The purpose of this research study is to test the utility of an ankle robot, when used during treadmill walking, in people with ankle weakness and foot drop from a peripheral nervous system injury due to neuromuscular or orthopedic injury.

Many individuals with central nervous system (CNS) injuries (e.g., a stroke) or peripheral nervous system (PNS) injuries (e.g., peroneal nerve injury, neuropathy, radiculopathy, and/or musculoskeletal injury) that affect their ankle movement have residual impairments that affect their walking and balance. These impairments include the disability "foot drop," which increases the risk for falling.

This study will focus on PNS injuries that cause foot drop.

Current therapy to address foot drop is limited primarily to the use of ankle foot orthoses (braces) that help keep the foot from hitting the ground to prevent falling. Also, some individuals with foot drop use functional electrical stimulation to the leg nerve to lift the foot. Regardless, none of these, or other existing, methods to address foot drop cures or even improves significantly the underlying neurological deficit behind this disability. Braces improve walking safety only while they are worn, and functional electrical stimulation does not work when it is turned off, or when the nerve has been severely damaged. Thus, the increased fall risk due to foot drop is generally considered life-long and incurable.

The investigators have developed a shoe-interfaced ankle robot with an adaptive control system, to assist an individual with ankle movement only as needed. Data from the investigators' previous studies on foot drop due to stroke show great promise for this ankle robot as a new rehabilitation tool for invididuals with foot drop, when used during treadmill walking. The investigators would like to utilize our findings from these stroke studies in learning how they can be used for PNS-related foot drop.

Inclusion Criteria:

1. Men and women, aged 18 to 88 years
2. Chronic foot drop and ankle weakness in one leg from a peripheral nervous system injury due to a neuromuscular or orthopedic injury
3. Ability to walk 10 meters and arise from a chair with no human assistance (but usage of usual assistive device[s] is permitted)

Exclusion Criteria:

1. Cardiac history of (a) unstable angina; (b) recent (less than 3 months) myocardial infarction, congestive heart failure (NYHA category II); and/or (c) hemodynamically significant valvular dysfunction
2. Hypertension that is a contraindication for a bout of walking training (greater than 160/100 mmHg on two assessments)
3. Medical history of (a) recent hospitalization (less than 3 months) for severe medical disease; (b) symptomatic peripheral arterial occlusive disease; (c) orthopedic or chronic pain conditions that significantly alter gait function; (d) pulmonary or renal failure; and/or (e) active cancer
4. Current participation in orthopedic or rehabilitation medical programs
5. Active deep venous thrombosis
6. Distal paretic leg skin lesions, infections, or soft tissue inflammation