Inferior Mechanical Properties of Spastic Muscle Bundles Due to Hypertrophic but Compromised Extracellular Matrix Material

ABSTRACT

The passive mechanical properties of small muscle fiber bundles obtained from surgical patients with spasticity (n=9) and patients without neuromuscular disorders (n=21) were measured to determine the relative influence of intra- and extracellular components.

For both patient types, tangent modulus was significantly greater in bundles compared to identical tests performed on isolated single cells (P<.05). However, the relative difference between bundles and single cells was much greater in normal tissue than spastic tissue. Tangent modulus of normal bundles (462.5±99.6 MPa) was 16 times greater than normal single cells (28.2±3.3 MPa), whereas tangent modulus of spastic bundles (111.2±35.5 MPa) was only twice that of spastic muscle cells (55±6.6 MPa).

This relatively small influence of the extracellular matrix in spastic muscle was surprising because spastic muscle cells occupied a significantly smaller fraction of the total specimen area (38.5±13.6%) compared to normal muscle (9568.8%). Based on these data, normal muscle extracellular matrix is calculated to have a modulus of 8.7 GPa and the extracellular matrix from spastic muscle of only 0.20 GPa.

These data indicate that spastic muscle, although composed of cells that are stiffer compared to normal muscle, contains an extracellular matrix of inferior mechanical strength. These data illustrate some of the profound changes that occur in skeletal muscle secondary to spasticity.