Drop foot, a loss of use of the muscles that lift the foot, can be caused by stroke, cerebral palsy (CP), multiple sclerosis (MS), or neurological trauma. The two major complications of drop foot are slapping of the foot after heel strike (foot slap) and dragging of the toe during swing (toe drag). The current assistive device is the Ankle Foot Orthosis (AFO), which though offering some biomechanical benefits, is non-adaptive and fails to eliminate significant gait complications. An Active Ankle Foot Orthosis (AAFO) is presented where the impedance of the orthotic joint is modulated throughout the walking cycle to treat drop foot gait. To prevent foot slap, a biomimetic torsional spring control is applied where orthotic joint stiffness is actively adjusted to mi-nimize forefoot collisions with the ground. Throughout late stance, joint impedance is minimized so as not to impede powered plan-tar flexion movements, and during the swing phase, a torsional spring-damper (PD) control lifts the foot to provide toe clearance. To assess the clinical effects of variable-impedance control, kinetic and kinematic gait data were collected on two drop foot parti-cipants wearing the AAFO. For each participant, zero, constant and variable impedance control strategies were evaluated, and the re-sults were compared to the mechanics of three age, weight and height matched normals. It was found that actively adjusting joint im-pedance significantly reduces the occurrence of slap foot, allows greater powered plantar flexion, and provides for greater biolo-gical realism in swing phase ankle dynamics. These results indicate that a variable-impedance orthosis may have certain clinical benefits for the treatment of drop foot gait compared to conventional AFO having zero or constant stiffness joint behaviors.

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