Prescribed Precision: Dosing AFO Stiffness for Maximum Outcomes

An AFO’s fundamental function is to manipulate the orientation and progression of force about the body to improve alignment and gait.

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by Kendall Brice, MS, CPO, The O&P EDGE November 1, 2025

Biomechanics is the core treatment target of orthotic care. In the realm of rehabilitation medicine, there have been calls to define the “active ingredients” that cause meaningful change in the treatment targets.^1,2 It is important to define these active ingredients to enhance targeted outcomes.

In his work, Dijkers implores the field of rehabilitation medicine to disaggregate treatments—to describe the subcharacteristics of interventions—in order to more completely identify the treatment, specify reasonable goals, and explain how the treatment affects the outcome.¹ It would not be incorrect to say, for example, that taking medicine reduces pain. In the same way, it is not necessarily incorrect to say that utilizing an AFO improves gait. While not particularly incorrect, these statements are incomplete. A more useful statement would be that taking 200mg of ibuprofen per day reduces inflammation or that a custom composite AFO of 7 Nm/degree bending stiffness worn while walking improves gait efficiency. The revised statements are valuable because they identify essential subcomponents of the intervention and their anticipated effect on an outcome of interest. There is a clear cause-and-effect relationship drawn in this framework, which then allows the identified components to be manipulated to achieve a desired effect on the outcome.

Dijkers goes on to say that once treatments are characterized in this format, they should be quantified to “link interventions to patient inputs and especially to outcomes.”¹ Originating from discussions in the physical therapy realm, essential dimensions of a treatment’s strength include adherence to a protocol, the degree to which it is tailored to specific patient characteristics, and intensity.¹ While originating outside of the practice of orthotics, this framework can be adapted to our practice in which essential dimensions of an AFO’s efficacy can include consistent decision-making protocol, individual optimization, and dosage.

The components, or ingredients, that produce a change in the treatment target—an aspect of a patient’s functioning—are known as active ingredients.² As established, the most immediate effect of an orthosis is to manipulate the alignment and the forces about the body. But the presence of an AFO in and of itself is too broad a category on which to attribute all outcomes. There are many complexities integrated into the design of an AFO, from the device’s alignment, stiffness, and trimlines to its aesthetic value, which might influence the patient’s desire to use it. To say that use of an AFO causes an outcome, while not incorrect, is not useful to clinical decision-making, troubleshooting, outcomes, or research.

Stiffness Is an Active Ingredient

Within the scope of AFO intervention, the rigidity, or stiffness, of the device should be considered an active ingredient. Measured often in academic settings by the Bi-articular Reciprocating Universal Compliance Estimator (BRUCE) apparatus, stiffness is quantified in Nm/degrees—essentially, how much force is required to deflect the AFO one degree (typically into plantarflexion or dorsiflexion). Although BRUCE is currently a research tool, the value it presents can extend to daily clinical practice by providing a universal language for stiffness, which is currently lacking when communicating across manufacturers or patient cases.

Emphasizing the significance of this variable, research has demonstrated that the mechanical stiffness of an AFO significantly affects metabolic, kinetic, kinematic, and physiologic outcomes. In 2019, Waterval et al. found that varying stiffness of AFOs to optimal levels led to nearly a 20 percent reduction in energy costs and a 25 percent increase in walking speed.³ These results distinguish AFO stiffness as not simply a design feature, but as a clinical lever that influences efficiency. Another study found that varying AFO stiffness caused changes in medial gastrocnemius activity, propagation of ground reaction forces through the limb, and ankle range of motion (ROM).⁴ Further, Waterval et al. reported in 2025 that a 0.5 Nm/degree change in stiffness can be considered clinically relevant.⁵ This level of granularity is equal to the change in force achieved by turning the adjustment collar on a torque wrench about half a turn. It is undeniable that stiffness plays a significant role in manipulating gait, the core treatment target of AFO intervention. Just as important as recognizing stiffness as an ingredient is realizing that too much or too little can compromise these benefits.

Nearly as ubiquitous in the research is the proposition that stiffness must be optimized to the individual. Recognizing the presence alone of mechanical stiffness fails to recognize the nature of its effect. Just as a medication must be dosed, the value of stiffness lies in its precise calibration rather than its presence alone.

Stiffness Must Be Dosed

There is no one-size-fits-all recommendation for the degree of stiffness an AFO must provide, and no singular threshold for efficacy across all clinical presentations.^3,5 The same level of rigidity that optimizes one patient’s gait might over-restrict another or fail to provide enough support for a third. In a study comparing AFOs provided with and without a tuning procedure, it was found that the stiffness-tuned AFOs increased energy efficiency twofold, and walking speed, endurance, and satisfaction all improved comparatively.⁶ Another study compared clinically tuned AFO stiffness outcomes versus bodyweight-based stiffness recommendations and found that the tuned AFO stiffness lowered energy costs, increased speed, and allowed greater ROM and power.⁷ For clinicians, these findings deliver a critical message: There is an optimal amount of stiffness required for each patient’s unique presentation, and a constellation of factors, more than bodyweight alone, determines the optimal level.^7,8

Dosage Strategy

This concept is essentially a “dose” or “the amount of active ingredient(s) expected to produce the desired effect.”⁹ The literature has established the existence of an optimization point—where more or less stiffness diminishes outcomes—imploring the profession to acknowledge the significance and sensitivity of this ingredient and kickstarting our ability to prescribe its optimal levels. Stiffness has a window of efficacy where too much or too little can both degrade function, and we need better tools to consistently identify and target that window.

Traditionally more nebulous, inherited, and intuited than a prescription drug’s chemical composition, the current practical methods of dosing stiffness, increasing thermoplastic thickness for adult versus pediatric patients, perhaps selecting firm, medium, or low stiffness categories on a composite AFO work order, are a start in the right direction. But the broad categories and qualitative names lack specificity and universality.

A resident once asked me how I could distinguish between firm stiffnesses of composite AFOs from multiple manufacturers, and I replied it was because I had personal comparative experience from ordering them in the past. While experience is invaluable, it takes time and is relatively undefined. I cannot say exactly how much stiffer one manufacturer’s firm is from another, and thus I have a limit to how precise my practice may be. Presented with a plethora of options for materials and more options for fabrication, clinicians are often left to make decisions based on previous experience and intuition. I believe we can shift the paradigm to developing a system that empowers clinicians to make experience-based decisions backed by expertise and precision.

Beyond inconsistent terminology in manufacturing, a more pressing issue stems from the same branch: AFOs are commonly provided with suboptimal stiffness.⁵ This means that without a system to measure, validate, predict, and optimize this active ingredient, outcomes are underrealized—not due to poor care, but due to lack of measuring a key biomechanical parameter. With the development of tools and systems, we have an opportunity to more fully realize a patient’s function and enhance outcomes past the horizon on which we currently sit.

Imagine, for a moment, having the clinical ability to precisely predict what stiffness level would be appropriate for each patient, to a fairly accurate degree, and specify this value in Nm/degrees to a fabricator. The procedure empowers clinicians by integrating their expertise into a prescribed dose of an active ingredient for the device’s success. The language is precise, increasing reliability and consistency of products across fabricators. And the AFOs would be comparable based on a universal unit of Nm/degrees. There would be no more guessing at coarse categories of stiffness, sometimes finding out at delivery that the stiffness is not quite optimal. There would be far fewer refabrications and less time spent tuning. The system would become more efficient and more effective. Potentially, the system could preference particular outcomes, reflecting the variation in specific goals among patients, which could range from efficiency, to stability, to walking speed, and more.

But how might we arrive at such a reality? There is work yet to be done, but a similar system is already commonplace in parallel clinical settings. When ordering a prosthetic foot, clinicians routinely encounter manufacturers’ configurator tools that recommend foot stiffness categories based on information provided about the patient such as foot size, weight, and activity level. Translating this model to AFO workflow and adding a quantifiable value to the recommendation is the next evolution in AFO provision.

A promising start to apply this concept and this sort of configurator tool within AFO intervention has been made in a collaboration among Waterval and colleagues just this year. Predicting optimal AFO stiffness by calculating the ankle power deficit for each patient, the researchers found that the predicted optimal stiffness value was not remarkably different from a clinically optimized stiffness.⁵ While their methods were more technical than is currently clinically feasible, these findings suggest a potentially valid method to precisely prescribe AFO stiffness.

What we can continue to strive for is a method of prediction and prescription of stiffness values, produced in a common and quantitative language of Nm/degrees. Many factors, as defined in continued research efforts, have been shown to accurately prescribe optimal and individualized stiffness values.⁹

Utilizing such a system in AFO care mirrors what is a daily event in clinical settings within the context of prosthetic feet, but it will require a shift in the language we use, the workflow we engage, fabrication infrastructure, clinical education, and reimbursement pathways. These changes are not trivial, but they are achievable, and the benefits are substantial. What clinicians can do until we achieve these industry shifts is to record stiffness values however available (categories, plastic thicknesses, materials, etc.), document the effects of changing stiffness, and utilize systems that foster the tuning process that research has shown to produce more optimal outcomes.

Call to Action

In the United States, patients using lower-limb orthoses face lower baseline mobility and higher anxiety and fatigue than lower-limb prosthesis users.¹⁰ Compared to the general US population, patients using lower-limb orthoses experience lower mobility, higher pain, more anxiety and fatigue, greater sleep disturbance, and reduced ability to participate in social roles.¹⁰ This gap represents an opportunity and an obligation: We cannot accept vague categories of stiffness when precision is within reach.

The evolution of the field has been called for and has been piloted. AFO stiffness must be defined, measured, and prescribed with precision and accuracy rather than intuition and convention. Advances in prescriptive algorithms have shown promising predictions of optimal stiffness values, foreshadowing a clinical reality where work orders specify a Nm/degree value over an adjectival category of firm or medium. We are currently operating within a framework that recognizes the importance of variable stiffness; the framework just needs enhanced specificity and a common, empirical unit with which to communicate.

Without biomechanical precision and efficiency, our impact will inevitably fade on more distant outcomes like satisfaction and quality of life. However, if we build precision into the fundamentals, the opposite can be true: The effects resonate further, stronger, and more consistently, as a ripple effect. So, for maximum effectiveness and efficiency, let our foundations be as precise and purposeful as possible. Let the baseline reflect the highest standard we are capable of delivering.

The stiffness of an AFO is an active ingredient in the treatment. The future of orthotic care requires, and is evolving toward one where prediction becomes prescription, and where precision-empowered potential becomes routine practice.

Kendall Brice, MS, CPO, graduated from Baylor College of Medicine’s Orthotics and Prosthetics Program and has a bachelor’s degree in kinesiology, sports medicine, from Rice University. Brice provided a broad spectrum of care from lower-limb prosthetics to scoliosis and craniosynostosis care and emphasized advanced lower-limb orthotic care.

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