For years, we have used a minimally invasive and biological variant of Achilles tendon lengthening using a needle inserted into the Achilles tendon so that it is weakened and can be gradually stretched to the desired length. So far, no negative effects, complications or tendon problems have been observed. A prospective study of a cohort of children and adolescents is planned where there is an indication for Achilles lengthening, such as spastic or non-spastic contracture and toe walking that results in an unwanted shortening of the Achilles tendon that makes walking difficult. We plan to include up to 50 children and adolescents over a two-year period and follow them closely for the first year after surgery. Pediatric physiotherapists will perform all clinical examinations to limit observer bias, and a radiologist will examine the tendon tissue with ultrasound before and one year after surgery to check anatomical conditions. The study will examine safety, effectiveness and feasibility of using the method, and anatomy, patient satisfaction and quality of life.