Design of active ankle foot orthotics for gait assistance and fall prevention

Abstract

Certain groups of people are more susceptible to experience fall than others, one of which being stroke survivors. Although the risk of stroke increases with age, a stroke can occur at any age, and the most common type of stroke inhibits blood flow to the brain.2 The brain is responsible for sending motor signals to muscles through the nervous system required for muscle movement. After a stroke, these signals get affected and can delay muscle, both kinetic and kinematic responses to perturbation.1 Partial or complete loss of muscle activity (paresis) in the lower limb or limbs can result in some cases. This loss of muscle activity leads to motor-issues such as Drop foot (i.e. inability to lift the impaired foot during swing phase of gait) or Spasticity (i.e. stiffness and tightening of muscles) of lower limb muscles. These issues lead to the loss of postural control3 increasing fall risk among those affected. Stroke is not uncommon either, with around 795,000 people in the United States suffering from stroke annually according to the Centers for Disease Control and Prevention (CDC). Falls are the common complaints that individuals with such impairment have during or after their rehabilitation phase. Statistics on after stroke falls4 reveal that 14%-65% patients fall during hospitalization and between 37%-73% fall during the first 6 months of discharge from hospital. The most commonly prescribed clinical remedy provided to tackle drop foot issue and improper gait is passive thermoplastic Ankle Foot Orthosis (AFO) that is designed to lock the paretic ankle joint at a certain angle, facilitate foot clearance during swing phase, ankle stability during stance phase and heel strike. While there are reported improvements of gait velocity, stride length and cadence(steps/min) shown after the use of such AFOs, studies5 show that continual constraints in the ankle joint adversely affects the compensatory stepping response, forward propulsion and proprioceptive sensory information. The primary contributor to the kinetic energy and the speed of the stepping leg is forward propulsion force which is generated by the plantar flexor muscles. Locking the ankle joint using rigid AFOs lead to impeded forward propulsion due to restricted plantar flexion and that can cause inhibited compensatory stepping response, inadequate foot clearance and improper gait. Therefore, there is a need for an Active Ankle Foot Orthotics (AAFO) which can deliver powered push off for a stronger plantar flexion, locking of the ankle joint during swing phase to prevent dragging of the paretic foot and a stable heel strike.