Kevin A. Kirby, DPM

Kevin A. Kirby, DPM We provide the most advanced podiatric care to our patients with an emphasis on the biomechanics of the foot and lower extremity.
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Dr. Kevin Kirby graduated from the California College of Podiatric Medicine in 1983 and completed his first year surgical residency at the Veteran’s Administration Hospital in Palo Alto, California. He spent his second post-graduate year doing the Fellowship in Podiatric Biomechanics at CCPM where he also earned his MS degree. Dr. Kirby has authored or co-authored 27 articles in peer-reviewed jour

nals, has authored or co-authored five book chapters, and has authored five books on foot and lower extremity biomechanics and orthosis therapy, all five of which have been translated into Spanish language editions. He has invented the subtalar joint axis palpation technique, the anterior axial radiographic projection, the supination resistance test, the maximum pronation test and the medial heel skive and lateral heel skive orthosis techniques. He has also created and developed the Subtalar Joint Axis Location and Rotational Equilibrium Theory of Foot Function and has co-developed the Subtalar Joint Equilibrium and Tissue Stress Approach to Biomechanical Therapy of the Foot and Lower Extremity. He has lectured internationally on 33 separate occasions in China, Spain, Belgium, New Zealand, Australia, England, Dominican Republic and Canada over the past 23 years on foot and lower extremity biomechanics, foot orthoses, and sports medicine. He has also lectured extensively throughout the United States. Dr. Kirby is a member of the editorial advisory board for the Journal of the American Podiatric Medical Association and a manuscript reviewer for the Journal of Biomechanics, Journal of Foot and Ankle Surgery, Medicine and Science in Sport and Exercise, Journal of Foot and Ankle Research and Journal of Sports Sciences. He is currently an Adjunct Associate Professor in the Department of Applied Biomechanics at the California School of Podiatric Medicine and has a full time podiatric biomechanics and surgical practice in Sacramento, California.

04/08/2026

Slow Motion Video of Ryan Hall Running at 2:03 Marathon Pace on Instrumented Treadmill at Southern Methodist University Locomotor Performance Laboratory

The Locomotor Performance Laboratory at Southern Methodist University (SMU) has done some amazing slow motion videos of elite distance and middle distance runners and sprinters. Below is a slow motion video of Ryan Hall, one of the fastest US marathoners of all time. Hall is seen here running on SMU's instrumented treadmill, which has a force platform embedded under the belt of the treadmill. He is running at a 2:03 marathon pace (4:41 mile pace).

On the right hand side of the video is a plot of ground reaction force (GRF) versus time which is synchronized to the video of Hall running on the treadmill, seen on the left. Note that the tall upward peak on the GRF vs time curve is the vertical component of GRF acting under Hall's foot. At this running speed, the magnitude of Hall's vertical GRF is about 500 pounds which, at his weight of approximately 135 pounds, equals about 3.7 times body weight (BW). In general, the faster the individual runs, the greater will be the vertical component of GRF for the runner.

The amount of time that Hall spends on the ground with each running step is only about 167 milliseconds (ms) or about 1/6 of a second. In general, sprinters will spend less time on the ground and slower runners will spend more time on the ground when running.
The lower, smaller peak seen on the GRF vs time curve is the anterior-posterior horizontal component of GRF. This curve is also called the fore-and-aft shear curve or the braking-propulsion curve.

Note that in the first half of Ryan Hall's support phase, the downward movement of this curve below the baseline represents his foot tending to slide forward relative to the ground, or a braking impulse. In the second half of Hall's support phase, the upward movement of this curve above the baseline represents his foot tending to slide backwards relative to the ground, or a propulsive impulse. All runners will exhibit this transition between initial braking and later propulsion in their horizontal anterior-posterior running shearing forces and is a normal part of the running gait cycle.

Finally, also note that Ryan Hall's footstrike is on his forefoot and is about 41 cm (16 inches) ahead of his hip. The foot always lands ahead of the hip joint and center or mass (CoM) of a runner that is running at constant velocity and on a level surface. If the runner, for some reason, tried to footstrike with their foot under their CoM, as some self-proclaimed "runnin formexperts" suggest, and especially at this 2:03 marathon pace, they would be thrown violently forward into a face-plant fall. The runner's footstrike must be ahead of the runner's CoM in order to maintain vertical balance during running and, again, this is a normal part of the running gait cycle.

04/07/2026

Danny Dreyer, Inventor of "Chi Running" Preaches "Don't Let Your Feet Land in Front of Your Hips"

Danny Dreyer, the inventor of "Chi Running" is shown in his video from 2009 on his running form technique which he has written books about, and given lectures on, all at the beginning of the Barefoot Running Fad that lasted about 5 years, from 2009-2014.

In his books, videos and courses on "Chi Running", Dreyer preaches about how runners shouldn't footstrike with their feet in front of their hips. He says that footstrike should occur under or even in back of the hip. This is contrary to all available scientific research on running biomechanics which clearly shows that, during running at constant velocity on level ground, the foot strikes well ahead of the CoM.

It is these self-proclaimed "experts in running form", who are not academics and don't understand running biomechanics, that continue to injure runners with their false methods of teaching proper running form. Please note, at the end of the video, I have included the front cover from Danny Dreyer's book on Chi Running.

Please note that on the cover of his book, Danny is shown doing the very same thing he says you shouldn't do when running...he is shown landing with his foot in front of his hips. Why are people still teaching this biomechanical nonsense to runners? Maybe it is because they don't understand the biomechanics of running?

Or maybe it is just because they want to make money off of those people who unfortunately believed all the hype that arose from the Barefoot Running-Minimalist Running Shoe-Anti-Rearfoot Striking Running Fads. Lesson to be learned? Be skeptical of all new fads in running and also be wary of the self-proclaimed "experts" who want to make money off of these new fads....at your own expense.

https://youtube.com/watch?v=rkUqkdPQHis

The Spring-Mass Model and Ground Reaction Force Vector of RunningModelling is used in science in an attempt to make an a...
04/06/2026

The Spring-Mass Model and Ground Reaction Force Vector of Running

Modelling is used in science in an attempt to make an activity easier to understand, define, quantify, visualize, or simulate by referencing it to existing and usually commonly accepted knowledge. For human running, the preferred model within the international biomechanics community for the past three decades is the Spring-Mass Model (Blickhan R: The spring-model for running and hopping. J Biomech, 22(11/12):1217-1227, 1989).

In the spring-mass model of running, the center of mass (CoM) of the individual is represented as a single mass that is attached to a leg-spring that contacts the ground with each running footstrike. The spring-mass model, like actual running, has the foot contact the ground in front of the CoM. Then, as the CoM progresses forward over the planted foot, the leg-spring compresses in the first half of the running support phase, then elongates in the second half of the running support phase. [The support phase in running is the phase in which the foot is on the ground and is analogous to the stance phase in walking.]

During the first half of the support phase, the leg-spring uses the kinetic energy of the falling CoM to compress and store potential energy as elastic strain energy within the tendons, ligaments and muscles of the lower extremity, or, in this model, the leg-spring. Then, in the second half of support phase, the leg-spring elongates, releasing the stored potential energy (i.e. elastic strain energy) as kinetic energy that then propels the CoM upwards and forwards.

This transfer of potential energy to kinetic energy, using the kinetic energy of a falling mass contacting the ground to first store potential energy to then be released next as kinetic energy, is the same potential-kinetic energy transfer mechanism used by bouncing balls and by a child bouncing along the ground on a pogo-stick.

In the spring-mass model, the ground reaction force (GRF) peaks when the CoM is directly over the planted foot. The GRF represents the mechanical interaction of the mass of the individual with the ground and the accelerations-decelerations of that mass relative to the ground at each instant of the support phase of running.

Note, in my video below, the GRF vector is illustrated at each phase of running gait, showing how the GRF vector will always align closely with the CoM of the body during running. This alignment of the GRF vector with the CoM of the runner causes the anterior-posterior shearing forces in running during the first half of support phase to be in a posterior direction, sometimes known as a "braking force".

This posteriorly-directed shear force during the first half of support phase does not necessarily slow down the runner as the name "braking force" suggests. Rather, this posteriorly-directed shear force is a necessary result of the foot needing to land in front of the CoM of the runner in order to allow the lower extremity (and foot) to have the time to store energy by "compressing the spring", as the body moves forward over the foot, which, in turn, allows the most metabolically-efficient running stride.

At the middle of the support phase, the anterior-posterior shearing force is at a magnitude of zero since the CoM is directly over the foot. With the CoM over the planted foot, the GRF vector is at its maximum. This phase of running gait, when shown on the GRF vs time curve from a force plate, is called the "active peak" or "propulsive peak".

In the second half of support phase, there will now be an anteriorly-directed shearing force on the foot (i.e. "propulsion force") since the CoM is now forward of the foot and the GRF vector always points toward the CoM of the runner. During the second half of the support phase of running, the foot and lower extremity is releasing its stored elastic strain energy within the leg-spring and converting it into a kinetic energy that now pushes the CoM of the runner upwards and forwards.

These posteriorly-directed shear forces in the first half of support phase and anteriorly-directed shear forces in the second half of support phase seen while running on a force plate are present during all forward running velocities, from a slow jog to an all-out sprint. The spring mass model of running predicts these anterior-posterior shear forces need to occur with running in order for proper potential-kinetic energy exchange to occur to optimize the metabolic efficiency of the runner and for the individual to be able to maintain upright balance during running.

Unfortunately, there is still a small number of vocal barefoot running-minimalist shoe wearing-anti-heel striking running zealots that teach that runners should have their foot under their CoM at the instant of footstrike in running. "Chi-Running" is one such example of a "running form" that teaches this running technique, "landing with your feet under your hips". In fact, it is impossible to run at a constant, non-zero forward velocity over level ground without the foot first striking the ground ahead of the CoM.

All in all, runners have their foot strike ahead of their CoM during running since this is the only way for the human body to maintain upright balance during all velocities of running and to use the potential-kinetic energy exchange within their feet and lower extremities to optimize the metabolic efficiency of running. Overall, the spring-mass model nicely predicts this experimental observation seen in runners over the five decades in which force plates have been able to accurately measure the three-dimensional GRF vector under the feet of runners during scientific running studies.

Podiatrists, and other foot-health professionals, should warn the public and their patients of the misinformation often taught by self-proclaimed "experts on running form" to avoid having them becoming injured by trying to run in a fashion that does not optimize their own running biomechanics. We need to ensure that accurate information is being relayed to our patients, and the running public in general, so that runners can better enjoy their activity, with less chance of becoming injured.
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Running Footstrike is Always Ahead of the Runner's Center of Mass The biomechanical importance of running footstrike bei...
04/05/2026

Running Footstrike is Always Ahead of the Runner's Center of Mass

The biomechanical importance of running footstrike being in front of the center of mass (CoM) in runners cannot be underestimated. Research from 15 years 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 at the 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 both superiorly and also posteriorly at the instant of footstrike (see photos below).

Since the resultant GRF vector acting on the shoe/foot of the runner always 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 fall violently forward at the instant of footstrike, falling onto their face.

In order to run properly when the runner's foot first strikes the ground, the resultant GRF vector must pass very close to being through the middle of their CoM, otherwise the runner's body would either suddenly lurch forwards or backwards. Runner's will instinctively place their foot so that the resultant GRF vector will pass nearly directly through their CoM in order to smooth the motions of their body over the ground and prevent any rotational moments on their body at footstrike which would tend to either rapidly accelerate their body forwards (and fall forwards) or rapidly accelerate their body backwards (and come to an abrupt stop). In other words, to maintain sagittal plane rotational equilibrium of the body's CoM at the instant of footstrike during running, and not be suddenly rotated forward at the instant of footstrike, the runner must place their foot ahead of their CoM during running.

Shown nelow also are my illustrations of runners at foot-strike, midsupport and toe-off. Notice how the resultant GRF vector "follows" the CoM of the runner from initial ground contact at footstrike with the GRF resultant vector pointing posterior-superiorly. Next, at the middle of midsupport, since the CoM is directly over the planted foot, the GRF vector is close to vertical. At toe-off, since the foot is pushing the body forward, the GRF resultant vector is pointed anterior-superiorly.

This change in three-dimensional position of the GRF vector during running relative to the foot means that there must be a posterior shearing component GRF vector in the first half of the support phase of running. The GRF vector then converts to having an anterior shearing component during the latter half of the support phase of running, as the foot pushes bacwards on the ground during late midsupport and propulsion.

This posterior shearing component of the GRF vector, contrary to many ill-informed, self-proclaimed "running form experts", does not slow the runner down. Rather, this posterior shearing component of GRF is a an absolutely necessary component of the GRF vector at footstrike and during the first half of the support phase of human bipedal running. Without it, the individual could not run with a smooth form and could not run with a minimum amount of accelerations and decelerations of their CoM and trunk during running.

This biomechanical mechanism of the human planting their foot ahead of their CoM during running, which is mediated by the central nervous system, allows the human bipedal animal to increase the metabolic efficiency of running. In addition, by planting the foot ahead of their CoM, the bipedal human can run with excellent upright stability to avoid injury,and to maintain their head in a relatively stable position to watch the passing terrain for obstacles or potentially dangerous situations while running.

Can Running Shoes "Return Energy" Back to the Runner's Body to Improve the Metabolic Efficiency of Running?The term "ene...
03/31/2026

Can Running Shoes "Return Energy" Back to the Runner's Body to Improve the Metabolic Efficiency of Running?

The term "energy return" has been used over the past few decades to describe how some running shoes may work for the runner to help them improve the metabolic efficiency of running. This question of energy return in running shoes has especially been one that has become more prominent with the advent of "supershoes", such as the Nike Vaporfly 4%/Next %/Alphafly shoes which have very thick, highly compliant and resilient midsoles that are surprisingly light in weight.

The performance increases in these shoes has been so significant (4% in a 2018 study by Hoogkamer et al), that new rules needed to be instituted the World Athletics governing body in January 2020 that specifically limited the thickness of the midsoles and the types and number of carbon-fiber plates within the midsoles of running shoes for competitions (Hoogkamer W, Kipp S, Frank JH, et al. A comparison of the energetic cost of running in marathon racing shoes. Sports Med, 48:1009–1019, 2018).

There has been considerable debate regarding Hoogkamer et al's research about how shoes such as the Nike Vaporfly 4% increases metabolic efficiency of running. The most reasonable biomechanical and physiological explanation for the performance increases in these supershoes can fairly easily be explained by the previous research on "tuned tracks" by Thomas McMahon and Peter Greene in the late 1970s (McMahon TA, Greene PR: Fast running tracks. Scientific American. 239:148-163, 1978; McMahon TA, Greene PR: The influence of track compliance on running. J Biomech, 12:893-904, 1979).

McMahon and Greene worked to improve race times and reduce the injury rates of athletes with their "tuned track" that was first constructed at Harvard University (see illustration below).

McMahon and Greene believed that the surface of a track could be specifically designed to deform at the same rate that the runner's leg compressed and their center of mass (CoM) lowered during the first half of support phase of running, and then elastically rebound as the runner's leg elongated again and their CoM raised toward toe-off and into the next running step. In other words, McMahon and Greene designed their Harvard tuned track to be deformed under the load of the runner hitting the track surface and then rebound as the runner's CoM was being propelled upwards and forwards toward the next step. Their research showed collegiate runners improving their mile times by about 5 seconds when running on their Harvard tuned track.

Using the tuned track mechanical analogy, the midsoles of these new supershoes, such as the NIke Vaporfly 4%/Next %/Alphafly, act biomechanically very similarly to McMahon's turned track. These shoe with thick, lightweight, highly compliant and resilient midsoles are able to deform a large amount as the load from the runner's body moves over it in the first half of the support phase of running.
In the second half of support phase, these supershoe midsoles are also able to then rapidly spring back to their un-deformed shape, which, in effect, helps propel the body upwards and forwards to the next running step.

Even though the midsoles of these supershoes do contain carbon-fiber or other types of stiffening plates within them, these plates likely work with the midsole materials of these shoes to improve their integrity over time and distribute the loads within the midsole more uniformly. The most likely estimate is that the carbon-fiber plates of the Nike 4% contribute only about 1% while the Pebax midsole foam of the shoe contributes 3% to the 4% metabolic energy improvements seen within this shoe.

Further research will be necessary to delineate how the carbon-fiber plates and midsole foams interact with each other. But, certainly, the thick, lightweight, compliant and resilient foam midsole materials that these supershoes all contain are the main factor allowing the majority of the metabolic energy and performance improvements seen in these shoes, not the carbon-fiber plates.

How Do the Plantar Fascia and Plantar Plate Cause Normal Digital Purchase Force?The plantar fascia and plantar plate for...
03/29/2026

How Do the Plantar Fascia and Plantar Plate Cause Normal Digital Purchase Force?

The plantar fascia and plantar plate form one continuous soft-tissue structure from the medial calcaneal tubercle, proximally, and to the base of the proximal phalanx of the lesser digits, distally. With loading of the plantar forefoot by ground reaction force (GRF), the forefoot will dorsiflex on the rearfoot which will cause a flattening and elongation of the longitudinal arch of the foot. In turn, the plantar fascia and plantar plate will come under tension forces due to this longitudinal arch elongation in order to resist further arch flattening and to help stabilize the longitudinal arch from flattening further.

The resultant increase in plantar fascia and plantar plate tension due to forefoot loading from GRF will also cause a metatarsophalangeal joint (MPJ) plantarflexion moment (i.e. a tendency to plantarflex the MPJ). As a result, the lesser digit proximal phalanx will plantarflex at the MPJ until the GRF under the digit is increased sufficiently to counterbalance the MPJ plantarflexion moment (see my illustration below). The result of this MPJ plantarflexion moment, therefore, is what is known as digital purchase force.

Rotational equilibrium within the sagittal plane at the MPJ will only occur once the MPJ plantarflexion moments originating from the tension force within plantar fascia and plantar plate is exactly counterbalanced by the MPJ dorsiflexion moment from GRF acting on the plantar aspect of the digit (assuming no flexor tendon tension forces). In this way, the passive plantar fascia and plantar plate force which automatically develop within the human foot with forefoot loading during the latter half of the stance phase of gait will also automatically cause a digital plantarflexion moment and the digit to have purchase force with the ground which helps stabilize the digit within the sagittal plane during weightbearing activities.

References:

Kirby KA: Understanding the biomechanics of plantar plate injuries. Podiatry Today, 30(4):30-39, 2017.

Kirby KA: Longitudinal arch load-sharing system of the foot. Revista Española de Podología, 28(2), 2017.

Kirby KA: New concepts in longitudinal arch biomechanics. Podiatry Today, 31(6):20-27, 2018.

How Does the Human Body Balance Itself While Standing on Both Feet?Upright standing on both feet (i.e. upright bipedal s...
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.

Standing Bipedal Balance Using "Ankle-Strategy": Part 2David Winter's analysis of how the standing bipedal human is able...
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).

Standing Bipedal Balance Using "Ankle-Strategy": Part 1David Winter, PhD, wrote extensively on the biomechanics of balan...
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/

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