June 2020

What If the Ankle Is More Than a Class 2 Lever?

By Thomas J. Cutler, CPO, FAAOP

A knowledge of the historic and philosophical background gives that kind of independence from prejudices of his generation from which most scientists are suffering. This independence created by philosophical insight is—in my opinion—the mark of distinction between a mere artisan or specialist and a real seeker after truth.

Albert Einstein, Letter to Robert Thornton, 1944

The realm of orthotics and prosthetics (O&P) is based on gaining leverage for patients and eking out power when strength is not plentiful. A physical therapist will work with a patient over time to gain power through strengthening exercises where the surgeon may be focused on securing a torn ligament or fracture in hours. But the toolbox of the orthotist-prosthetist is an array of resources designed to optimize the dark side of biomechanics: Reactive Biomechanics—those passive forces that take energy, create leverage, and return more power in an instant. For the orthotist-prosthetist, biomechanics is currently all about levers and leverage.

But there appears to be a multidisciplinary disconnect around the biomechanics of the ankle that is becoming increasingly burdensome as we see the rise of post-traumatic arthritis of the ankle (PTAA), most likely caused by sprains or other trauma incurred 10 or more years earlier.1,2 As this societal burden continues to rise and our ability to “fix” it remains limited, perhaps it’s time to  consider the words of physicist Dr. Carlo Rovelli, director of the Quantum Gravity Group at Aix-Marseille University, who said “…before experiments, measurements, mathematics, and rigorous deductions, science is above all about vision. Science begins with a vision. Scientific thought is fed by the capacity to ‘see’ things differently than they have previously been seen.”3

Historically, the mechanical advantage of the human ankle has been understood in medical and research communities to be a class 2 lever.4 And this can be explained by a commonly-overlooked premise of osteokinematics that requires the observer to describe movement in anatomical position. In short, we’ve been asked to discuss joints from only one perspective.5 But just as the ankle can move in many directions, is it possible to gain insights by recognizing ankle forces from both active and reactive perspectives?6 Indeed, when Zelik et al reanalyzed previously published data, they found “…>30% of the energy changes during the push-off phase of walking were not explained by conventional joint- and segment-level work estimates, exposing a gap in our fundamental understanding of work production during gait.”7

So, in addition to the classic textbook presentation of a class 2 lever, couldn’t the ankle also be understood in the reactive direction to gravity as a class 1 lever? Otherwise, shouldn’t it be explicitly stated that Newton’s third law of motion is “equal and opposite and identical”? But it obviously isn’t, despite being represented as such in biomechanics research.  Since both the action and the reaction of gravity apply to the ankle at all times, would a modern understanding of the ankle incorporate both lever classes? Perhaps we need to challenge what we thought we knew about the ankle to come up with better, more cost-efficient, patient-friendly solutions.

A Question

To that end, a survey was conducted at the 2017 Annual Meeting of the American Association of Orthopaedic Surgeons and its findings presented at the 2019 Annual Meeting of the American Orthotic & Prosthetic Association.8 Survey takers were provided with illustrated depictions of the 3 known lever classes and asked which lever class applied to the human ankle. They were allowed to check any that apply. The explicit provision of all 3 lever classes was intended to provide a full range of biomechanical options. Respondents totaled 50; 58% (n=29) identified as MD/DO; 24% (n=12) identified as affiliated with manufacturers; and 18% (n=9) identified as engineers, researchers, medical students, or teaching faculty.

Results to the survey8 question were as follows: Overall, 50% selected Class 1, 22% selected Class 2, 26% selected Class 3, and one MD/DO reported “No idea.” Of the MD/DO category, 60% selected a Class 1 lever, with 20% each selecting Class 2 and Class 3 levers. Of the manufacturer category, respondents were equally split among the three choices (4 in each lever class). Of the category with the balance of 9 respondents, 4 selected a Class 1 lever, 2 selected a Class 2 lever, and 3 selected a Class 3 lever.

Figure. Unmeasured positive work. Push-off kinetics are not explained by conventional joint- and segment-level biomechanical measures, based on our re-analysis of previously published data (Zelik and Kuo, 2010). At 1.4 m s−1 there is about 24 J of total positive energy change (Δenergy) during Push-off, which reflects the redirection of the body’s center-of-mass (COM) velocity, and also the motion of segmental masses relative to the COM (termed Peripheral energy change). We can compare this total energy change estimate with the mechanical work computed from commonly used joint- and segment-level estimates. We observe that the Push-off work performed by the stance limb joints and foot segment – work performed rotationally (represented by green arrows) about the hip, knee and ankle joints [from 3 degree-of-freedom (DOF) inverse dynamics] and work performed by the foot [a combination of metatarsophalangeal (MTP) joint rotations and other deformations within the foot and shoe] – only sums to about 16 J. Thus, these conventional measures fail to account for 8 J (33%) of the Push-off kinetics. Reproduced with permission from Zelik K, Takahashi KZ, Sawicki GS. Six degree-of-freedom analysis of hip, knee, ankle and foot provides updated understanding of biomechanical work during human walking. J Exp Biol. 2015;218(Pt.6): 876-886. Available at https://jeb.biologists.org/content/218/6/876.long

Unexpected Findings

This study was initially conducted with the assumption that if provided with depictions of all 3 lever classes, clinicians would unanimously select the lever class designated as correct from textbooks and the American Society of Biomechanics. That is, if provided with clear depictions of each lever class, subjects would select class 2 levers 100% of the time. It was therefore highly unexpected to have only 22% of respondents select this answer. It was further surprising to note that this response rate fails to reach the 33% rate typical of random chance.

Interestingly, the manufacturers who would not be classified as clinicians with the same biases, had response rates that did reflect random chance.

These results indicate that medical professionals with significant experience develop a heuristic awareness of the reactive biomechanics of the ankle. According to the results, practitioners intuitively understand how it functions reactively as a class 1 lever. In fact, the results indicate that in selecting the reactive configuration at 3 times the rate they selected the active configuration, surgeons with practical experience see the ankle differently from its understanding in academia.

Before proposing an inclusive resolution, it must be noted that a “rocker” as it relates to biomechanics, is jargon because it is a misnomer. Despite having gained wide usage and application, “rocker” was used initially by Dr. Jaqueline Perry in her research as a simile. Thus, she provided a literary tool and not an engineering mechanism. Rather than seeing only a single type of ankle leverage, Divergent Reactive Leverage addresses the simile and incorporates an understanding of both types of leverage at play.

While gravity is naturally the first recognized force in the equation, Reactive Biomechanics requires modeling the reactive (normal) force as a class 1 lever prior to modeling the applied ankle force of the plantarflexors. With the 4-fold increase in moment arm of the reactive toe lever, error rates found in research (Figure)9 with current ankle modeling are eliminated and a new appreciation of the lower extremity is possible.

Modeling indications for Divergent Reactive Leverage are straightforward, but as of yet, unexplored.

Thomas J. Cutler, CPO, FAAOP, is a prosthetist, biomechanist, innovator, and researcher at Limb.itless, LCC. In Visalia, California. He welcomes questions and comments about Divergent Reactive Leverage and Reactive Biomechanics. He can be reached at cutlercpo@gmail.com.

  1. Delco ML, Kennedy JG, Bonassar LJ, Fortier LA. Post-traumatic osteoarthritis of the ankle: a distinct clinical entity requiring new research approaches. J Orthop Res. 2017;35(3):440-453.
  2. Ewalefo SO, Dombrowski M, Hirase T, et al. Management of posttraumatic ankle arthritis: literature review. Curr Rev Musucloskel Med. 2018:11(4):546-557.
  3. Rovelli C. The Architecture of the Cosmos. In Seven Brief Lessons on Physics. Riverhead Books: 2016; pg. 24.
  4. Brockett CL, Chapman GJ. Biomechanics of the ankle. Orthop Trauma. 2016;30(3):232- 238.
  5. Neumann DA. Kinesiology of the Musculoskeletal System: Foundation for Rehabilitation. 3rd Ed. Mosby: 2016; pg. 491.
  6. Amis J Front. Surg. 38, 110, 2016Amis J. The split second effect: the mechanism of how equinus can damage the human foot and ankle. J Exp Biol. 2016;3:38.
  7. Zelik K, Takahashi KZ, Sawicki GS. Six degree-of-freedom analysis of hip, knee, ankle and foot provides updated understanding of biomechanical work during human walking. J Exp Biol. 2015;218(Pt.6): 876-886.
  8. Cutler TJ. Reactive biomechanics: study shows lack of consensus about ankle function, and an inclusive resolution with divergent reactive leverage is proposed. Poster presented at American Orthotic and Prosthetic Association. San Diego, California: Sept. 26-28, 2019.
  9. Dumas R, Branemark R, Frossard L. Gait analysis of transfemoral amputees: errors in inverse dynamics are substantial and depend on prosthetic design. IEEE Trans Neural Syst Rehabil Eng. 2017;25(6):679- 685.

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