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What A Recent Study Gets Right And Wrong About Root Biomechanics

This author takes a critical look at recent kinematics research, which questions established tenets of Root biomechanics. 

The biomechanics theories of Merton Root, DPM, and colleagues have been questioned publicly by myself, and many of my international biomechanics colleagues for three decades. The paper by Jarvis and colleagues is just the latest paper that does not support many of the ideas promoted by Root and colleagues.1 

Twenty years ago, Hylton Menz, BPod(Hons), PhD, DSc, an Australian researcher, moderated a forum discussion of podiatric biomechanics researchers about the problems with Root’s measurement system inaccuracies and about how podiatrists could improve on Root’s static morphologic measurement system.2 I further outlined the problems with the Root measurement system in a previous article in Podiatry Today, “Are Root Biomechanics Dying?,” from eight years ago. 3 Therefore, the international podiatric medical community has long believed that some of the ideas taught over the years by Root and colleagues need to be more critically evaluated in today’s world of evidence-based medicine.

It appears that now, with the recent publication of the kinematics research by Jarvis and coworkers, along with the publication of previous research from as early as 1994, that substantial research evidence points to the fact that the validity of the theories proposed by Root and coworkers is questionable.1,4–7 However, the study by Jarvis and colleagues does have some significant issues that lead me to believe that these researchers may have overstated some of the conclusions from their research.

A Closer Look At The Study’s Flaws

First of all, the study by Jarvis and colleagues was on asymptomatic patients, not injured patients with pathologies.1 Therefore, we don’t know if the Root measurements did correlate to any mechanically-based pathologies of the foot and lower extremity since the 100 patients studied were symptom-free, having no foot and lower extremity pathologies. Secondly, the research by Jarvis and coworkers was a kinematics (i.e., position and motion) study and was not a kinetics (i.e., forces and moments) study. In other words, the researchers used no pressure insoles or pressure mats to study temporal and segmental plantar pressure patterns during walking. No electromyogram studied contractile activity of the muscles of the foot and lower extremity during walking. In addition, researchers used no inverse dynamics analysis study to determine the internal moments acting within the joints of the foot and lower extremities during gait. 

The problem with the kinematics-only study of Jarvis and colleagues is that even though there was no significant correlation of foot motions to the Root static measurements, the researchers did not measure the external forces and pressures acting on the foot, nor did they attempt to measure the muscle activity or internal forces, stresses or moments acting within the structural components of the foot and lower extremity during gait. Since abnormal magnitudes of external and internal forces, moments, pressures and stresses are the cause of the mechanically-based pathologies that podiatrists see in the office on a daily basis, and not abnormal foot motion, measuring the kinetics of the foot and lower extremity is absolutely essential to understand if foot and lower extremity “deformities” may or may not cause musculoskeletal injury. Thus, using kinematics-only research to suggest elimination of a clinical evaluation system for treating musculoskeletal injuries that are directly caused by abnormal forces and moments seems unwise at best.

What You Should Know About The ‘Preferred Movement Path’ Theory

How then could the motions of feet with different Root “deformities” of the foot and lower extremity still have similar movement patterns during walking gait? The answer to this question comes from the biomechanics researcher Benno Nigg, PhD, who first proposed his “preferred movement path” theory in 1999 and further expanded upon this theory in subsequent papers.8-10 Nigg and coworkers have proposed that the skeleton has a preferred movement path for a given movement task and that any intervention that supports the preferred movement path will reduce muscle activity. Nigg and colleagues also propose that any intervention that counteracts the preferred movement path will increase muscle activity. 

Since the central nervous system (CNS) controls the motion of the foot and lower extremity, it is possible that the CNS may also attempt to produce a preferred movement path regardless of which “foot deformities” exist from one individual in comparison to another. If the CNS does alter the efferent neural activity to the lower extremity musculature depending on the structure of the foot and lower extremity, these alterations in EMG activity will cause differences in the external and internal forces and moments from one individual to another. Over time, mechanically-based pathologies may develop in the individual’s foot and lower extremity in response to any “foot deformities” that the CNS needs to adapt to in order to produce a preferred movement path. Even though Nigg’s preferred movement path theory has not yet been applied within this “foot deformity” context, the concept that different foot structures may produce similar movement paths during walking gait certainly merits further research in the future.

It is obvious that the research by Jarvis and colleagues is only one of many research studies that have questioned the ideas that Root and colleagues first introduced over 45 years ago. Not only did Jarvis and colleagues study only asymptomatic people, these researchers had no way of knowing whether the CNS was actively attempting to create a preferred movement path of walking even though the feet of the study participants may have had different Root “foot deformities.” As such, abnormal magnitudes of plantar pressures, abnormal external and internal forces and moments, and abnormal internal stresses may have been acting on and within the individuals with different foot “deformities” in the Jarvis study, but this is not known due to the researchers not measuring the kinetic parameters. Since abnormal forces and moments, and not “abnormal motion,” are the cause of the injuries that podiatrists treat on a daily basis, it would make sense to wait until better kinetic studies happen to determine if alterations in foot morphology do correlate to pathological levels of abnormal forces, pressures, stress and moments before recommending that all podiatrists abandon measuring the structure of the foot and lower extremity as a precursor to making treatment decisions for their patients.

With all these facts in mind, I believe it would be unwise to discontinue using all the Root measurements without further evidence that these measurements don’t affect the kinetics of gait and are not responsible for mechanically-based pathologies of the foot. I have personally noted that the Root measurements only occasionally show good correlation to gait kinematics in the thousands of patients I have examined over the past three decades. Since I have lectured on this problem with Root and colleagues’ system for decades, the study by Jarvis and colleagues does not surprise me.1 

However, in some cases, Root and colleagues’ measurements do correlate to pathology. Therefore, we, as a profession, need to do the research to determine which Root measurements best predict pathology and gait function, which don’t have any relation to pathology or gait function, and discard the measurements that don’t correlate to pathology or gait function. I am certain, however, that the Root measurements need to be seriously questioned since in my clinical experience, they fall short of predicting either pathology or gait function of individuals in many cases.

Why The Tissue Stress Approach Is A Better Approach To Orthotics

My main problem with Root and colleagues’ measurements, a system I taught during my biomechanics fellowship at the California College of Podiatric Medicine (CCPM) to hundreds of podiatry students from 1984–1985, is that it relies too heavily on calcaneal bisections. Excellent research from 16 years ago by LaPointe and coworkers has shown that visual bisection of the calcaneus produces errors ranging from 3 to 6 degrees.11 This is a serious problem for the Root measurement system since the calcaneal bisection is the main reference marker for determining resting calcaneal stance position, neutral calcaneal stance position, the degree of “rearfoot varus/valgus deformity” and the degree of “forefoot varus/valgus deformity.” Anecdotally, during my biomechanics fellowship, I saw differences in calcaneal bisections of up to 3 to 5 degrees between different biomechanics faculty at CCPM and up to 5 to 10 degrees between different fourth-year podiatry students.

A much better approach for making foot orthoses for patients and an approach recommended within the paper by Jarvis and colleagues is the tissue stress approach.1 In the tissue stress approach, the clinician concentrates not on heel bisections, subtalar joint neutral position or “foot deformities,” but concentrates rather on the location and functions of the patient’s injured tissues, and then designs a prescription foot orthosis based on reducing the stress on the injured tissue, improving gait function and not causing any other pathologies to occur. The tissue stress approach eliminates the need to perform accurate calcaneal bisections and instead focuses on treating the injured tissues of the patient, which greatly simplifies and improves the therapeutic effectiveness of the foot orthosis prescription process.12-16

Dr. Kirby is an Adjunct Associate Professor within the Department of Applied Biomechanics at the California School of Podiatric Medicine at Samuel Merritt University in Oakland, Calif. He is in private practice in Sacramento, Calif.


1.   Jarvis HL, Nester CJ, Bowden PD, Jones RK. Challenging the foundations of the clinical model of foot function: further evidence that the Root model assessments fail to appropriately classify foot function. J Foot Ankle Res. 2017; 10(1):7.

2.   Menz HB (moderator), Kirby K, Cornwall M, Rome K, Tinley P, Murphy N, Keenan A: Clinical measurement of the lower extremity-where to from here? Austral J Podiatr Med. 1997; 31(3):95-99.

3.   Kirby KA. Are Root biomechanics dying? Podiatry Today. 2009; 22(4):58-65.

4.   McPoil TG, Cornwall MW. The relationship between subtalar joint neutral position and rearfoot motion during walking. Foot Ankle Int. 1994; 15(3):141-145.

5.   Garbalosa JC, McClure MH, Catlin PA, Wooden M. The frontal plane relationship of the forefoot to the rearfoot in an asymptomatic population. J Orthop Sports Phys Ther. 1994; 20(4):200-206.

6.   Cornwall MW, McPoil TG. Effect of ankle dorsiflexion range of motion on rearfoot motion during walking. J Am Podiatr Med Assoc. 1999; 89(6):272-277. 

7.   Buldt AK, Murley GS, Levinger P, Menz HB, Nester CJ, Landorf KB. Are clinical measures of foot posture and mobility associated with foot kinematics when walking? J Foot Ankle Res. 2015; 8(1):63.

8.   Nigg BM, Nurse MA, Stefanyshyn DJ. Shoe inserts and orthotics for sport and physical activities. Med Sci Sports Exerc. 1999; 31(7 Suppl):S421-S428.

9.   Nigg BM. The role of impact forces and foot pronation: a new paradigm. Clin J Sport Med. 2001; 11(1):2-9.

10.                 Nigg BM, Baltich J, Hoerzer S, Enders H. Running shoes and running injuries: mythbusting and a proposal for two new paradigms: ‘preferred movement path’ and ‘comfort filter.’ Br J Sports Med. 2105; 49(20):1290–4. 

11.                 LaPointe SJ, Peebles C, Nakra A, Hillstrom H. The reliability of clinical and caliper-based calcaneal bisection measurements. J Am Podiatr Med Assoc. 2001; 91(3):121-126.

12.                 McPoil TG, Hunt GC. Evaluation and management of foot and ankle disorders: Present problems and future directions. J Orthop Sports Phys Ther. 1995; 21(6):381-388.

13.                 Kirby KA. Foot and Lower Extremity Biomechanics II: Precision Intricast Newsletters, 1997-2002. Precision Intricast, Inc., Payson, AZ, 2002, pp. 11-18.

14.                 Kirby KA, Spooner SK, Scherer PR, Schuberth JM. Foot orthoses. Foot & Ankle Specialist. 2012; 5(5):334-343.

15.                 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.

16.                 Kirby KA. Prescribing orthoses: Has tissue stress theory supplanted Root theory? Podiatry Today. 2015; 34(4):36-44.

Editor’s note: Parts of this article originally appeared in Dr. Kirby’s comments for the News & Trends article “Study Questions Validity Of Root Model When Prescribing Orthoses,” which appeared in the May 2017 issue of Podiatry Today.

Online Exclusives
Kevin A. Kirby, DPM



Dr. Kirby states that a much better approach to making foot orthoses is the tissue stress approach. Unfortunately none of the advocates of the tissue stress approach have provided clinicians and prescription foot orthotic manufactures with clear guidelines for practicing the tissue stress approach to foot orthotic therapy. In fact, there is considerable debate and disagreement amongst advocates of the tissue stress approach as to how to examine the patient, cast the foot and how to prescribe and manufacture foot custom orthoses. For example, Jarvis et. al. advocating eliminating open chain ROM, forefoot to rearfoot relationship, etc. in the treatment of patients. However, Dr. Kirby has previously stated that he does not necessarily agree with this conclusion. This tells us the tissue stress approach is poorly defined and lacks the guidelines and standards that would make it a teachable and therefore credible approach to the treatment of lower extremity related pathology. In making his case, Dr. Kirby uses the argument that heel bisection errors are a major problem with the Root approach. If we look at how most prescription foot orthoses are manufactured, the manufacturer must somehow orient the cast in the frontal plane in order to manufacture the device. This is a simple fact that can’t be ignored. Using a heel bisection is the most common method for orienting the cast in the frontal plane when making functional orthoses. The heel can be corrected (i.e. positioned) in a prescribed inverted, everted or vertical orientation with the degree of inversion or eversion being determined by the prescribing practitioner. The significance of the frontal plane orientation of cast and resulting orthotic, and our reliance on heel bisection should not be understated. So if the practitioner doesn’t bisect the heel, the orthotic manufacturer will have to use some other method of orienting the cast in the frontal plane. Since the frontal plane is the only plane that is used to orient the cast when manufacturing a functional foot orthotic, I am interested in how Dr. Kirby suggests practitioners and orthotic manufacturers orient the cast in a more consistent fashion than can be done with the use of heel bisections.

Thank you Dr. Kirby for beginning the discussion of truly analyzing research that would contend that there are no applications for the Root principles. Many people assume that if a study is published in a reputable journal by reputable researchers, that the research is totally valid and the conclusions by the writers are also totally valid. Thank you for pointing out some of the flaws in the Jarvis article. Unfortunately many people believe that the Root principles constitute a single theory or a single concept. Nothing could be further from the truth. I find in Root, great insights, however there are many things that are not in the Root concepts. Thus I am always looking for additional concepts to add. There are many principles of Root that I follow and there are some that I partially follow, some that I have made adaptations to, and some that I do not follow at all. One example is that I find it almost impossible to clinically define ankle joint equinus. Root did not originate the idea of tight Achilles tendon being a cause of foot dysfunction. He did elaborate and add his own thoughts to it. However the unreliability in taking ankle joint dorsiflexion measurements is overwhelming to me. When I do take them I usually take two measurements, first a passive measurement and the other and active measurement. What we really need to accurately measure ankle joint dorsiflexion is not a single number, but a graph of passive tension in the Achilles tendon versus ankle joint dorsiflexion position. Notice that I do not criticize the concept that there is such a pathology as ankle equinus, only that the methods of measurement that had been utilized are not adequate for finding out when it really exists. Unfortunately in your discussion above, I find that you also fall into a trap of not fully representing the literature. An example is how you utilize the LaPointe et al. (JAPMA 2001) article to advocate against calcaneal bisection’s. It is true that LaPointe did show that visual calcaneal bisection lines are not reliable. It is also true that Lapointe DID find that utilizing a caliper instrument produced reliable calcaneal bisection lines. I would think that a truly honest utilization of the Lapointe article would be to encourage practitioners to start using instruments in making calcaneal bisection lines. In a similar fashion those who are some of the most vocal critics of Root fail to consider literature that does support Root. As an example those who use the unreliable argument to say that goniometric measurements should be discarded, usually fail to mention studies that have shown good reliability, e.g. Freeman et al. (The Foot, 2007) in which a special instrument was used and shown to have good reliability to take subtalar joint measurements. Failure of a discipline to develop instruments that can take accurate measurements is more of a sad commentary on the intellectual capacity of the discipline, and does not constitute a valid critique of the concepts that have been advocated to be measured. Finally it important to note that Root tried to make some correlations between function and form, which I find still have validity, however it is important to realize that in podiatric circles, we may have our own version of the Heisenberg uncertainty principle, where you can’t know precisely both position and motion. I would like to see Dr. Roy Lidtke resurrect his student paper that he wrote on using chaotic attractors for describing gait. For anyone to say that they only need Root for practicing clinical biomechanics is a great slap in the face to Dr. Root himself, for he was one that was always advocating for further development of ideas. We seem to have a lot of advocates of one theory or another that now are best described by the adage, “When the only tool you have is a hammer, the whole world looks like a nail.” Tissue stress may be able to help one understand peroneal tendinitis, but I have yet to see it help in understanding why hammertoes develop. I believe that it is time for the profession to move on from the Root-right-or-wrong debate, and instead focus on better development of measurement instruments and techniques and also incorporating all valid research and ideas to find the best clinical practices. A science with no measurements is no science. And a measurement system with no identification of zero is no measurement system. Best wishes, to all in their continuing endeavors to make the treatment of foot disorders more scientific.
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