A lower-limb prosthesis uses a fitted interface and structural components to transfer loads between the wearer and the ground. Some feet include flexible elements that store energy during loading and return part of it during unloading. This animation illustrates a bending keel, and the calculator compares a simple spring-work estimate with an entered returned-energy value.
Artificial Leg and Foot Interactive Calculator
Compare force, deflection and returned energy in a simple spring model, with peak force expressed relative to entered body weight.
Equation Used
The worked example estimates the keel input energy as triangular spring work, Ein = 0.5 Fpeak delta, then divides the measured toe-off energy by that stored energy to get etareturn. The ratio is not a clinical acceptance threshold or a specification for a particular prosthetic foot.
- Carbon-fibre keel behaves as a linear elastic spring during loading.
- Force and displacement must describe the same loading path for the spring-work estimate to apply.
- Energy values use SI units with delta converted from mm to m.
- Efficiency check is for passive foot energy return only, not full gait metabolic cost.
Load transfer and a flexible keel
The socket interfaces with the residual limb, while the structural assembly connects to the prosthetic foot. Depending on the limb level and design, the assembly can also include a prosthetic knee. The simplified view highlights a socket, pylon and flexible foot element.
During the illustrated loading cycle, the keel bends and the pylon moves downward. During unloading, the keel returns toward its initial shape. The heel and toe remain on the test surface, so this is not a complete walking sequence or a simulation of knee control, socket pressure or balance.
Components and roles
- Socket and interface: connect the wearer to the prosthesis.
- Suspension: helps maintain that connection through movement.
- Pylon and connectors: transmit structural loads.
- Knee, where present: provides the required joint function for that prosthesis.
- Foot and keel: provide support and deformation according to the design.
A passive elastic element returns energy that was supplied to it; it does not create net energy over a complete loading cycle. Real force-displacement behavior can be nonlinear and can differ between loading and unloading. A single force and deflection pair cannot capture that whole curve.
Understanding energy storage without prescribing a device
The Amputee Coalition describes several prosthetic-foot designs and notes that selection depends on factors including amputation level, weight, activity, goals and individual needs. Energy storage is one feature among many, not a standalone measure of whether a device suits its wearer.
This calculator can demonstrate mechanical work and a returned-energy ratio. It cannot select a foot category, align a socket, determine whether a device is suitable for running, or predict comfort, stability or metabolic effort.
Fitting and component adjustments belong with the wearer’s prosthetist and clinical team. Pain, skin problems, instability or an unfamiliar noise need assessment of the person and device; they cannot be diagnosed from the energy ratio or treated with a generic alignment or fastener setting.
A linear-spring work estimate
Convert entered deflection from millimeters to meters: δ = δ_mm/1000. For force increasing linearly from zero to F along the matching displacement, Stored Energy E_in = ½Fδ joules. More generally, input work is the area under the loading force-displacement curve, ∫F dδ.
Return = 100E_out/E_in percent for positive E_in. Energy Lost = E_in − E_out joules. BW Load = F/(m × 9.80665), where m is entered body mass in kilograms. This last result is peak force expressed as a multiple of body weight.
The same loading/unloading boundary and work definition must be used for E_in and E_out. Combining a whole-body force peak with an unrelated local keel displacement does not automatically measure energy stored in the foot. A force plate alone does not supply every displacement and moment needed for a complete prosthetic-work measurement.
If entered E_out exceeds the estimate, the tool shows Return above 100%, a negative Energy Lost value and an inconsistency notice. That calls for checking the inputs, measurement boundary and model assumptions; it is not proof that a passive foot generates energy. When E_in is zero, the displayed Return is zero by software convention, but the physical ratio is undefined.
Worked example: an illustrative spring comparison
Enter m = 75 kg, F = 850 N, δ = 18 mm and E_out = 5.1 J. These are hypothetical model inputs, not measurements or specifications for a named product or wearer.
E_in = ½ × 850 × 0.018 = 7.65 J. Return = 100 × 5.1/7.65 = 66.6667%, displayed as 66.7%. Energy Lost = 7.65 − 5.1 = 2.55 J. BW Load = 850/(75 × 9.80665) ≈ 1.156.
If only E_out changes to 10 J, the input-energy estimate remains 7.65 J. Return becomes 130.71895% and Energy Lost becomes −2.35 J. These entries are inconsistent with a passive cycle having no other energy input under the stated work model.
If only body mass changes, the BW Load tile changes while the energy tiles stay fixed. Force, deflection and returned energy are separate inputs rather than predictions based on body mass. Neither example certifies foot performance or suggests a fitting adjustment.
Mechanical quantities and clinical questions
| Quantity shown | Meaning | Does not establish |
|---|---|---|
| Stored Energy | Triangular loading-work estimate. | Complete measured foot or limb work. |
| BW Load | Entered force divided by body weight. | Component rating or allowable activity. |
| Return | Entered returned energy relative to the estimate. | Comfort, walking economy or clinical suitability. |
| Energy Lost | Difference between the two entered/model energies. | A diagnosis of damage or poor fit. |
Comparing specific components requires their documented capabilities and an individualized assessment. This tool does not rank brands, assign activity classifications or set replacement intervals.
Spring-energy questions
No. The ratio describes one mechanical comparison. Fit, function, goals and the full clinical assessment also matter.
The model enters force, deflection and returned energy independently. Body mass is used only to normalize force by body weight.
That is the area of a triangular loading curve for a linear spring starting from zero force. A nonlinear or preloaded curve requires its actual work integral.
No. It means the entered return exceeds this input-work estimate. Inconsistent measurements or an inadequate model can also produce that result.
No. It has no clinical, socket-contact or gait-diagnostic model. Those concerns require assessment by the clinical team and prosthetist.
No. This educational calculation provides no fitting prescription, alignment value, torque setting or repair procedure.
No. It shows a simplified load-and-unload cycle with the heel and toe on a test surface.
No. Mechanical returned energy in this model is different from the person’s metabolic energy expenditure during walking.
Patient and technical resources
Amputee Coalition: Prosthetic Feet, an overview of designs and selection considerations. The calculator’s linear-spring comparison is an educational mechanics model, not a substitute for individualized prosthetic care.
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