Botulinum toxin injections are a common form of treatment for children with cerebral palsy to help reduce muscle tightness and help them walk without resorting to surgery. While the treatment is beneficial, it requires that the therapy be repeated every six months, which can be costly and may eventually result in the therapy not being effective over time. The study being conducted at Shriners Hospitals for Children, Philadelphia, will examine the effects of botulinum toxin injections, not only on walking, but on the changes it causes in the muscle, brain and spinal cord. The changes in the muscle, brain and spinal cord, called neuromuscular plasticity, have not been extensively examined in response to botulinum toxin, and could provide insight into how botulinum toxin works and how to make it more effective. This study will focus on children with cerebral palsy between the ages of 4 and 12 years, with only one leg involved and who would normally be receiving botulinum toxin injections of the lower leg to help improve walking. The children will be asked to come into the hospital for a full day before they receive the injection, and then again after four and twelve weeks to look at the changes in the muscle, brain and spinal cord. This will involve recording them while they walk, measurements of the muscle reflexes, and recording their brain activity while the move their ankle (also known as functional magnetic resonance imaging or fMRI). A group of children with typical development will also be asked to participate in the study to act as a comparison group. The researchers believe that examining the changes in the muscles, brain and spinal cord in response to the injection will aid in the understanding how better to use botulinum toxin to achieve better or longer lasting changes.