Energy Storage Characteristics of Leaf and Bioinspired Saddle Shaped Springs for Ankle–Foot Prostheses

Walking is a rhythmic exchange of energy. With every step, the human body stores mechanical energy as the foot lands and releases it to push the next step forward. For people who have lost a foot or part of a leg, artificial limbs must replicate this delicate process. The most common solution today is a prosthetic foot made from a single, curved piece of composite material, often resembling a leaf. This "leaf spring" bends under the weight of the body, storing energy like a coiled spring, and then snaps back to help the wearer move. While these devices are lightweight and effective, they have a fundamental limitation: because they bend in only one direction, the stress concentrates in a narrow band. This localized strain can lead to material fatigue and limits how much energy the device can safely store and return.

Researchers at the University of Central Florida and Mississippi State University have looked to nature for a better design. They turned their attention to the mantis shrimp, a marine predator known for its incredibly powerful strikes. The shrimp's club-like appendage is reinforced by a saddle-shaped structure that is curved in two directions at once. This "doubly curved" geometry allows the shell to distribute forces across its entire surface rather than focusing them in one spot. Inspired by this biological efficiency, the team set out to see if a prosthetic foot built with a similar double-curved shape could outperform the traditional single-curved leaf spring. Their goal was not just to make a foot that bends, but one that stores more energy, lasts longer, and moves more like a real human ankle.

To test this idea, the team created two virtual models of prosthetic feet: one using the standard single-curved leaf design and another using a new, bioinspired saddle shape. They carefully adjusted the materials and dimensions of both models so that they would feel equally stiff to a user, ensuring a fair comparison. Using powerful computer simulations, they applied the same forces that a person exerts while walking, specifically the pressure that occurs when the foot rolls forward and the ankle bends upward. They then measured how much energy each design could store and how the internal stresses were distributed throughout the material.

The results revealed a clear advantage for the double-curved design. When both springs were bent to the same angle, the saddle-shaped spring stored significantly more energy than the traditional leaf spring. Specifically, within the range of motion used during normal walking, the saddle design held up to 16 percent more strain energy. Even more impressive was the efficiency of the material itself. The researchers found that the saddle spring packed roughly three times more energy into every unit of its volume compared to the leaf spring. This means that for the same amount of material, the new design could do much more work, or conversely, it could achieve the same performance with less material, making the prosthetic lighter.

Beyond raw energy storage, the study highlighted a critical difference in how the two shapes handle stress. In the traditional leaf spring, the force of bending creates a sharp concentration of stress in the middle of the curve, a weak point where cracks are likely to start over time. In contrast, the saddle spring distributed the stress much more evenly across its entire surface. This uniform distribution suggests that the new design would be far more resistant to the repetitive wear and tear of daily walking, potentially lasting longer and offering greater reliability for the user. The double curvature also allowed the structure to stiffen naturally as it bent, a behavior that closely mimics the way a biological ankle becomes stiffer as it pushes off the ground, providing a more natural walking experience.

The researchers validated their computer models against real-world tests of the traditional leaf spring, confirming that their simulations were accurate. While the saddle spring design has not yet been built and tested on a human subject, the simulations provide strong evidence that this bioinspired geometry offers a superior mechanical solution. By leveraging the unique properties of a doubly curved surface, this new approach could lead to the next generation of prosthetic feet that are not only lighter and more energy-efficient but also more durable and closer to the natural function of the human body.