Muscle contractures that occur after upper motor neuron lesion are often surgically released or lengthened. However, surgical manipulation of muscle length changes a muscle's sarcomere length (L_s), which can affect force production. To predict effects of surgery, both macro- (fascicle length (L_f)) and micro- (L_s) level structural measurements are needed. Therefore, the purpose of this study was to quantify both L_s and L_f in patients with cerebral palsy (CP) as well as typically developing (TD) children. Soleus ultrasound images were obtained from children with CP and TD children. L_f was determined and, with the joint in the same position, CP biopsies were obtained and formalin fixed, and L_s was measured by laser diffraction. Since soleus L_s values were not measurable in TD children, TD L_s values were obtained using three independent methods. While average L_f did not differ between groups (CP = 3.6 ± 1.2 cm, TD = 3.5 ± 0.9 cm; p > 0.6), L_s was dramatically longer in children with CP (4.07 ± 0.45 µm vs. TD = 2.17 ± 0.24 µm; p < 0.0001). While L_f values were similar between children with CP and TD children, this was due to highly stretched sarcomeres within the soleus muscle. Surgical manipulation of muscle-tendon unit length will thus alter muscle sarcomere length and change force generating capacity of the muscle.

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