Kevin A. Kirby, DPM

03/23/2026

How Does the Human Body Balance Itself While Standing on Both Feet?

Upright standing on both feet (i.e. upright bipedal standing) seems so simple to us because it is a task we have learned to do over time from our first year of lifes. However, the task of maintaining balance in an upright human is not a simple or easy task. Rather, maintenance of upright bipedal standing requires a complicated interplay between the afferent input to the central nervous system (CNS), integration by the CNS, and CNS efferent output to the muscles of the lower extremity.

The most common position of the center of mass (CoM) of the body during upright bipedal standing is with the CoM anterior to the ankle joint axis (left). In this position of standing, gravitational acceleration on the CoM will tend to accelerate the CoM forward causing an ankle joint dorsiflexion moment. If the CNS did not respond to the forward acceleration of the CoM with increased muscle activity, then the individual would fall forward flat onto their face.

The CNS "knows" that if it increases the contractile activity of the gastrocnemius and soleus muscles while the CoM is anterior to the ankle joint axis, the ankle joint plantarflexion moment created by gastroc-soleus activity will nicely counterbalance the ankle joint dorsiflexion moment caused by the CoM being anterior to the ankle joint. Most people will positions their CoMs during relaxed bipedal standing with their CoM anterior to the ankle joint since the gastrocnemius-soleus muscles are the largest muscles of the leg and can quite easily maintain an upright balance with their relatively large tension-producing forces within the Achilles tendon.

However, the CoM must be balanced somewhere above the confines of the borders of both feet (i.e. grey area in illustration on right) or the individual will not be able to remain balanced in an upright position, and will fall. Sensory input from the eyes, vestibular apparatus (within the inner ear), plantar skin, ankle joint capsule and leg tendon stretch receptors are continually sent by afferent nerves to the CNS in order for the CNS to integrate this sensory information in order to maintain proper firing of the lower extremity muscles and maintain balance in the standing human. Loss of any of these multiple afferent sensory organs can greatly affect human bipedal standing balance. The podiatrist must be aware of the importance of CNS activity on all weightbearing activities to better understand the biomechanics and neurophysiology of the foot and lower extremity.

03/22/2026

Standing Bipedal Balance Using "Ankle-Strategy": Part 2

David Winter's analysis of how the standing bipedal human is able to balance their center of mass (CoM) over their feet by changing the anterior-posterior location of their center of pressure (CoP) [CoP is the center of ground reaction force (GRF) on the plantar foot], is one of the best ways to begin to understand the significance of central nervous system (CNS) control of human weightbearing activities (Winter, David A.: A.B.C. (Anatomy, Biomechanics and Control) of Balance During Standing and Walking. Waterloo Biomechanics, Waterloo, Ontario, Canada, 1995).

In the previous example from yesterday, the center of gravity (CoG), which represents the position of the CoM relative to the ground, was anterior relative to the CoP which caused the CoM to be accelerated forward. Balance would be lost by anterior movement of the CoM in front of the toes which would cause the individual to lose balance and fall forward.

In the example below, now the CNS has recognized that the CoM has been accelerated forward and loss of balance will soon occur. In response to this anterior movement of the CoM relative to the plantar foot, the CNS will increase the contractile activity of the gastrocnemius-soleus complex which, in turn, creates an ankle joint plantarflexion moment and a large forward shift in the CoP on the plantar aspect of the foot.

The forward shift in the CoP on the plantar foot, now ahead of the CoM and CoG, will first decelerate the forward movement of the CoM and then will start to accelerate the CoM backward. This forward and backward oscillation of the CoM of the body by ankle joint dorsiflexion and ankle joint plantarflexion is known as the "ankle-strategy of standing balance".

Previous research papers have shown that this ankle-strategy of standing balance consistently seen in human subjects uses the inverted pendulum model of the CoM rotating about the ankle joint axis as a means maintaining upright balance (Robinovitch SN, Heller B, Lui A, Cortez J. Effect of strength and speed of torque development on balance recovery with the ankle strategy. Journal of Neurophysiology. 2002 Aug 1;88(2):613-20).

http://www.physiology.org/doi/pdf/10.1152/jn.2002.88.2.613

I first wrote about the subject of CNS control of upright balance and Dr. Winter's balance research over 25 years ago in my August 2000 Precision Intricast newsletter "Maintenance of Balance in Relaxed Bipedal Standing" which was published in my 2nd Precision Intricast Newsletter Book (Kirby KA: Foot and Lower Extremity Biomechanics II: Precision Intricast Newsletters, 1997-2002. Precision Intricast, Inc., Payson, AZ, 2002, pp. 134-137).

03/21/2026

Standing Bipedal Balance Using "Ankle-Strategy": Part 1

David Winter, PhD, wrote extensively on the biomechanics of balance. It is recommended that all podiatrists and other foot-health clinicians read Dr. Winter's work on standing balance and how the central nervous system (CNS) coordinates balance for the standing human (Winter, David A.: A.B.C. (Anatomy, Biomechanics and Control) of Balance During Standing and Walking. Waterloo Biomechanics, Waterloo, Ontario, Canada, 1995).

The CNS, by changing the timing and magnitude of efferent signals to the muscles which cross the ankle joint, changes the position of the center of pressure (CoP) on the plantar foot relative to the position of the center of mass (CoM) of the body in order to maintain balance during relaxed bipedal standing. Winter defined the center of gravity (CoG) as being the place on the ground which is directly under the CoM of the body at any point in time.

In the illustration below, the CoM and CoG are anterior to the ankle joint. However, in this case, the posterior musculature of the calf (e.g. gastrocnemius-soleus muscles) that can cause an ankle joint plantarflexion moment do not have much contractile activity so that the resultant concentration of ground reaction force (GRF) on the plantar foot at the CoP [CoP is defined as the point where the concentration of GRF is located] is only slightly anterior to the ankle joint axis, making the CoP posterior to the CoG.

As a result, gravitational acceleration acting on the CoM will cause the body to start to sway forward at the ankle joint with the CoM being accelerated anteriorly. If the CNS does not respond within a very short time to this forward acceleration of the CoM, upright balance will be lost and the body will fall forward. However, if upright balance is to be maintained, the CNS will immediately increase the contractile activity of the gastrocnemius-soleus muscles which will move the CoP forward of the CoM and CoG which will first slow the anterior acceleration of the CoM, then will next accelerate the CoM posteriorly.

This movement of the CoP anteriorly or posteriorly at the ankle joint by the CNS in response to the position of the CoM and CoG relative to the plantar feet by to the CoG to maintain standing balance is known as the "Ankle-Strategy of Balance". Much previous research has been performed in the past on standing balance and tracking of the CoM and CoP in order to better understand how the CNS maintains balance

I first wrote about this subject and Dr. Winter's balance research over a quarter-century ago in my August 2000 Precision Intricast newsletter "Maintenance of Balance in Relaxed Bipedal Standing" (Kirby KA: Foot and Lower Extremity Biomechanics II: Precision Intricast Newsletters, 1997-2002. Precision Intricast, Inc., Payson, AZ, 2002, pp. 134-137).

For further reading on the subject, here is one paper from 1999 that studied standing balance (Gatev P, Thomas S, Kepple T, Hallett M. Feedforward ankle strategy of balance during quiet stance in adults. The Journal of physiology. 1999 Feb 1;514(3):915-28).

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2269093/

03/17/2026

Proximal and Distal Structures in First Ray Dorsiflexion Stiffness

In order to increase the dorsiflexion stiffness of the first ray (i.e. prevent first ray hypermobility) during weightbearing activities, the body uses both active factors that are mediated directly by the central nervous system (CNS) and passive factors that are not mediated or altered by the CNS. Active factors, such as contractile activity of the peroneus longus (PL), posterior tibial (PT), flexor hallucis longus (FHL), abductor hallucis (ABH), flexor hallucis brevis (FHB), and adductor hallucis (ADH) muscles, may be voluntarily or involuntarily controlled by the CNS in order to either increase or decrease the first ray dorsiflexion stiffness depending on the functional needs of the individual’s foot.

Passive factors that increase first ray dorsiflexion stiffness are those factors that are not controlled by the CNS and include the tension forces generated within the plantar fascia and the plantar ligaments of the first ray when ground reaction force (GRF) is exerted on the plantar forefoot. Both the active and passive factors that mechanically affect first ray dorsiflexion stiffness are necessary to allow normal function of not only the first ray but also the rest of the foot and lower extremity during weightbearing activities (Kirby KA: Foot and Lower Extremity Biomechanics III: Precision Intricast Newsletters, 2002-2008. Precision Intricast, Inc., Payson, AZ, 2009, pp. 73-84).

For each of these passive and active factors to contribute toward increasing the dorsiflexion stiffness of the first ray, they must, by definition, also be able to generate first ray plantarflexion moments. Increases in the contractile activity of the PL, PT, FHL, ABH, FHB and ADH will generate a first ray plantarflexion moment which will, in turn, increase first ray dorsiflexion stiffness by actively resisting the first ray dorsiflexion moments from GRF. Passive increases in tension forces within the plantar aponeurosis and plantar ligaments of the first ray, which occur as a result of plantar loading of the forefoot, will also generate a first ray plantarflexion moment which will, in turn, increase first ray dorsiflexion stiffness and actively resist first ray dorsiflexion moments (see my illustrations below).

Feet with a lower medial longitudinal arch will have a decreased 1st ray plantarflexion moment arm available for the 1st MPJ compression force generated by the tension force within the plantar fascia, abductor hallucis and other tendons that attach to the sesamoids and hallux. As a result, this lower longitudinal arch will produce much less 1st ray plantarflexion moment when compared to the same 1st MPJ compression force in a foot with a higher medial longitudinal arch due to the longer 1st ray plantarflexion moment arm that is present in the higher-arched foot.

The function of the first ray and first MPJ may be better understood by further dividing the tension load-bearing structures that increase first ray dorsiflexion stiffness into two categories that are dependent on where these muscles, tendons and ligaments insert onto their respective osseous structures in the first ray and hallux. Many of the muscles, tendons and ligaments that produce a first ray plantarflexion moment do not exert tension force directly on the osseous structures of the first ray, but rather exert increased tensile force directly through their insertions onto osseous structures which are distal to the first metatarsophalangeal joint (MPJ). Included in these structures, are the FHL, which inserts on the base of the hallux distal phalanx, and the ABH, FHB, ADH and medial slips of the central component of the plantar aponeurosis, all of which insert onto the base of hallux proximal phalanx via the medial and lateral sesamoids.

All of the these structures will exert a posteriorly-directed force on either the proximal phalanx or distal phalanx of the hallux which will, in turn, result in an increase in posteriorly-directed force acting onto the anterior articular surface of the first metatarsal head from the base of the hallux proximal phalanx.

The other tension load-bearing structures that produce a first ray plantarflexion moment do so by exerting increased tension force directly through their insertions onto the osseous structures of the first ray, or, in other words, by inserting proximal to the first MPJ. Included in these structures are the PL, which inserts on the base of the first metatarsal and first cuneiform, the PT, which inserts onto the navicular and first cuneiform, and the plantar ligaments of the first ray including the plantar ligaments of the navicular-first cuneiform joint, first cuneiform-first metatarsal joint and, to a lesser extent, the ligaments between the first and second rays. Since all of these tension load-bearing structures directly attach to the osseous structures of the first ray, they can therefore directly act to either actively or passively generate a first ray plantarflexion moment so that first ray dorsiflexion stiffness may be increased as is necessary.

One of the interesting aspects about this division of tendons and ligaments that produce a first ray plantarflexion moment is that it highlights the fact that many of the structures that increase first ray plantarflexion moment exert their stabilizing mechanical effect on the first ray as a result of their ability to cause increased compression forces at the first MPJ. For example, if the FHL, FHB, ABH, ADH tendons and medial slip of the central component of the plantar aponeurosis were all transected, these structures could then no longer contribute to increasing first ray plantarflexion moment and to increasing first ray dorsiflexion stiffness. As a result, the first ray would lose important load-support function with transaction of these structures.

In addition, the amount of first ray plantarflexion moment caused by tension forces within these structures which insert distal to the 1st MPJ are directly related to the degree of first ray declination angle which is also correlated to the height of the medial longitudinal arch (MLA) of the foot. For example, in a foot with a high MLA (e.g. pes cavus), the posteriorly-directed force on the first metatarsal head resulting from the increase in tension forces from these structures will have a longer moment arm to produce first ray plantarflexion moment than in the foot with a low MLA (e.g. pes planus). Therefore, for a given amount of first MPJ compression force caused by the tension forces within these structures which insert distal to the first MPJ, the foot with a higher MLA will have an increased first ray moment arm, an increased first ray plantarflexion moment and increased first ray dorsiflexion stiffness when compared to the foot with a lower MLA.

[Reprinted with permission from : Kirby KA: May 2009 Precision Intricast Newsletter, "Proximal and Distal Structures in First Ray Dorsiflexion Stiffness" in: Kirby KA: Foot and Lower Extremity Biomechanics IV: Precision Intricast Newsletters, 2009-2013. Precision Intricast, Inc., Payson, AZ, 2014, pp. 45-46.]

03/12/2026

Interview from three years ago that may be of interest to some.

Dr. Kevin Kirby graduated from the California College of Podiatric Medicine in 1983 and completed his first year surgical residency at the Veteran's Administ...

03/11/2026

The Myths of Subtalar Joint Neutral Biomechanics: Part 3 - When the Calcaneus is Everted More than Two Degrees, It Will Continue Pronating Until the Maximally Pronated Position of the Subtalar Joint is Reached

I heard this claim about the 2 degree everted position of the "calcaneal bisection" being some type of magic tipping point for the foot from not only Dr. Merton Root but also from Drs. John W**d and William Orien on numerous occasions when I was in podiatry school. The explanation I remember being taught to me as a podiatry student at the California College of Podiatric Medicine from 1979-1983 was that the 2 degree everted position of the calcaneus somehow pushed the subtalar joint (STJ) into a maximally pronated position. The implied meaning was that the "calcaneal bisection" was so important that it somehow totally controlled the pronation forces acting on the foot.

This conjecture from the originators of Root Biomechanics is so wrong in so many ways. However, let me try to briefly summarize what mechanical factors are important and unimportant in causing STJ pronation moments and the problems with this "2 degree everted calcaneus equals more pronation" notion:

1) Calcaneal bisections are very difficult to replicate from one examiner to another and within the same examiner with the range of error of bisecting the calcaneus being about +/- 5 degrees. Therefore, who knows what the "correct heel bisection" really is? Answer? It depends on who is drawing the line on back of the heel.

Therefore, given the range of error in calcaneal bisections, one examiner could draw the heel bisection so that the calcaneus is 4 degrees everted in relaxed calcaneal stance position (RCSP) while another examiner could draw the heel bisection so that the calcaneus is 2 degree inverted in RCSP. Which one is right? Neither. There is not a single accurate way to reliably draw "calcaneal bisections" on a live human foot.

Calcaneal bisections only can reliably serve as markers for frontal plane motion of the calcaneus during standing and walking, and for balancing foot orthosis casts as a frontal plane reference tool. Otherwise, calcaneal bisections are not predictive of foot function.

2) The shape of the posterior calcaneus is only one of the many factors that determines the rotational forces (i.e. moments) acting across the STJ, and it is not a very important factor at that. The factors that most importantly determine STJ moments during RCSP are the relative location of the plantar calcaneus and relative location of the weightbearing portions of the plantar forefoot to the STJ axis spatial location (Kirby KA: Subtalar joint axis location and rotational equilibrium theory of foot function. JAPMA, 91:465-488, 2001) since ground reaction forces acting on the plantar foot produce the large external STJ pronation and supination on the foot during weightbearing activities.

If the STJ axis is medially deviated, there will be an over-abundance of STJ pronation moments which will tend to maximally pronate the STJ in RCSP, regardless of how the clinician drew their "calcaneal bisection" on the posterior calcaneus. The "calcaneal bisection" could be 2 degrees inverted, vertical or 4 degree everted in RCSP and the STJ may or may not be maximally pronated and may or may 2 degrees inverted from the maximally pronated STJ position. Does the "calcaneal bisection matter? Not really.

However, if the STJ axis is laterally deviated, there will be an over-abundance of STJ supination moments which will tend to supinate the STJ in RCSP toward its midrange position, regardless of how the clinician drew their "calcaneal bisection" on the posterior calcaneus.

In this case, the "calcaneal bisection" is not an indicator of foot function, but rather only gives the clinician a way to determine if the calcaneus can pronate further from RCSP by performing the Maximum Pronation Test (Kirby KA, Green DR: Evaluation and Nonoperative Management of Pes Valgus, pp. 295-327, in DeValentine, S.(ed), Foot and Ankle Disorders in Children. Churchill-Livingstone, New York, 1992) or by performing the Coleman Block Test.

The bottom line is that the "calcaneal bisection" is simply not accurate enough or important enough of an biomechanical factor in foot function to possibly be used as a precise indicator of whether the STJ will pronate further in RCSP, or not. The only factor that determines whether the STJ will pronate further in RCSP is if there is an overall increase of the summation of external STJ pronation moments (from ground reaction force) and internal STJ pronation moments (from muscle and ligament tension forces), relative to the external and internal STJ supination moments acting while the individual is standing in RCSP.

Therefore, the notion that once the calcaneus has everted 2 degrees the STJ will pronate to the maximally pronated position of the STJ is simpy not true, and is another fallacy of Subtalar Joint Neutral Biomechanics that was taught to generations of podiatry students and podiatrists by Merton Root and his colleagues with not a piece of scientific research evidence to support their conjectures.

03/10/2026

The Myths of Subtalar Joint Neutral Biomechanics: Part 2- Merton Root Was the First to Coin the Term "Neutral Position" for the Subtalar Joint

Podiatric legend has it that Dr. Merton Root was the first to think up the idea of the neutral position of the subtalar joint (STJ), while standing in the shower one day in 1954:

“One morning in 1954, just by luck I guess, I was standing in the shower without any thought abut the foot and all of a sudden the concept of neutral subtalar joint position flashed into my mind. I could hardly wait to get to the office to substantiate it. That’s what turned out to be the key to my being able to contribute to podiatry.
M.L. Root, 1989”

(from: Lee WE: Podiatric biomechanics: an historical appraisal and discussion of the Root model as a clinical system of approach in the present context of theoretical uncertainty. Clinics Pod Med Surg, 18 (4):555-684, 2001.)

As shown in the photo below from a 1944 paper, Merton Root was not the first to come up with the idea of STJ neutral position. Actually, the concept of STJ neutral position seems to have been first mentioned within the medical literature as early as 1944 by W. Sayle Creer, an Orthopaedic Surgeon from Salford Royal Hospital in England. This was when Merton Root was 22 years old and also was 4 years before Merton Root started podiatry school (Creer WS. Some Foot Faults Related to Form and Function. British journal of industrial medicine. 1944 Jan;1(1):54).

Here is a photo from Creer's 1944 paper showing the reference to STJ neutral position and the link to the paper below. [Thanks to Dr. Simon Spooner for alerting me to Creer's 1944 paper where he first describes STJ neutral.]

As is obvious from reading Creer's 1944 paper, 10 years before Merton Root's "shower epiphany", Dr. Root was not the first to coin the term STJ "neutral". However, Root likely came up with his idea on his own 10 years after Creer first described the "neutral position" of the STJ within the medical literature.

https://pmc.ncbi.nlm.nih.gov/articles/PMC1035558/pdf/brjindmed00269-0058.pdf

03/09/2026

The Myths of "Subtalar Joint Neutral" Biomechanics: Part 1-The Vertical Calcaneus is Normal or Ideal

In 1971, Drs. Merton Root, William Orien, John W**d and Robert Hughes published a book that listed what they called their "Eight Biophysical Criteria for Normalcy". Root et al's notions were that these criteria represented the "ideal physical relationship of osseous segments of the foot and leg for the production of maximum efficiency during static stance or locomotion" (Root ML, Orien WP, W**d JH, RJ Hughes: Biomechanical Examination of the Foot, Volume 1. Clinical Biomechanics Corporation, Los Angeles, 1971).

Here are Root et al's speculations as to what constituted the "Eight Biophysical Criteria for Normalcy":

A. The distal one-third (1/3) of the leg is vertical.
B. The knee, ankle and subtalar joints lie in transverse planes parallel to the supporting surface.
C. The subtalar joint rests at its neutral position.
D. The bisection of the posterior surface of the calcaneus is vertical.
E. The midtarsal joint is locked in its maximum position of pro­ nation. {Therefore, the forefoot is locked against the rear­ foot during stance).
F. The plantar forefoot plane parallels the plantar rearfoot plane, and both parallel the supporting surface. In this po­sition, the sagittal bisection of the posterior surface of the calcaneus is perpendicular to the plantar plane of the foot.
G. Metatarsals 2, 3 and 4 are in a totally dorsiflexed position, and the plantar surface of the metatarsal heads describe a common plane parallel to the supporting surface.
H. Metatarsals 1 and 5 are maintained in such a position that the plantar surface of these heads lie in the same trans­verse plane as the metatarsal heads of 2, 3 and 4.

These ideas published by Root and colleagues included no references to previous scientific research. In fact, they seem nothing more than ideas made up by clinicians who were trying to create a model of foot function and a model of foot and lower extremity structural classification.

Unfortunately, these conjectures have been handed down year after year, accepted as scientific fact, when, in reality, there is no experimental or research support for these "Eight Biophysical Criteria for Normalcy". Podiatrists around the world have been taught these speculations with the assumption that they are true even though no research supports Root et al's supposition that these 8 "Criteria" are either normal or ideal.

Over the next few posts, I will present the various problems, inaccuracies, inconsistencies and errors with the Subtalar Joint Neutral (STJN) model of biomechanics advocated by Root and colleagues. It is my hope that the modern podiatrist and foot-health clinician can clear their mind of many previously taught ideas that, today, simply don't makes biomechanical sense and are not supported by the scientific research literature.

The first problem with STJN biomechanics is the notion that the calcaneus must be in the vertical position while in relaxed calcaneal stance position (RCSP) in order for the foot to be ideal or to function normally during gait.

Calcaneal bisections are notoriously problematic clinically. The range of error in bisecting the calcaneus from one examiner to another is likely to be +/- 4-5 degrees since the posterior calcaneus is covered by skin, fat and tendon and can't truly be accurately "bisected". Unpublished informal studies during my Biomechanics Fellowship often resulted in Biomechanics Professors drawing heel bisections on the same calcaneus about 3-4 degrees apart from each other. Students were much worse at agreeing on their heel bisections with much larger errors resulting. Whose heel bisection is then "correct"? No one knows and we really don't need to worry since it really doesn't matter.

The absolute position of the posterior calcaneus in relation to the ground is only a fair indicator of the inherent pronation and supination moments acting across the subtalar joint axis during static stance. The position of the medial calcaneal tubercle (i.e. where ground reaction force [GRF] acts on the plantar calcaneus) and the plantar forefoot in relation to the subtalar joint axis (STJA) help determine the external STJ pronation-supination moments. The spatial location of the posterior calcaneus (i.e. where Achilles tendon tension acts on the calcaneus) in relation to the STJA also determines the majority of the pronation-supination moments acting on the calcaneus during static stance (Kirby KA: Subtalar joint axis location and rotational equilibrium theory of foot function. JAPMA, 91:465-488, 2001).

The posterior calcaneus, or what any one clinician guesses as its "bisection" is irrelevant as a frontal plane marker for either being "ideal" or "normal" due to the range of measurement error and the lack of the biomechanical importance of the posterior calcaneus in determining foot function. A line on the back of the heel can only be reliably used as a way to assess frontal plane motion of the foot relative to the ground and to indicate how to orient a cast of the foot within the frontal plane for orthosis manufacture.

Therefore, the presumption by Root and colleagues that a vertical posterior calcaneus is ideal or normal not only doesn't make biomechanical sense, but also has never been supported by any research literature. lt is simply time to stop thinking that this half-century notion is true and stop teaching it to podiatry students as being true. It is not true, it is a podiatric myth.

03/08/2026

Peroneal Tendon Bowstringing During Bipedal Standing Indicates Tonic Peroneal Muscle Activity

Pes cavus deformity will commonly be associated with a laterally deviated subtalar joint (STJ) axis. When ground reaction force (GRF) acts on the plantar aspect of a foot with a laterally deviated STJ axis, there will be greater magnitudes of STJ supination moment generated from GRF than when a foot has a more normally-located, or medially deviated, STJ axis. Therefore, the result of GRF acting across a STJ axis which has a laterally deviated STJ axis is a tendency for the STJ to supinate excessively whenever it stands, walks or runs.

When an individual has a foot with a laterally deviated STJ axis, the central nervous system (CNS) of the individual recognizes this tendency to "over-supinate" during weightbearing activities. "Over-supination" can cause injuries, such as inversion ankle sprains, and the prime functions of the CNS furing walking, running and other weightbearing activities is to avoid injury, avoid pain and improve the metabolic efficiency of the activity.

Thus, when a foot with a laterally-deviated STJ axis performs weightbearing activities, the CNS will increase the tonic activity of the peroneal muscles since the peroneus brevis and peroneus longus muscles are the only two muscles of the foot and lower extremity which can actively pronate the STJ. An increase in CNS-controlled peroneal muscle activity in these feet with laterally deviated STJ axes will increase the internal STJ pronation moment which is necessary to counterbalance the excessive STJ supination moments from GRF acting on a laterally deviated STJ axis.

This increase in CNS-controlled peroneal muscle activity is, therefore, necessary to prevent the individual from suffering an inversion ankle sprain with each step and to allow the plantar forefoot to remain in a plantigrade position on the ground. As a result of this increase in magnitude in peroneal muscle activity and peroneal tendon tension forces in feet with laterally deviated STJ axes, these feet will often develop peroneal tendinitis and/or peroneal tendinopathy.

The increased CNS-controlled contractile activity of the peroneal muscles can be easily seen on examination of feet with significant laterally deviated STJ axes during relaxed bipedal standing by inspecting the peroneal tendons at the posterior-lateral ankle. When viewing the foot from the posterior and lateral aspect, both the peroneus brevis tendon and the peroneus longus tendon can be either visualized or palpated to determine whether they are under significant tension, or "bowstrung". If one only views the foot from anteriorly while standing, or does not pay close attention of the peroneal tendons as they pass posterior to the lateral malleolus during standing, the very obvious peroneal tendon bowstringing will not be appreciated.

On the contrary, in the case of feet with normal STJ axis locations, the peroneal muscles will not be active during standing and peroneal tendon bowstringing will not be evident. In feet with medially deviated STJ axis locations, the posterior tibial muscle is often tonically active during standing to resist the excessive STJ pronation moments from the medially deviated STJ axis.

Understanding how the CNS typically responds to lateral and medial deviation of the STJ axis in the bipedal human is one of the keys to appreciating how peroneal tendinopathy can develop in the feet with laterally deviated STJ axes and how posterior tibial tendon dysfunction can develop in feet with medially deviated STJ axes. Such an appreciation of the biomechanics of the foot and lower extremity will allow the podiatrist or foot-health professional to become a better healer for their patients.

References:

Kirby KA: Subtalar joint axis location and rotational equilibrium theory of foot function. JAPMA, 91:465-488, 2001.

Kirby KA: Foot and Lower Extremity Biomechanics III: Precision Intricast Newsletters, 2002-2008. Precision Intricast, Inc., Payson, AZ, 2009, pp. 165-166.

Kirby KA: Foot and Lower Extremity Biomechanics IV: Precision Intricast Newsletters, 2009-2013. Precision Intricast, Inc., Payson, AZ, 2014, pp. 103-104.

Fuller EA, Kirby KA: Subtalar joint equilibrium and tissue stress approach to biomechanical therapy of the foot and lower extremity. In Albert SF, Curran SA (eds): Biomechanics of the Lower Extremity: Theory and Practice, Volume 1. Bipedmed, LLC, Denver, 2013, pp. 205-264.

Kirby KA: Foot and Lower Extremity Biomechanics V: Precision Intricast Newsletters, 2014-2018. Precision Intricast, Inc., Payson, AZ, 2018, pp. 101-102.