Muscle strength changes following multi-level surgery in cerebral palsy and the impact of rehabilitation on functional recovery are largely unknown. The aim of this study was to quantify lower limb muscle strength changes in children with spastic diplegia after multi-level orthopaedic surgery and to compare the efficacy of progressive resistance strengthening (RS) versus active exercise (AE). Twenty children with spastic diplegia (mean age 12.5 years) participated in this prospective randomised controlled trial. Participants underwent multi-level orthopaedic surgery. Routine physiotherapy commenced immediately after surgery. At 6 months post-operatively, children were randomly assigned to the resistance strengthening RS (n=11) or active exercise AE (n=9) group and received intensive physiotherapy for 6 weeks. Gait, motor function and maximum isometric muscle strength in five lower limb muscle groups were measured pre-operatively and at 6 months (before and after intensive physiotherapy) and 12 months. As part of the study, we developed and validated a myometry protocol. Despite kinematic improvements, there was significant reduction of muscle strength (p<0.05) in all muscle groups at 6 months post-operatively. Following 6 weeks of intensive physiotherapy, both groups showed significant improvement in muscle strength, GMFM scores and gait parameters. Resistance training showed some advantages over active exercise. At 1 year after surgery, strength did not reach pre-operative values in some muscle groups but the benefit from physiotherapy was maintained. In conclusion, we quantified objectively the reduction in muscle strength 6 and 12 months following multi-level surgery. Furthermore, we demonstrated significant improvement in muscle strength, gait and function following post-operative strength training.