Wearable robots are a promising approach to augment healthy or restore impaired locomotion in humans. Efficient human walking is made possible by precisely coordinated ankle propulsion. The ankle plantar flexors provide the majority of the mechanical work for the step-to-step transition and much of this work is delivered via elastic recoil from the Achilles’ tendon - serving as an energy savings mechanism. Even though the plantar flexors play a central role in propulsion, body-weight support and swing initiation during walking, very few assistive devices have focused on aiding ankle plantarflexion.

 

Our goal was to develop a portable ankle exoskeleton taking inspiration from the passive elastic mechanisms at play in the human triceps surae-Achilles’ tendon complex during walking. The challenge was to use parallel springs to provide ankle joint mechanical assistance during stance phase but allow free ankle rotation during swing phase. To do this we developed a novel exoskeleton, consisting of a lightweight composite frame and a ‘smart-clutch’ that can engage and disengage a parallel spring based only on ankle kinematic state. This primary system is purely passive - containing no motors, electronics or external power supply. We also developed a secondary clutch with the addition of a nanoservo to control the engagement timing of the ‘smart-clutch’ to provide passive assistance in more dynamic locomotion such as changing or impaired gaits.

 

Initial testing of the device shows the exoskeleton provides significant torque during stance (28% (+/-4.4 SE) of the normal ankle joint moment using an intermediate spring stiffness (150Nm/rad)) while not hindering swing dynamics (Figure 1.). After a period of ~21min walking in the exoskeleton both plantarflexor muscle activation and whole-body net metabolic power fall below values measured during normal walking without the exoskeleton (8.3% (+/-5.3 SE)  reduction in integrated EMG and 6.7% (+/- 4.3 SE) reduction in metabolic power) using a slightly more compliant spring (n=5, 110Nm/rad).

This ‘energy-neutral’ ankle exoskeleton shows clear benefits to healthy populations but could also provide a relative cheap, simple solution to restore symmetry and reduce metabolic energy expenditure of walking in populations with weak ankle plantar flexors (e.g. stroke, spinal cord injury, normal aging).

 

Future work will build upon these results by adding more study participants and measuring invivo muscle dynamics with ultrasound measurments.