodation for nearly all patients. Plantar fascial accommodations are one of the most orthosis modifications I make in foot orthoses for my patients. Unless the podiatrist or foot-health clinician has a grinder in their office and knows how to use it properly, they will likely struggle with becoming proficient at foot orthosis therapy.

Tissue Stress Approach vs Subtalar Joint Neutral Approach to Mechanical Foot Therapy Dr. Merton Root popularized the concept that the neutral position of the subtalar joint (STJ) has special significance in foot orthosis therapy. He recommended casting feet in STJ neutral, balancing foot orthoses mostly at the heel vertical position and didn't advocate forefoot extensions to custom foot orthoses. His ideas of how to treat feet with custom foot orthoses is often known alter...Continue reading

Basic Biomechanics Terminology and Concepts: Moments This is my second lecture on biomechanics terminology covering the subject of rotational forces, or moments. Moments can be either external, generated by external forces acting on the foot, such as ground reaction force, or internal moments, generated by muscle, tendon, ligament or fascial tension forces. Moments can either accelerate rotation (i.e. angular acceleration) in the direction of the moment, or decelerate rotati...on in the direction opposite to the direction of the applied moment. Moments acting in equal and opposite directions across a joint axis help produce joint stability. Newton's second law of motion is used in the lecture to help explain how muscle moments, along with gravitational acceleration and ground reaction forces, help us produce the motions necessary to performs all types of sport and other weightbearing activities. In addition, examples of moments acting across the ankle joint and subtalar joint are used to explain the accelerations, decelerations and stability at these important joints of the foot and lower extremity. Muscle testing is also mentioned in regards to how muscle strength is not exactly what we are testing when clinically assessing "muscle strength" in our patients. Overall, without a good understanding of the concepts of joint moments and how they are produced and altered by muscle, tendon, ligament and fascial tension forces, ground reaction forces, and foot orthoses and shoes, the podiatrist and foot-health clinician will have an extremely difficult time appreciating how foot and lower extremity pathologies are created and how these pathologies can be healed with custom foot orthoses

Basic Biomechanics Terminology and Concepts: Force In the following series of posts, I thought it might be helpful for many of those following along to define and describe the basic terminology used within the International Biomechanics Community to describe the biomechanics of the foot and lower extremity. Having a complete understanding of these terms is critical for the podiatrist and foot-health professional to develop a deeper appreciation of foot and lower extremity bio...mechanics. The first term that I will define and describe is "force". Force is defined as a mechanical disturbance or load in which the action of one object on another causes the object to move and/or deform. Force is a vector quantity and has four important components: 1) magnitude 2) line of action 3) a direction, and 4) point of application. Force measured in Newtons . 1.0 pounds =4.5 N. By analyzing the forces acting both on and within the foot, the clinician and researcher can gain a much better understanding of why a foot moves, or doesn't move, during gait. In addition, by better understanding forces, the clinician will better appreciate how shoe insoles, pads, pre-made orthoses, custom foot orthoses, and strapping can perform their various therapeutic functions for the foot and lower extremity.

Excessive Lateral Midsole Compression Set in Running Shoes The shoes shown below are only 4 months old and cost $125. After 4 months of wear, both shoes had developed excessive lateral midsole compression set which resulted in the footbed and heel counter of the shoes being deformed to a 10 degree varus angulation relative to the ground. Compression set is a known characteristic of EVA foams whereby the midsole foam material loses thickness over time due to repetitive compression of the midsole. The question is, should running shoe companies start providing the consumer with a "Durability Rating System" for their $100+ shoes to make them aware at how quickly their shoes deteriorate over time??

Why Do Low-Arched Feet Have More Functional Hallux Limitus? After over 36 years of clinical observation, I have consistently found that individuals with low-arched feet have an increased tendency to have functional hallux limitus (FnHL). FnHL is defined as a condition where there is reduced hallux dorsiflexion on weightbearing, but normal hallux dorsiflexion during non-weightbearing examination. The most coherent biomechanical explanation for this observation that feet with ...Continue reading

Subtalar Joint Axis Locator on Patient with Medially Deviated STJ Axis Here is another video from Dr. Simon Spooner of a patient walking with the STJ Axis Locator attached to her foot. This patient has a medially deviated STJ axis. Please note the significant internal rotation and plantarflexion of the STJ axis during the late midstance phase of walking. Since the majority of ground reaction force (GRF) has transferred from the plantar calcaneus to the plantar forefoot by late midstance, the magnitude of STJ pronation moments from GRF are markedly greater in late midstance due to the progressive medial deviation of the STJ axis through midstance. Is this the foot that would eventually develop posterior tibial tendon dysfunction over time? Why?

Video of Subtalar Joint Axis Locator in Action The Subtalar Joint Axis Locator is a device that Dr. Simon Spooner and I worked together on to see if the subtalar joint (STJ) axis could be tracked using a relatively simple mechanical device. I sent a drawing of a proposed STJ Axis Locator model to Dr. Spooner in 2001, and then Simon came to my private office in Sacramento from the UK for a short visit to do a radiographic study on the STJ Axis Locator. Our paper was publishe...d in 2006 (Spooner SK, Kirby KA: The subtalar joint axis locator: A preliminary report. JAPMA, 96:212-219, 2006). In the video below, even though the STJ Axis Locator is only attached by a clamping mechanism to the calcaneus, by carefully aligning the STJ Axis Locator with the predetermined posterior and anterior exit points from the foot at, respectively, the posterior lateral calcaneus and anterior dorsal talar neck, it can be seen how closely the anterior wand of the STJ Axis Locator can track the movements of the STJ axis exit point. It can be seen that the STJ Axis Locator remarkably tracks the STJ axis exit point anteriorly at the talus, even though it is not physically attached in any way to the talus. The points of no rotation of the posterior and anterior exit points of the STJ axis from the posterior calcaneus and anterior dorsal talar neck are predetermined using the method first described in 1994 by Drs. Jack Morris and Lester Jones (who were both biomechanics professors of mine at CCPM). Those who are not familiar with this paper, and don't understand these concepts, should read the paper by Morris and Jones to better familiarize yourself with these important concepts (Morris JL, Jones LJ: New techniques to establish the subtalar joint's functional axis. Clinics Pod Med Surg., 11(2):301-309, 1994).

Cadaver Research Study Measures Both First Ray and Second Ray Stiffness In 2002, a master's degree candidate from Penn State University Biomechanics Lab, Andrew Fauth, performed the first scientific investigation of the dorsiflexion stiffness of the first and second rays on cadaver feet. He eventually published the study in 2004 along with coauthors Andrew Hamel and Neil Sharkey (Fauth AR, Hamel AJ, Sharkey NA: In vitro measurements of first and second tarsometatarsal joint s...Continue reading

Biomechanics and Use of Morton's Extensions in Custom Foot Orthoses A Morton's Extension is a forefoot extension plantar to the 1st metatarsal head of a foot orthosis. The Morton's Extension is named for Dudley Joy Morton (1884-1960), who was a physician, anatomist and anthropologist. Morton wrote one of the earliest books on foot anatomy and function, "The Human Foot, It’s Evolution, Physiology and Functional Disorders", in 1935. In 1927, Morton applied for a patent for ...a compensating insole that focused on elevating the first metatarsal head and preventing pronation compensation for a short, hypermobile first ray. The accompanying illustration of the "Morton's extension" is shown from Morton's patent that was granted to him in 1932. The Morton's extension is useful for increasing the ground reaction force (GRF) plantar to the 1st metatarso-phalangeal joint (MPJ) in cases where there is a shortened or dorsiflexed first metatarsal from congenital malformation, trauma or surgical complications. Commonly, when the first metatarsal is shortened or elevated, the plantar 2nd MPJ which may become symptomatic over time. In these cases, the Morton's extension may be very helpful in reducing excessive GRF on the plantar 2nd MPJ and reducing any pain and swelling in the plantar 2nd MPJ. A Morton's extension may also be used along with a forefoot extension plantar to the 4th and 5th MPJs. This technique of accommodating the 2nd and 3rd MPJs may be used in cases of pain and/or swelling plantar to the 2nd and 3rd MPJs, a condition that is often called "central metatarsalgia". Also, in cases of painful hallux limitus deformity, where there is pain with dorsiflexion of the hallux during non-weightbearing examination of the patient's foot, a Morton's extension can be used to limit hallux dorsiflexion during gait and decrease the pain of the hallux limitus deformity. However, care must be taken when using a Morton's extension, since an excessively thick Morton's extension may cause a significant increase in GRF plantar to the 1st MPJ. This increase in plantar 1st MPJ GRF may not only cause a hallux limitus to occur and limit the propulsive phase of gait, but may result in over-supination of the foot during gait which may lead to lateral metatarsal or lateral midfoot pain over time.

Biomechanics Terminology: Force Force is an important concept for podiatrists and other foot-health professionals to understand. It is forces which produce the motions of the human body and produce the injuries we see in our clinics. In this video, I cover the biomechanics definition of force, Newton's Laws of Motion, the four components of a force vector, the three types of force and what forces can do to objects. In addition, the concepts of ground reaction force, external and internal forces, and axial and eccentric loading forces are also covered.

New Finite Element Study on Stress-Relieving Function of Plantar Arch Strapping on Plantar Fascia During Running. A new thesis has just been published by a PhD candidate, Chen Linwei, at the Hong Kong Polytechnic University, which reviewed the biomechanics of the plantar fascia during running and used finite element analysis to study the effects of plantar arch strapping on stresses within the plantar fascia. https://theses.lib.polyu.edu.hk///1/991022379157403411.pdf... This PhD thesis also used my paper on the Longitudinal Arch Load-Sharing System to explain the function of the plantar fascia and how it worked with the other passive and active elements of the foot and lower extremity to maintain and optimize longitudinal arch function (Kirby KA: Longitudinal arch load-sharing system of the foot. Revista Española de Podología, 28(2), 2017). It is good to see other researchers being able to use these concepts in their studies in order to better explain the biomechanical function of the foot and lower extremity during weightbearing activities. Very interesting read!

Subtalar Joint Axis Locator Dr. Simon Spooner and I first collaborated on building the first Subtalar Joint Axis Locator over two decades ago, back in 2001. We did a radiographic study using the STJ Axis Locator which was eventually published in 2006 (Spooner SK, Kirby KA: The subtalar joint axis locator: A preliminary report. JAPMA, 96:212-219, 2006). https://www.youtube.com/watch?v=_94CATh7ZDk&t=6s... Now, somebody else is using our design to digitally measure the STJ axis spatial location. Hopefully further progress will be made in refining this device to allow it to be used better in a clinical setting.

Medial Heel Skive Orthoses Found to Help Patellofemoral Syndrome The Medial Heel Skive Orthosis Technique was first published in the podiatric medical literature in 1992 after I had invented and started using the technique in about 1989 (Kirby KA: The medial heel skive technique: improving pronation control in foot orthoses. JAPMA, 82: 177-188, 1992). The Medial Heel Skive Orthosis Technique has been used in a study of foot orthosis treatment of patellofemoral syndrome first... published in 2017. Custom foot orthoses with medial heel skive modifications were used in 14 patients with patellofemoral syndrome and found to improve knee symptoms. Data analyses indicated significant (P=0.008) improvements in weight bearing pain after 4 weeks of using the orthosis with the medial heel skive technique (Bahramian F, Aminian G, Bagherzadeh M, Fardipoor S, Kashani V. The Effect of Custom Made Foot Orthoses Fabricated With Medial Heel Skive Technique on Pain and Function in Individuals With Patellofemoral Pain Syndrome. Iranian Rehabilitation Journal. 2017; 15(1):37-42). The Medial Heel Skive Technique is a powerful addition to the orthosis modifications the podiatrist and foot-health clinician can use to treat their patients with pronation-related pathologies. Understanding the function of the Medial Heel Skive Orthosis Technique is critical to achieving the best results with custom foot orthoses.

Development of Medial Heel Skive Orthosis Technique I first came up with the idea for my Medial Heel Skive Technique in about 1988 after considering how a medial heel (i.e. varus) wedge added to a cork and leather orthosis worked for patients with pronation-related pathologies. I then decided to try and find a way to produce this same varus heel cup shape in a polypropylene orthosis without going to the considerable extra work required by the Blake Inverted Orthosis Techni...que. The result of my experimentation with positive cast modifications, along with the owner of Precision Intricast Orthosis Lab, Paul Rasmussen, was the Medial Heel Skive Orthosis Technique. The attached video goes into more detail on the development of this technique that is now used worldwide on both custom and premade foot orthoses. https://doi.org/10.7547/87507315-82-4-177

Foot and Lower Extremity Scientific Research, Past and Present I just had an article published in the November 2020 issue of Podiatry Today magazine titled "Emphasizing The Principles of Good Research Studies in the Age of Instant Online Access and an Increased Volume of Studies". Hope you enjoy the article. https://www.podiatrytoday.com/emphasizing-principles-good-r

Effects of Subtalar Joint Rotational Motion on Subtalar Joint Axis Spatial Location One of the most important concepts when discussing subtalar joint (STJ) biomechanics is to clearly define and differentiate between STJ rotational motion and STJ axis spatial location. STJ rotational motion is defined as the position of the STJ within its range of rotational motion (e.g. maximally pronated, 4 degrees from maximally pronated, maximally supinated). On the other hand, STJ axis ...Continue reading

I am giving a virtual lecture for the National Podiatry Congress in Portugal on November 14, 2020. Should be a good conference!

Merry Christmas to all my friends and colleagues around the world. The Kirby family wishes you all a very Happy Holidays and, hopefully, a better 2021 for all of us!

I recently had an article published on my thoughts regarding the past and present of foot and lower extremity research, in the November 2020 issue of Podiatry Today. It is remarkable how things have changed over my past four decades of entering the podiatric profession in this regard.

I will be giving two lectures for the virtual conference being organized by the Canadian Federation of Podiatric Medicine on November 12-14, 2020. My first 45-minute lecture will be on "The Biomechanics of Plantar Heel Pain" and my second 45-minute lecture will be on "Root Biomechanics is No Longer Alive: How Should Podiatry Move Forward?" Looks like a great conference with many excellent speakers! https://www.podiatryinfocanada.ca/2020-Conference-Schedule

I will be giving the keynote lecture for a virtual conference organized by The College of Podiatry in the UK on November 26, 2020. The title of the lecture is "The Biomechanics and Treatment of Plantar Fat Pad Atrophy Symptoms in the Geriatric Foot." https://cop.org.uk/conference/copc21 https://twitter.com/CollegePodi/status/1322153794445156352

The Rise and Fall of the Barefoot Running and Minimalist Running Shoe Fads Here is my most recent Precision Intricast Newsletter, for November 2020, titled "The Rise and Fall of the Barefoot Running and Minimalist Running Shoe Fads". Happy reading. All five of my Precision Intricast Newsletter Books may be purchased at the Precision Intricast website at www.precisionintricast.com/shop ++++++++++++++++++++++++++++++++++++++++++...Continue reading

Working on four lectures for virtual podiatry conferences to be held in Portugal, the UK and Canada, all within two weeks of each other in November. And doing it all from the comfort of our motorhome parked at a Pacific Ocean beach campground here in Northern California. Cool!

Neutral Suspension Negative Casting Technique The neutral suspension negative casting technique was first described by Root et al as a method to obtain a three-dimensional (3D) impression (i.e. negative cast) of the plantar aspect of a patient's foot in order to make custom foot orthoses. During the neutral suspension negative casting technique, the foot is held with the subtalar joint in its neutral positiion and with the lateral column of the foot dorsiflexed against resis...tance (Root ML, Weed JH, Orien WP: Neutral Position Casting Techniques, Clinical Biomechanics Corp., Los Angeles, 1971). The resulting negative cast, made from two 5" x 30" extra-fast setting plaster splints, is then used by the custom-foot orthosis laboratory to make 3D positive casts of the patient's foot. The positive casts of the foot, made by pouring plaster into the negative casts are then ultimately used to make custom foot orthoses for the patient. Optical 3D scans of the plantar foot may also be used to obtain a 3D image of the plantar foot in order to make custom foot orthoses. Multiple modifications may be made to the negative and positive cast and/or scanned 3D image of the plantar foot in order to accomplish the goals of custoom foot orthosis therapy for the patient. I use a number of variations in positioning of the subtalar, midtarsal and midfoot joints of the foot during negative casting technique in order to either increase or decrease medial longitudinal arch height or otherwise slightly alter the 3D shape of the negative cast for the resultant orthosis. These variations in negative casting technique allows me to slightly vary the shape of the negative cast from patient to patient in order to optimize the biomechanical function of resultant custom foot orthoses for each patient.

Haile Gebrselassie Demonstrates that Proper Footstrike is Forward of Center of Mass This ultra slow-motion video shows one of the greatest distance runners of all time, Haile Gebrselassie, of Ethiopia. Note that Gebrselassie's footstrike is always forward of his center of mass (CoM), even though he has a midfoot strike pattern. The biomechanical importance of running footstrike being in front of the center of mass (CoM) in runners cannot be underestimated. Research from 9 yea...rs ago clearly shows that the running footstrike in elite distance runners is well-ahead of the CoM of the runners, averaging 33-34 cm ahead of their CoM in the male runners and 31-33 cm ahead of their CoM in the female runners (Hanley B, Smith LC, Bissas A. Kinematic variations due to changes in pace during men's and women's 5 km road running. International Journal of Sports Science & Coaching. 2011 Jun;6(2):243-52). Unfortunately, many self-proclaimed "running-form experts" still wrongly preach that runners should footstrike under their CoM in order to run with "proper running form". That is not only untrue, but is also a biomechanical impossibility. At footstrike during running, whether the footstrike is forefoot, midfoot or rearfoot, the foot must be ahead of the CoM during level, constant-velocity running. This is a biomechanical necessity. The reason why the running footstrike biomechanically needs to be ahead of the runner's CoM during level, constant-velocity running is due to the fact that at all running speeds, the angle of the ground reaction force (GRF) vector points posterior-superiorly at the instant of footstrike. If the runner landed with their foot under the CoM, as many self-proclaimed "running-form experts" suggest, the runner would fall flat on their face attempting to run with this abnormal footstrike pattern. In other words, if runners were to try to run the way many self-proclaimed "running-form experts" suggest, the runner would rotate violently forward at the instant of footstrike, falling flat on their faces. At constant running velocity on level surfaces, the foot always strikes ahead of the CoM of the body since this is a biomechanical necessity for human bipedal running. It is biomechanically impossible to run at a constant velocity on level surfaces at the running speeds that Haile Gebrselassie displays here. The foot must strike ahead of the CoM of the body during runing....unless, of course, the individual is running in place at a running velocity equal to zero!

Abductory Twist: What is It and Why Does It Occur? Abductory twist is a sudden abduction motion of the foot at the time of heel-off during walking gait. Abductory twist is a relatively common gait abnormality that occurs due to the external rotation motion of the pelvis above the foot during late midstance not being matched by corresponding subtalar joint supination and tibial external rotation during late midstance. As a result of this "mismatching" of transverse plane rota...tions of the pelvis and lower extremity during late midstance, the resultant increase in elastic strain energy occurring within the muscles, tendons and ligaments of the hip, knee and lower extremity will be resolved into this rapid medial movement of the heel at the time of heel-off. The reason why the abductory twist occurs just after the instant of heel-off is because the frictional forces between the heel of the foot and the ground are no longer present after heel-off to prevent external rotation motion of the foot relative to the ground. The best example of an abductory twist is seen in the last few steps of the left foot of this slow motion video. It is often commonly clinically seen with late midstance pronation which makes good biomechanical sense because increased subtalar joint pronation moments in late midstance will tend to result in both late midstance pronation and abductory twist.

The Windlass and Reverse Windlass Effects of Hicks The Windlass and Reverse Windlass Effects were first described 66 years ago by John H. Hicks in his classic paper on the biomechanics of the plantar fascia in 1954 (Hicks JH: The mechanics of the foot. II. The plantar aponeurosis and the arch. J Anatomy. 88:24-31, 1954). Hicks noted that hallux dorsiflexion, in both live feet and cadaver feet, produced the mechanical effect of raising of the medial longitudinal arch and plant...arflexing the first ray. He also found that during relaxed bipedal standing, hallux dorsiflexion produced subtalar joint supination and external rotation of the leg. Hicks called this mechanical linking of hallux dorsiflexion to first ray plantarflexion, medial longitudinal arch elevation, subtalar joint supination and external leg rotation the "Windlass Effect". Hicks described in his paper how the hallux and the plantar fascia were a mechanical analogue to a windlass winding a cable (i.e. plantar fascia) around the drum (i.e. first metatarsal head) of the windlass. Hicks also noted that a "Reverse Windlass Effect" occurred in both live and cadaver feet where the digits all plantarflexed with the application of loading force being applied onto the plantar metatarsal heads. Hicks determined that the "Reverse Windlass" was due to the plantar loading force on the metatarsal heads producing a flattening and lengthening of the longitudinal arch of the foot which, in turn, increased the tension within the plantar fascia and plantarflexion of the digits. Hicks also noted that cutting of the plantar fascia in cadaver feet eliminated both the Windlass and Reverse Windlass Effects. Podiatrists and foot-health clinicians should understand the windlass and reverse windlass effects that Hicks described 66 years ago since the plantar aponeurosis has very important biomechanical functions for the human foot during weightbearing activities. Plantar fascial ruptures and plantar fasciotomies will alter the function of the plantar fascia and cause a multitude of pathological conditions as a result of the alterations within the biomechanical function of the plantar fascia. For those who would like to further reading on the biomechanical functions of the plantar fascia, I wrote an article on the 10 functions of the plantar fascia six years ago which is listed below. https://www.podiatrytoday.com/understanding-ten-key-biomech.

Biomechanics Terminology: How Are Stress and Pressure Similar Yet Different From Each Other? The concepts of stress and pressure in biomechanics can be confusing for many podiatrists and foot-health professionals since the terms are measured in the same units, Pascals (Pa), but describe quite different mechanical phenomena. Let's take a close look at both of them. The term "stress" refers to the internal ability of an object to resist deformation and, as such, is measured in... units of force divided by the cross-sectional area of the material. Typically, stress is measured in Newtons (N) per square meter or Pascals (Pa). For example, if the posterior tendon has 100 N of tension force in it and it has a cross-sectional area of .000025 m^2, the tension stress within the posterior tibial tendon would be 100 N/.000025 m^2 =4,000,000 Pa or 4.0 MegaPascal (MPa) or in English units, 580 pounds/inch^2. Stress, therefore, refers to a concentration of internal resisting force within an object, or specifically within the tissues of the foot and lower extremity, a concentration of internal resisting force within a tendon, ligament, bone, cartilage, fascia, muscle or skin. The term "pressure" is also measured as a force divided by a cross-sectional area but this term is generally reserved for an force acting externally on an object or an "external force". Also, pressure refers to the force acting perpendicular to the external surface of the object. The perpendicular component of an external force is also known as a normal force. Pressure, like stress, is also measured in Pascals (Pa). For example, on the plantar foot, a neuropathic diabetic ulcer with a pressure measured at 500 kiloPascals (kPa) means that the external force from ground reaction force on the plantar foot is concentrated into a small enough surface area of the foot to cause 500 kPa of pressure which let to a plantar skin ulceration. Therefore, both "stress" and "pressure" are measurements of a force divided by a cross-sectional/surface area so that more force over a smaller cross-sectional/surface area will equal greater magnitudes of stress and/or pressure. As such, both stress and pressure are measured in the same units, Pa, but stress generally refers to an internal resisting force within an object whereas pressure refers to an external force acting over a given surface area of that object. Both stress and pressure are important terms in biomechanics and, as such, the podiatrist and foot-health professional should intimately understand these terms so that they can better appreciate how external and internal forces within the foot and lower extremity can cause injury and how therapeutic options for treating these injuries function to heal our patients.

Definitions in Biomechanics: What is Stress? Stress is defined in biomechanics as the amount of force acting within an object over a given cross-sectional area of that object. Higher magnitudes of force acting over smaller cross-sectional areas will lead to higher values of stress. Lower magnitudes of force acting over larger cross-sectional areas will lead to lower values of stress. Stress may also be thought to be the ability of an object to develop internal resistance t...o loading forces. If the molecules within an object can develop large internal resistance to loading forces, then it will be capable of developing higher stresses. If the molecules within an object can only develop a small amount of internal resistance to loading forces, then it will be capable of developing only low stress values. Obviously a steel rod, since its molecules can develop quite large internal resistance to loading forces, is able to develop quite high stress values. However, a rubber band, since its molecules can only develop small amounts of internal resistance to loading forces acting upon it, is able to develop relatively small amounts of stress. There are five basic types of stress: Tension stress internal resistance to elongation Compression stress internal resistance to shortening Torsional stress internal resistance to twisting Shear stress internal resistance to preventing sliding of layers of object relative to each other Bending stress internal resistance to bending Stress () is measured as a force (F) acting over a unit cross-sectional area (A) of material, or stress: = F/A. Stress measured in pascals (Pa) = N/m2.

Plantarflexion Taping for Plantar Plate Tears The digital plantarflexion taping technique for plantar plate tears involves using cloth adhesive tape to restrict the dorsiflexion motion of the metatarsophalangeal joint of the affected digit to only 10-15 degrees of dorsiflexion during the propulsive phase of walking or running. The digital plantarflexion taping technique was first described within the medical literature in 2002 by Gerard Yu and colleagues (Yu GV, Judge MS, Hu...dson JR, Seidelmann FE: Predislocation syndrome: Progressive subluxation/dislocation of the lesser metatarsophalangeal joint. JAPMA, 92(40:182-199, 2002). Three 8-9" pieces of 1/2" cloth adhesive tape are used to prevent the proximal phalanx of the affected digit from dorsiflexing during propulsion and stretching the plantar plate. I prefer 1/2" Johnson and Johnson "Zonas" brand tape since it is well tolerated and less compliant than K-tape. If the clinician decides to use the more compliant (i.e. more stretchy) K-tape, the digit will need to be taped in a more plantarflexed position than when using cloth adhesive tape for the plantarflexion taping technique. By preventing excessive digital dorsiflexion during gait with digital plantarflexion taping, excessive tension force on the plantar plate will be prevented to allow the plantar plate to become less inflamed during activities. As a result, the plantar plate will heal more rapidly over time as the patient continues their normal daily weightbearing activities. Digital plantarflexion taping, along with twice daily plantar icing therapy (i.e. two 20 minute icing sessions per day), custom foot orthoses to accommodate the affected metatarsophalangeal joint, wearing thicker soled cushioned shoes and avoiding barefoot activities greatly accelerates the healing of plantar plate pathologies. Cortisone injections and even plantar plate repair surgery may be required if other conservative therapy options fail at rendering the patient asymptomatic. I had an article published on the anatomy, biomechanics and injury mechanics of the plantar plate in Podiatry Today magazine in 2017 (Kirby KA: Understanding the biomechanics of plantar plate injuries. Podiatry Today, 30(4):30-39, 2017). https://www.podiatrytoday.com/understanding-biomechanics-pl