Horse soleus muscle: postural sensor or vestigial structure?

The researchers investigated the soleus muscle in horses, which is relatively small compared to other hind leg muscles, to determine its function. They hypothesized that its composition, primarily of slow-twitch fibers, suggested a role in proprioception rather than force generation during movement.

Methodology and Purpose

• The purpose of the study was to understand the role of the diminutive soleus muscle in horses, which is smaller compared to other accompanying muscles. The investigation aimed at determining whether this muscle is vestigial (non-functional or useless) or serves some essential function.
• The research involved performing histochemical (using ATPase), immunocytochemical (for myosin isoform identification), and SDS-PAGE analysis on the horse’s soleus muscle. This helped in establishing that it is majorly composed of type I, presumed slow-twitch fibers.

Findings and Discussion

• The analysis showed that out of the 13 adult horses studied, only one had type II muscle fibers in its soleus muscle. This highlighted a high percentage of slow-oxidative fibers and led to the hypothesis that the muscle could play a significant role as a proprioceptive function, acting more as a sensory organ than a force contributor during locomotion.
• The researchers then tested their hypothesis by examining the muscle spindle content in three whole soleus muscles. The spindle index was found to be around 12, which didn’t entirely support the hypothesis as it was significantly lower than the spindle values reported for other mammalian proprioceptive muscles (about 40-50 spindles per gram of muscle mass).
• However, they did not completely dismiss their hypothesis, suggesting that other factors such as the number of motoneurons and muscle unit size may be instrumental in interpreting these data.

Implications and Possible Further Research

• The study offers a new perspective on the function of the smaller soleus muscle in horses. While the evidence doesn’t entirely support the initial hypothesis, it doesn’t eradicate the possibility of the muscle playing a pivotal role in proprioception.
• This research encourages further work on the topic, calling for more comprehensive studies considering more parameters like motoneuron number and muscle unit size to provide deeper insights into the functionality of the horse’s soleus muscle.