Imagine a suspension bridge.
It has to remain flexible enough to absorb powerful gusts of wind while staying strong enough to support thousands of cars every day. Too flexible, and it becomes unstable. Too rigid, and it loses its ability to adapt.
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Designing a prosthetic foot presents a surprisingly similar challenge.
Every step places contradictory mechanical demands on the foot within fractions of a second. As the heel touches the ground, the foot needs to be compliant enough to absorb impact and adapt to the surface. A moment later, it must become stable enough to support body weight before helping propel the body forward.
Our biological ankles perform this balancing act effortlessly. Prosthetic feet have spent decades trying to reproduce it.
But before understanding why this challenge is so difficult to solve, we first need to answer a simple question:
What exactly is prosthetic foot stiffness, and why does it matter so much?
Prosthetic foot stiffness describes how much a prosthetic foot resists bending when forces are applied during walking.
Although stiffness is often viewed as a purely mechanical property, its effects are very tangible for the user. It influences how stable, comfortable and natural walking feels throughout every step.
In this context, stiffness refers to resistance to deformation, while compliance describes the ability to deform under load. A more compliant structure is therefore generally less stiff.
At first glance, stiffness might seem like a simple choice between two extremes:
In reality, walking is far more sophisticated than that.
The human ankle does not operate with a single level of stiffness. Its mechanical behaviour changes continuously throughout the gait cycle, adapting to what the body needs at each moment.
Early in the step, a certain level of compliance helps soften contact with the ground and adapt to the surface. As the body moves over the foot, stability and support become increasingly important. During push-off, the foot must efficiently support forward progression.
In other words, efficient walking depends less on having one perfect stiffness than on having the right stiffness at the right moment.
That is precisely what makes prosthetic foot design such a complex engineering challenge.
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Most prosthetic feet are available in different stiffness categories, typically selected according to the user's weight and activity level. This is an essential starting point.
However, choosing the right stiffness is only part of the story.
Walking is a dynamic process. The mechanical demands placed on the foot constantly evolve from one phase of gait to the next. A foot that is too stiff may limit natural movement and make progression feel less fluid. One that is too compliant may struggle to provide enough stability when the body moves over it.
The biological ankle continuously balances these opposing requirements without us ever noticing.
This ability to transition smoothly between flexibility and stability is one of the defining characteristics of human gait and one of the reasons prosthetic gait biomechanics remains such a fascinating engineering challenge.
If changing mechanical behaviour throughout a step is so important, why is it still such a difficult challenge for passive prosthetic feet?
The answer lies in the way many conventional passive prosthetic feet are designed.
Many Energy Storage and Return (ESR) prosthetic feet rely primarily on one or more flexible composite elements that act like springs. As the user loads the foot during walking, these elements bend and store mechanical energy before releasing part of it later in the step to support forward progression.
This principle has transformed prosthetic foot design over the past decades.
However, it also comes with an inherent engineering challenge: the same flexible structural elements may be expected to contribute to several functions that naturally compete with one another.
The foot needs to:
In other words, the same structure may be asked to behave both like a suspension system and a structural frame.
This creates a mechanical compromise.
A more compliant ankle can allow greater motion and better adaptation to the ground. However, increased flexibility may also reduce standing stability and lead to greater postural sway.
Conversely, a stiffer ankle can improve static balance by limiting body oscillations. During walking, it may also support step symmetry and a controlled rollover pattern. However, excessive stiffness can restrict movement, reduce adaptability and make progression feel less fluid.
Neither approach is universally better.
Every mechanical adjustment changes the walking experience differently. The challenge is therefore not simply deciding how soft or stiff a prosthetic foot should be.
It is determining how its stiffness should behave throughout the entire gait cycle.
Prosthetic foot stiffness is therefore about much more than choosing the right material.
Even with advanced composite materials, a single structure that must simultaneously provide flexibility, stability, support and energy return will always have to balance competing mechanical demands.
Increasing flexibility may improve adaptability but reduce stability. Increasing rigidity may improve support but limit natural progression.
The question is not simply:
How stiff should the prosthetic foot be?
It is:
How should that stiffness evolve as the mechanical demands of walking change?
That shifts the problem from material science to architecture.
And that is where Lunaris takes a different approach.
Instead of asking one structure to perform contradictory functions, Lunaris separates them.
Its endoskeletal architecture is inspired by biomechanics: bones provide structural support and soft tissues manage movement, flexibility and energy. Each structure has a dedicated role but works together.
Lunaris applies the same logic. Its rigid internal structure is responsible for:
Meanwhile, the spring structure, working together with the ankle, independently manages:
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Rather than relying on the same element to provide both support and flexibility, these functions work alongside each other.
Support no longer comes at the expense of flexibility and flexibility no longer has to compromise stability. This changes the mechanical logic of the prosthetic foot.
Instead of asking one component to solve every problem, each element is designed to perform a specific mechanical role.
Walking is not a repetitive movement where the body asks the foot to behave the same way from start to finish.
Quite the opposite.
During a single step, mechanical demands change continuously.
As the foot first contacts the ground, a certain level of compliance helps absorb impact and adapt to the surface. A few moments later, as the body's centre of mass moves over the foot, stability becomes increasingly important. Finally, during push-off, the foot needs to efficiently support forward progression.
The body is not looking for one ideal level of stiffness.
It is looking for the right mechanical response at the right moment.
This is one of the reasons the biological ankle performs so efficiently. Its behaviour continuously adapts throughout the gait cycle instead of remaining mechanically constant.
Lunaris follows the same philosophy.
Rather than maintaining the same mechanical response from heel strike to toe-off, its architecture allows stiffness to increase progressively as walking unfolds.
Early in the step, the spring remains relatively compliant, helping the foot adapt to the ground and creating a smoother rollover.
As loading increases and the body progresses forward, stiffness gradually increases to provide greater support and stability when mechanical demands become higher.
At first, this may sound counterintuitive.
How can the same spring become stiffer without changing its material?
The answer is surprisingly simple.
Imagine holding a ruler over the edge of a table.
With most of its length free, it bends easily.
Now shorter the free length of the ruler. Without changing the ruler itself, you've made it noticeably harder to bend. It becomes even stiffer.
Nothing about the material has changed.
Only the effective length available to bend has become shorter.
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That simple mechanical principle is at the heart of Lunaris' bumper mechanism.
As the user moves through the gait cycle, an additional contact point—the bumper—progressively engages with the spring.
This gradually shortens the blade’s effective working length. As a result, the spring naturally becomes stiffer as loading increases.
The material remains exactly the same. The spring itself does not change.
Only the way it is mechanically supported evolves throughout the step.
This allows stiffness to increase precisely when greater support is needed, rather than remaining permanently soft or permanently stiff.
Instead of forcing a compromise between flexibility and stability, the architecture helps coordinate both.
This is where the design moves beyond simply storing and returning energy.
It begins to guide how the foot behaves throughout walking.
By separating support from movement, and by allowing stiffness to change progressively during walking, Lunaris approaches this challenge from a different perspective.
Rather than asking one structure to do everything, each component contributes to a specific function.
The result is a mechanical system designed to support the changing demands of every step.
Ground contact can feel smoother. Progression can feel more fluid. Moving across everyday environments—from flat pavements to uneven paths or gentle slopes—can feel less mechanically constrained because the foot is not relying on the same fixed response throughout the entire gait cycle.
The design of a prosthetic foot cannot be reduced to choosing between soft and stiff.
Walking requires compliance, stability, support and progression at different moments within the same step.
The biological ankle achieves this through continuous adaptation.
Lunaris approaches the same challenge through architecture.
By assigning support, movement, flexibility and progressive stiffness adaptation to dedicated mechanical elements, it moves away from a design driven by a fixed compromise and towards a more biomimetic way of thinking.
Ultimately, walking is not defined by a single stiffness value.
It is defined by the body’s remarkable ability to continuously adapt, step after step.
Prosthetic foot stiffness describes how strongly a prosthetic foot resists bending or deformation when forces are applied during standing and walking. It influences how the foot absorbs impact, supports body weight, adapts to the ground and contributes to forward progression.
Prosthetic foot stiffness influences stability, comfort, ground adaptation and rollover during walking. A foot that is too compliant may provide insufficient support, while one that is too stiff may restrict movement and adaptability. The appropriate mechanical response also changes throughout the gait cycle.
The mechanical needs of a prosthetic foot evolve throughout each step. Greater compliance can help during initial ground contact, while support and stability become increasingly important as the body moves over the foot. During later stance and push-off, a firmer response can help support forward progression.
Some prosthetic feet maintain a relatively fixed mechanical response, while others use their geometry or architecture to create progressive stiffness. In Lunaris, a bumper progressively engages with the spring, reducing its effective bending length and increasing stiffness as loading increases.
Progressive stiffness means that a prosthetic foot becomes increasingly resistant to bending as it is loaded. This allows the foot to remain relatively compliant early in the step while providing greater support as mechanical demands increase.
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