That's the discouraging finding of a study published online today in the journal Muscle and Nerve.
Spasticity refers to an involuntary increase in muscle stiffness, which is often caused by damage to the part of the brain that controls movement. It's estimated about 500,000 Americans have spasticity caused by cerebral palsy.
Spasticity can also be caused by degenerative diseases, stroke and head or spinal cord injuries.
It's often difficult for doctors to treat a person with spasticity, due partly to a lack of information about the mechanical, physiological and biochemical features of spastic muscle.
While the increased stiffness in spastic muscles has previously been linked to solely to brain or spinal cord damage, this new study found spastic muscle cells are shorter and stiffer than normal muscle cells.
The study says that spastic muscle cells develop passive tension at much shorter lengths and their elastic modulus (a measure of material stiffness) is greater than that found in normal muscle cells.
The study researchers collected muscle fibers from healthy people and from children with cerebral palsy to compare 38 normal muscle fibers and 15 spastic muscle fibers. They used mechanical tests to measure the fibers' elastic properties and also measured fiber stress.
Here's where you can learn more about spasticity.