This trial will test the effectiveness of BTX injections versus placebo to reduce plantar-flexor spasticity and improve function and mobility in children with spastic diplegia, a common form of cerebral palsy. The study will evaluate the effects of the treatment across five domains that relate to disability: pathophysiology, impairment, functional limitation, disability, and societal limitation.

Forty children, ages 4 to 12 years, will be recruited and randomized into one of two groups. Group A will receive BTX. Group B will receive a placebo injection. Participants will be assessed at the beginning of the trial and at Weeks 3, 8, 12, and 24. Assessments include quantitative electromyographic kinesiology measurements, electromechanical measurement of joint torque across the ankle joint using the Spasticity Measurement System, Gross Motor Function Measure, physical exam parameters, energy expenditure using the Energy Cost Index, kinematic gait analysis, and the Canadian Occupational Performance Measure.

Participants are followed for a total of 6 months. Participants initially randomized to Group B will have the opportunity for BTX treatment after 24 weeks.