Microscopic features and transient contraction of palpable bands in canine muscle.

Ten pairs of biopsies were excised from gracilis and semitendinosus muscles of 4 mongrel dogs. These were compared with the histological findings reported in painful spots and muscle hardenings of human muscles. Test biopsies sampled a palpable band. Paired control biopsies sampled a portion of the same muscle where it showed no palpable hardening. Processed sections were randomized and read blind. Sections stained with aqueous toluidine blue showed no convincing metachromasia. Sections stained with trichrome showed no proliferation of endomysial connective tissue. Test sections stained with hematoxylin and eosin showed no proliferation of nuclei and no increase in the number of central nuclei in muscle fibers compared to control sections. No convincing histological difference was found. During surgery under Nembutal anesthesia, rubbing palpation of the exposed muscle elicited a transient contraction of a bundle of muscle fibers several milli-meters in diameter. This corresponded in time and position to the palpable band in the dog muscle. The band-like hardness palpated in these canine muscles appears to be caused by a circumscribed transient muscular contraction rather than histologically demonstrable structural changes.