Spasticity secondary to cerebral palsy can cause wrist flexion contractions. However, the precise muscular changes that occur during spastic wrist flexion contracture are not known. To investigate muscle adaptation after spasticity, sarcomere length was measured in flexor carpi ulnaris (FCU) muscles from patients with severely spastic wrist flexion contractures (n=6) as well as patients with radial nerve injury (n=12). Sarcomere length was measured by laser diffraction from a small bundle isolated from the FCU using a custom-designed 5 mW laser device. Spastic FCU muscles had extremely long sarcomere lengths with the wrist fully flexed (3.48 ± 0.44 µm) compared to the FCU muscles of patients with radial nerve injury (2.41±0.31 µm). In three of the patients with spastic wrist flexion contractures, the slope of the FCU sarcomere length-joint angle relationship was measured and found to be, essentially, normal (0.017±0.005 µm/°, n=3) suggesting that serial sarcomere number (and therefore muscle fiber length) was unchanged in spite of the dramatic absolute sarcomere length change. These results indicate that spasticity results in a major alteration of normal muscle-joint anatomical relationships.