Horse soleus muscle: Postural sensor or vestigial structure?

The soleus muscle of horses is rather diminutive with respect to the overall size of adjacent synergist muscles in the hind limb of the horse. Whether or not such a muscle might be vestigial or may be providing some essential function has not been determined. We have studied the horse's soleus muscle using histochemical (ATPase), immunocytochemical (myosin isoform identification), and SDS-PAGE analysis to demonstrate that it is largely composed of 100% type I, presumed slow-twitch fibers. Only one soleus muscle studied (out of 13 adult horses) contained any type II muscle fibers. Given this consistent high percentage of slow-oxidative fibers, we hypothesized that the soleus muscle could have a significant role in proprioceptive function, essentially functioning as a proprioceptive organ instead of a significant force-generating muscle during locomotion. We tested this by examining three whole soleus muscles and assessing their muscle spindle content, which proved to have a spindle index of about 12. This value provided equivocal support for the hypothesis since it did not approach values reported for other mammalian proprioceptive muscles that were approximately 40-50 spindles per gram of muscle mass. Other parameters, such as motoneuron number and muscle unit size, may be useful in understanding these data.