Introduction: Ankle joint hyper-resistance in children with spastic cerebral palsy (SCP) is commonly treated with Botulinum Toxin-A (BoNT-A) injections in the medial gastrocnemius (MG) combined with lower-leg casting. The overall aim of this combined treatment is to reduce spasticity and increase range of motion (1). Since hyper-resistance assessment mainly focuses on the joint level, whereas the treatment is directed at the muscle, it is worthwhile investigating the individual effects of BoNT-A and casting on MG and tendon lengths, to provide insight into the working mechanisms and to help improving treatment efficacy. Research question: What are the effects of BoNT-A injections and lower-leg casting on the MG and tendon lengths, at resting position and maximum dorsiflexion, in children with SCP? Methods: Children with SCP were assigned by minimization to receive either two weeks of lower-leg casts (n = 12, mean age 8.27 years; GMFCS-level I-III) or MG BoNT-A injections (n = 11, mean age: 6.75 years; GMFCS-level I-III). Data was acquired by 3D-freehand-ultrasound (2) at baseline and two weeks post-intervention with the knee in flexion (30.9° ± 3.7°) and the ankle in resting position and maximum dorsiflexion (maxDF). The same assessor extracted muscle and tendon lengths from the 3D reconstructions twice, and the standard error of measurement (SEM) was quantified. Muscle tendon unit (MTU) length was calculated as the summation of muscle (ML) and tendon length (TL). The change in ML and TL between rest and maxDF was used to calculate extensibility. Within-group treatment effect was evaluated with Wilcoxon signed rank tests and treatment differences, with Mann-Whitney U tests. Post-treatment changes were considered significant when >SEM and p < 0.05. Results: At baseline, groups did not differ for age, joint angles and lengths. Post-casting, resting angle, maxDF, MTU and TL at maxDF significantly increased. While two weeks post-BoNT-A-injection only MTU length and ML at rest significantly increased. There was no treatment effect on the extensibility. The change in maxDF, and ML at rest were significantly larger post-casting compared to post-BoNT-A. Similarly, the post-treatment change in MTU length at maxDF was significantly larger after casting compared to BoNT-A. Discussion: The results suggest that two weeks casting resulted in increased maxDF and MTU-length by increased TL (or compliance). This confirms previous research on the effects of ankle foot orthoses on MG morphology (3). BoNT-A on the other hand, affected the muscle's resting length, however without gain in extensibility or MTU-length. This emphasizes the requirement to combine both treatments, but also cautions the use of stretching casts for having adverse effects on the tendon. The treatment-effects on the MTU on the long-term and their carry over effect to gait is material for further investigation.