Spasticity causes a fundamental rearrangement of muscle-joint interaction.

Sarcomere length was measured in flexor carpi ulnaris (FCU) muscles from patients with severely spastic wrist flexion contractures (n = 6), as well as from patients with radial nerve injury and a normally innervated FCU (n = 12). Spastic FCU muscles had extremely long sarcomere lengths with the wrist fully flexed (3.48 +/- 0.44 microm) compared to the FCU muscles of patients with radial nerve injury (2.41 +/- 0.31 microm). 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 microm/degree, 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 that has not previously been recognized to our knowledge. We hypothesize that the results are explained either by the inability of muscle fibers to add serial sarcomeres in response to growth, or the selective loss of FCU muscle length secondary to the central nervous system lesion.