The SoftFoot Pro at the Cybathlon: Kinematic, Metabolic, and User Performance Evaluation

Background: Prosthetic feet are essential for restoring independent mobility in individuals with lower-limb loss. However, most commercial prosthetic feet rely on elastic elements and rigid, flat soles, limiting adaptability to uneven terrain and compromising user stability. Originally developed for robotic applications, the SoftFoot introduced an adaptive sole architecture inspired by the biomechanics of the human plantar fascia to improve ground conformity and gait stability. Building on this concept, we introduced the SoftFoot Pro at Cybathlon 2024 — a prosthetic counterpart that integrates a compliant adaptive sole with energy storage capabilities at the ankle joint through an agonist-antagonist mechanism. This design emulates the synergistic action of muscles, tendons, and the plantar fascia in the human shank-ankle-foot complex. This study evaluates the kinematic and metabolic performance of the SoftFoot Pro. Methods: During a dedicated pre-competition training session, the official SoftFoot Pro team pilot completed the Cybathlon Leg Race track twice using three prosthetic feet: (i) the Triton foot (energy storage only), (ii) the original SoftFoot (adaptive sole only), and (iii) the SoftFoot Pro. Kinematic and metabolic data were collected using the Xsens MVN Awinda and Cosmed K5 system. The evaluation was complemented by questionnaires assessing locomotor performance, usability, cognitive load, and user experience. Results: The SoftFoot Pro demonstrated greater ankle mobility than the original Soft- Foot and the Triton across various tasks. Stride length and gait velocity were comparable to the Triton and higher than with the original SoftFoot. The Soft- Foot Pro revealed the fastest circuit completion time, with a metabolic cost of transport comparable to the Triton and lower than the original SoftFoot. Questionnaires reported higher perceived mobility and lower cognitive and physical effort with the SoftFoot Pro, compared to both the original SoftFoot and the Triton, highlighting its functional and user experience advantages. Conclusions: This single-subject study quantitatively evaluated adaptive prosthetic feet in Cybathlon tasks simulating daily activities. Integrating an ankle joint with an agonist-antagonist energy recycling system improved mobility and reduced mental and physical effort, matching the performance of commercial carbon fiber feet while preserving the adaptive sole’s advantages. The Cybathlon was the catalyst for advancing the innovation and validation of our adaptive prosthesis.