Here we present the design of a novel unpowered ankle exoskeleton that is low profile, lightweight, quiet, low cost to manufacture, intrinsically adapts to different walking speeds, and does not restrict non-sagittal joint motion; while still providing assistive ankle torque that can reduce demands on the biological calf musculature. This work is an extension of the previouslysuccessful ankle exoskeleton concept by Collins, Wiggin, and Sawicki. We created a device that blends the torque assistance of the prior exoskeleton with the form-factor benefits of clothing. Our design integrates a low profile under-the-foot clutch and a soft conformal shank interface, coupled by an ankle assistance spring that operates in parallel with the user's calf muscles. We fabricated and characterized technical performance of a prototype through benchtop testing and then validated device functionality in two gait analysis case studies. To our knowledge, this is the first ankle plantarflexion assistance exoskeleton that could be feasibly worn under typical daily clothing, without restricting ankle motion, and without components protruding substantially from the shoe, leg, waist or back. Our new design highlights the potential for performance-enhancing exoskeletons that are inexpensive, unobtrusive, and can be used on a wide scale to benefit a broad range of individuals throughout society, such as the elderly, individuals with impaired plantarflexor muscle strength, or recreational users. In summary, this work demonstrates how an unpowered ankle exoskeleton could be redesigned to more seamlessly integrate into daily life, while still providing performance benefits for common locomotion tasks.

We aimed to determine a method for prescribing a standing prosthetic leg length (ProsL) that results in an equivalent running biological leg length (BioL) for athletes with unilateral and bilateral transtibial amputations (UTTA and BTTA, respectively). We measured standing leg length of ten non-amputee (NA) athletes, ten athletes with UTTA, and five athletes with BTTA. All athletes performed treadmill running trials from 3 m/s to their maximum speed. We calculated standing and running BioL and ProsL lengths and assessed the running-to-standing leg length ratio (Lratio) at three instances during ground contact: touchdown, mid-stance, and take-off. Athletes with UTTA had 2.4 cm longer standing ProsL than BioL length (p = 0.030), but up to 3.3 cm and 4.1 cm shorter ProsL than BioL length at touchdown and mid-stance, respectively, at speed 3-11.5 m/s. At touchdown, mid-stance, and take-off, athletes with BTTA had 0.01–0.05 lower Lratio at 3 m/s (p < 0.001) and 0.03–0.07 lower Lratio at 10 m/s (p < 0.001) in their ProsL compared to the BioL of NA athletes. During running, ProsL were consistently shorter than BioL. To achieve equivalent running leg lengths at touchdown and take-off, athletes with UTTA should set their running-specific prosthesis height so that their standing ProsL length is 2.8–4.5% longer than their BioL length, and athletes with BTTA should set their running-specific prosthesis height so that their standing ProsL lengths are at least 2.1–3.9% longer than their presumed BioL length. Setting ProsL length to match presumed biological dimensions during standing results in shorter legs during running.

Passive-elastic prosthetic feet are manufactured with different numerical stiffness categories that are prescribed based on the body mass and activity level of the user, but the mechanical properties, such as the stiffness values and hysteresis are not typically reported by the manufacturer. Since the mechanical properties of passive-elastic prosthetic feet can affect the walking biomechanics of people with transtibial or transfemoral amputation, characterizing these properties would provide objective values for comparison and aid the prescription of prosthetic feet. Therefore, we characterized the axial stiffness values, torsional stiffness values, and hysteresis of 33 different categories and sizes of a commercially available passive-elastic prosthetic foot model, the Össur low-profile (LP) Vari-flex with and without a shoe. We measured axial stiffness from compression and torsional stiffness from dorsiflexing and plantarflexing the prostheses. In general, a greater numerical prosthetic foot stiffness category resulted in increased heel, midfoot, and forefoot axial stiffness values, increased plantarflexion and dorsiflexion torsional stiffness values, and decreased heel, midfoot, and forefoot hysteresis. Moreover, a greater prosthetic foot size resulted in decreased heel, midfoot, and forefoot axial stiffness values, increased plantarflexion and dorsiflexion torsional stiffness values, and decreased heel and midfoot hysteresis. Finally, adding a shoe to the LP Vari-flex prosthetic foot resulted in decreased heel and midfoot axial stiffness values, decreased plantarflexion torsional stiffness values, and increased heel, midfoot, and forefoot hysteresis. In addition, we found that the force-displacement and torque-angle relationships were better described by curvilinear than linear equations. Ultimately, our results can be used to objectively compare LP Vari-flex prosthetic feet to other prosthetic feet in order to inform their prescription and design and use by people with a transtibial or transfemoral amputation.

Athletes with transtibial amputation (TTA) use running-specific prostheses (RSPs) to run. RSP configuration likely affects the biomechanics of such athletes across speeds. We determined how the use of three RSP models (Catapult, Sprinter and Xtend) with three stiffness categories (recommended, ±1), and three heights (recommended, ±2 cm) affected contact length ( L c ), stance average vertical ground reaction force ( F avg ), step frequency ( f step ) and asymmetry between legs for 10 athletes with unilateral TTA at 3–7 m s −1 . The use of the Xtend versus Catapult RSP decreased L c ( p = 2.69 × 10 −7 ) and F avg asymmetry ( p = 0.032); the effect on L c asymmetry diminished with faster speeds ( p = 0.0020). The use of the Sprinter versus Catapult RSP decreased F avg asymmetry ( p = 7.00 × 10 −5 ); this effect was independent of speed ( p = 0.90). The use of a stiffer RSP decreased L c asymmetry ( p ≤ 0.00033); this effect was independent of speed ( p ≥ 0.071). The use of a shorter RSP decreased L c ( p = 5.86 × 10 −6 ), F avg ( p = 8.58 × 10 −6 ) and f step asymmetry ( p = 0.0011); each effect was independent of speed ( p ≥ 0.15). To minimize asymmetry, athletes with unilateral TTA should use an Xtend or Sprinter RSP with 2 cm shorter than recommended height and stiffness based on intended speed.