Muscle biology of contractures in children with cerebral palsy

Muscular contractures are routinely observed in children with cerebral palsy. The natural progression of gait leads to a reduction in passive range of motion. Here we discuss the physiological properties of skeletal muscle tissue and the recent advances in the biological basis of contractures. Skeletal muscles are highly organized structures composed of muscle cells, i.e., myofibers, arranged in parallel and series. Myofibers in turn are made up of the basic contractile proteins, actin, and myosin that form sarcomeres. Sarcomere length and force production are intricately associated such that at very long and short sarcomere lengths, there is a reduction in force-generating capacity. During normal postnatal development, stretch-induced longitudinal skeletal muscle growth by addition of sarcomeres is mediated by bone growth. In children with cerebral palsy, sarcomere lengths are overstretched, and sarcomere number is lower, associated with a limitation in joint range of motion, suggesting reduced ability for muscle growth. Increase in muscle extracellular matrix content and increase in passive mechanical stiffness of fibers and fiber bundles are also observed. Satellite cells are resident stem cells indispensible for postnatal development, repair, and regeneration of skeletal muscles. The satellite cell population is dramatically reduced in contractured muscles. Overall these findings suggest that impaired muscle growth and contractures in children with cerebral palsy are related to a reduced muscle stem cell number.