Normally, nerve fibers carry electrical impulses through the spinal cord, providing communication between the brain and the arms and legs. In people with spinal cord injury, some fibers may be destroyed at the site of injury, while others remain connected but do not work correctly to carry electrical impulses. As a result, subjects with an incomplete spinal cord injury may have spasticity which is muscle spasms or muscle stiffness that makes movement difficult. Fampridine-SR is an experimental drug that increases the ability of the nerve to conduct electrical impulses. This study will examine the effects of Fampridine-SR on moderate to severe lower-limb spasticity, as well as the effects on bodily functions such as bladder control, bowel function and sexual function. The study will also examine the possible risks of taking Fampridine-SR.