Problem

Ankle control deficiencies are highly prevalent neuromuscular disorders, however, the standard of care is primitive in comparison to advancements in science and technology. As an alternative approach, powered orthoses are highly anticipated, mostly due to their intelligence in interacting with complex ankle-dynamics while generating assistive motion.

Ankle Control. Efficient and effective walking is an extremely important task because mobility is associated with functional independence and participation in society. Insufficient ankle control is often one of the consequences of several neuromuscular disorders including cerebral palsy (CP), stroke, incomplete spinal cord injury, and multiple sclerosis, which collectively affect 8 to 10 million people in US alone. The ankle has three major roles during walking: 1) stability; 2) forward progression, which is mostly done during the push-off phase of gait using ankle plantarflexor muscles; and 3) clearing the foot off the ground during swinging of the leg forward, using ankle dorsiflexor muscles. Increased chance of falls and inefficient and slow walking speeds are major consequences of ankle control deficiencies.

Powered vs Passive Solutions. The standard of care for CP is a rudimentary solution, a bulky rigid (or semi-rigid) uncomfortable L-shaped structure, called passive ankle-foot orthoses (AFO). These orthoses constrain the ankle-motion’s degrees of freedom, consequently, often cause disuse atrophy of muscles and make the user physically dependent on the device^1,2. Furthermore, long-term use of these type of AFOs may induce adverse neural adaptations with gradual reduction of muscle activity over time; and may contribute to the development of contractures³. There are several studies assessing the efficacy and side-effects of AFOs, for example, Ring et al.⁴ noted that AFOs might block normal ankle kinematics and prevent active ankle stability during gait. The inhibition of sensory feedback that is needed for motor control was mentioned as another side-effect of the conventional AFOs⁵ It has also been shown that the efficacy of passive AFOs, in the long run, is fairly limited⁶.

Thus, non-mechanized/passive AFOs can increase the time needed in physical therapy to compensate for these unwanted long-term effects.

Powered orthoses are an alternative approach. they provide higher independence, and they also can be used for re-educating the neuro-motor system, which will further decrease the dependence of the subject on the orthosis⁷. Therefore, during the last five decades several powered AFOs have been developed by researchers and many important results have already been achieved^8–11. The actuation mechanisms of the most recent designs can be divided into two main groups; AFOs powered by pneumatic actuators^12–14 and AFOs powered by series elastic actuators -that is an elastic component in series with a DC motor to improve the torque control and increase the mechanical compliance of the actuator^15,16. Most of these mechanized AFOs, however, require rigid mechanical structures which restrict the natural motion of the joint. Rigid structures often result in pressure sores on users’ limb. Additionally, their bulky, heavy, and noisy design diminishes user acceptability and comfort, which inhibit their commercialization. We have confirmed such notions through 176 stakeholder interviews. Acceptability and comfort issues are mostly caused by using the aforementioned actuators, which are usually designed for industrial purposes and not necessarily for interfacing with human limbs. Thus, actuators specifically designed to mimic biological muscles, i.e., artificial muscles, are receiving greater attention. In a novel approach, Park et al. designed a soft powered AFO, which closely mimicked the musculoskeletal structure of the ankle joint¹². Unlike most powered AFO’s, this bio-inspired orthosis can assist motion in both sagittal (plantarflexion/dorsiflexion) and frontal planes (eversion/inversion). Due to using pneumatic artificial muscles, however, the device required a heavy (1 kg) and noisy air compressor to provide pressurized air, needed for actuation.