Four prosthetic ankles, eight terrain tasks: a study finds variable stiffness matched carbon-fiber returns on flat ground and was preferred going downhill

Seven people with below-knee amputation wore four different prosthetic ankles and completed eight terrain tasks. The results split across conditions.

The open-access study, published in Scientific Reports by a team that includes Hugh Herr, PhD, compared two quasi-passive ankle designs against two energy-storage-and-return feet across those conditions. The O&P EDGE reported on it.

Three categories, not two

Most discussions of prosthetic feet treat the options as two categories: passive devices and powered devices.

Passive energy-storage-and-return feet, usually called ESR, are the clinical standard for most people with transtibial amputation. They store mechanical energy as a carbon-fiber blade bends under load and release it at push-off. No sensors, no motors, no terrain adjustment.

Powered prosthetic ankles do adjust. They use motors and batteries to respond to the ground surface, and the functional benefit on slopes and stairs has been established in clinical research. The cost, weight, and difficulty of getting them covered by insurance are also well documented.

Quasi-passive devices sit between those two. They use mechanical elements, typically springs or damping systems, that respond to load or movement velocity without a motor or battery. The design goal is some terrain adaptability without the complexity and cost of a fully powered system. This study asked whether that goal is achievable, and at which tasks.

Study design

The four ankle types were two quasi-passive (one with variable stiffness, one with variable damping) and two ESR ankles. Seven participants with unilateral transtibial amputation each wore all four. The walking conditions were: three level-ground speeds, a figure-eight overground pattern, a 5-degree incline, a 5-degree decline, stairs ascending, and stairs descending. Five of the eight conditions were biomechanically instrumented with motion capture and force plates. Subjects rated each ankle after each task.

The design is a randomized, repeated-measures crossover, meaning each participant served as their own control across all four devices.

Results by condition

Variable damping was the least preferred approach in every condition, in both subject ratings and biomechanical measurements. That held across flat ground, slopes, and stairs.

On level terrain, the variable-stiffness ankle and both ESR ankles received similar subject ratings. The biomechanical data showed a gap at faster walking speeds: ESR ankles produced higher peak power and more center-of-mass push-off work at the trailing edge of the step. Variable stiffness did not close that gap at speed. At slow to moderate pace, the measured differences were small.

The clearest result for variable stiffness came on the 5-degree decline. Subjects preferred it over both ESR ankles on that task. The biomechanical metrics did not show a statistically significant difference between the two approaches, so the force-plate data did not fully explain what subjects were experiencing. A variable-stiffness mechanism can reduce resistance passively as load increases on a downhill surface, which the researchers noted may account for the perceived difference even when output measurements looked similar. They did not resolve the discrepancy, and neither does this piece.

What the study adds

The researchers concluded that a quasi-passive variable-stiffness prosthesis can achieve similar level-ground walking performance to ESR ankles at slow and moderate speeds, and that the design shows functional promise on sloped terrain.

Seven participants is enough to detect differences between conditions. It is not a large enough sample to generalize broadly, and the 5-degree terrain slope tested is moderate. The results do not tell any individual user which ankle to wear. That depends on their activity level, residual limb characteristics, socket fit, and what their insurer covers.

What the study does contribute is a peer-reviewed, open-access evidence base for a device category that has mostly been supported by theoretical arguments rather than randomized trials. When quasi-passive devices come up in a coverage conversation or a fitting discussion, a published randomized crossover trial in Scientific Reports is a more durable kind of support than a device brochure.

The full study title is “A comparison of passive and quasi-passive prosthetic ankles over varying speeds and terrains.” It is available open access in Scientific Reports.

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This article describes published research findings. It is not medical or device-selection advice. Talk with your prosthetist about ankle options appropriate for your situation.