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Hyper-resistance at the joint is one of the most common symptoms in children with cerebral palsy (CP). There are both neural and non-neural factors contributing to the observed joint hyper-resistance. Non-neural alterations to the musculoskeletal system such as a reduced muscle length, increased tendon length and an increase in joint stiffness are commonly treated by physiotherapy interventions such as stretching exercises. However, the effectiveness of these interventions in terms of improvements in function is of a low magnitude and the response of spastic muscles to stretch is poorly understood. Therefore, the main aim of the programme of work presented in this thesis was to increase the effectiveness of stretching interventions at the ankle joint. To achieve this, it is important to understand the behaviour of the medial gastrocnemius muscle and Achilles tendon in response to a stretch applied at the joint. For this first sub-goal, three experimental studies were performed. First, the relationship between joint rotation and muscle and tendon contribution during a single stretch is described. We show that a smaller Achilles tendon moment arm leads to a decrease in muscle lengthening during joint rotation. Additionally, we show that the relative stiffness of the muscle to the tendon is increased in children with CP, which further limits lengthening of the muscle during joint rotation. Secondly, we document the acute effect of stretching on medial gastrocnemius fascicle lengthening properties. It was found that medial gastrocnemius muscle stiffness cannot explain the increased ankle joint range of motion acutely following passive stretching. These studies all indicate that the lengthening stimulus to the muscle is reduced in children with CP. Therefore, to increase the effectiveness of stretching exercises, an intervention was designed with the aim to increase this stretching stimulus seen by the muscle. By performing progressive resistance training, we showed an increase in the stiffness of the tendon, and thus a reduction in the relative stiffness of the muscle to the tendon. Therefore, the amount of stretch seen by the muscle was increased. We have shown that due to this combined intervention a remodelling of muscle is possible, as we showed muscle fascicle length to increase. Even though functional benefits were limited, the proof of principle was demonstrated and future work should explore this model of intervention further.