Individuals with an upper motor neuron syndrome, e.g., stroke survivors, may have a pathological increase of passive ankle stiffness due to spasticity, that impairs ankle function and activities such as walking. To improve mobility, walking aids such as ankle-foot orthoses and orthopaedic shoes are prescribed. However, these walking aids generally limit the range of motion (ROM) of the foot and may therewith negatively influence activities that require a larger ROM. Here we present a new ankle-foot orthosis 'Hermes', and its first experimental results from four hemiparetic chronic stroke patients. Hermes was designed to facilitate active ankle dorsiflexion by mechanically compensating the passive ankle stiffness using a negative-stiffness mechanism. Four levels of the Hermes' stiffness compensation (0%, 35%, 70% and 100%) were applied to evaluate active ROM in a robotic ankle manipulator and to test walking feasibility on an instrumented treadmill, in a single session. The robotic tests showed that Hermes successfully compensated the ankle joint stiffness in all four patients and improved the active dorsiflexion ROM in three patients. Three patients were able to walk with Hermes at one or more Hermes' stiffness compensation levels and without reducing their preferred walking speeds compared to those with their own walking aids. Despite a small sample size, the results show that Hermes holds great promise to support voluntary ankle function and to benefit walking and daily activities.

The stiffness of an Ankle-Foot-Orthosis (AFO) that aims to assist walking affects the gait biomechanics of patients with impaired gait. In patients with equinus (spastic paresis of the lower leg), impaired gait is a consequence of an increased passive ankle joint stiffness (originated from calf muscles) in combination with reduced active muscle strength. Though standard AFOs affect clinically relevant improvements of gait parameters, their designs interfere with the range of motion of the ankle joint. We hypothesize that, by lowering the total passive ankle joint stiffness with the AFO, patient's active range of motion will increase while supporting the patients' muscle forces during gait. We propose a novel AFO design with negative stiffness (nAFO) produced by a spring-loaded CAM follower mechanism. The aim of the device is to compensate for the passive stiffness caused by the calf muscles. This study describes the design, evaluation and walk-ability of the prototype nAFO. Results of the evaluation showed the required compensatory negative stiffness -57.4Nm. Rad^-1 (in patients up to 76Nm. Rad-¹) to balance plantar-flexion torque along the range of motion for walking (0.44rad [25°] plantar-flexion to 0.33rad [19°] dorsi-flexion). Assessment on a healthy subject showed passive compensation up to 43.87%. During gait, Tibialis Anterior muscle forces were supported by the nAFO, as observed by a reduced electromyographic signal during swing phase. Though hysteresis of the device has to be reduced, the possibility to compensate for high passive joint stiffness shows promise to increase the active range of motion of the ankle of patients with equinus.