Background: Children with cerebral palsy (CP) present altered gait patterns and electromyography (EMG) activity compared to typically developing children. To temporarily reduce muscular activity and to correct the abnormal muscle force balance, Botulinum Toxin type A (BTX-A) injections are used. Research question: What is the effect of BTX-A injections on dynamic muscle forces during gait, when calculated using an EMG-constrained approach?. Methods: Retrospective data of ten typically developing (TD) and fourteen children with spastic diplegic CP were used for musculoskeletal modeling and dynamic simulations of gait, before and after BTX-A treatment. Individual muscle forces were calculated using an EMG-constrained optimization, in which EMG of eight muscles was used as muscle excitation signal to constrain the muscle activation patterns. Paired t-tests were used to compare average modelled muscle forces in different phases of the gait cycle pre- and post-BTX-A, summarized in the muscle profile score. Two-sample t-tests were used to determine significant differences between TD and pre- and post-BTX-A modelled muscle forces. Results: For most muscles, the force was decreased in CP compared to TD children in all phases of the gait cycle, both before and after BTX-A treatment. Differences in muscle forces before and after BTX-A treatment were limited, with only few significant differences between pre- and post-BTX-A. Compared to a standard static optimization approach, imposing the EMG activity increased modelled muscle forces for most muscles. Significance: Our findings indicate that BTX-A treatment has a limited effect on the muscle balance in CP children. Besides that, the use of EMG-constrained optimization is recommended when studying muscle balance in children with CP.@en