Spastic muscle cells are shorter and stiffer than normal cells.

The mechanical properties of isolated single muscle fiber segments were measured in muscle cells obtained from patients undergoing surgery for correction of flexion contractures secondary to static perinatal encephalopathy (cerebral palsy). "Normal" muscle cells from patients with intact neuromuscular function were also mechanically tested. Fiber segments taken from subjects with spasticity developed passive tension at significantly shorter sarcomere lengths (1.84 +/- 0.05 microm, n = 15) than fibers taken from normal subjects (2.20 +/- 0.04 microm, n = 35). Elastic modulus of the stress-strain relationship in fibers from patients with spasticity (55.00 +/- 6.61 kPa) was almost double that measured in normal fibers (28.25 +/- 3.31 kPa). The fact that these muscle cells from patients with spasticity have a shorter resting sarcomere length and increased modulus compared with normal muscle cells suggests dramatic remodeling of intracellular or extracellular muscle structural components such as titin and collagen. Such changes in muscles of patients with spasticity may have implications for therapy.