Comparison of muscle spindle concentrations in large and small human epaxial muscles acting in parallel combinations.

A small short muscle acting across a joint in parallel with vastly larger and longer muscles is clearly unable to play more than a minimal mechanical role in such a "parallel muscle combination" (PMC). This research investigates a feed back role for the small muscles of PMCs, proposing a significantly higher muscle spindle concentration therein to be consistent with this role. Epaxial PMCs (semispinalis and multifidus versus rotatores brevis) from the C5-C6, T6-T7, and L4-L5 regions of three 36-week-old male fetuses and two adult cadavers were removed and fixed in Carnoy's fluid. Tissue samples were embedded in paraffin, cut into 10 microns thick sections perpendicular to the muscle's longitudinal axis and stained by Harris' hematoxylin and eosin. Representative tissue sections were projected onto a sterological grid and the percentage volume of spindles determined. Data were analyzed with Student's unpaired t test. In all PMCs, rotators brevis spindle percentage volumes ranged from 4.58 to 7.30 times higher than those of multifidus and semispinalis. Differences in mean spindle percentage volumes between large and small members of all PMCs were significant (P less than .0001). Our findings are consistent with the notion of a "kinesiological monitor" or feedback role for rotatores brevis.