Many factors affect the ability to move. The ones we are mainly interested in investigating include muscle weakness, spasticity, and dynamic balance. To quantify strength or spasticity, a force transducer is needed such as a Biodex isokinetic dynamometer which is a standard rehabilitation assessment and training device, or a more simple handheld dynamometer can be used that the person presses against as force is recorded. For the spasticity tests, a person or a machine such as the Biodex moves a joint through an arc of motion (e.g. elbow, knee, or ankle) at a range of speeds from 5-240 degrees per second while resistance to motion is being recorded or graded subjectively. EMG and joint position may be collected simultaneously during the passive movements. The dynamic balance tests will be performed on a force plate or using a clinical balance testing device, the Equitest, by Neurcom. To determine if electrical stimulation (ES) improves limb movement, walking, and balance, a device such as the WalkAide may be used in patients who have some dysfunction in their lower extremities. Only persons with central nervous system injuries who have a foot drop gait pattern will be given the opportunity to use this device for 3-6 months. The WalkAide is worn below the knee and delivers individually programmed ES to aid in muscle function of the ankle.