Elbow extensor muscles of the horse: postural and dynamic implications.

This research explores the composition and roles of the elbow extensor muscles in horses, stating that there are two main types of muscles. Specifically, the larger long and lateral heads of the triceps brachii, which are mainly type II or fast muscles, and the smaller medial head and anconeus, which contain mainly type I or slow fibers. The former are believed to play a crucial part in dynamic activity, while the latter contribute to maintaining posture.

Muscle Composition in Horses’ Elbow Extensors

This research presents an analysis of the horses’ elbow extensor muscles using histochemical and immunohistochemical methodologies. The findings categorize these muscles into two major groups differentiated based on their morphological characteristics. Specifically:

• The triceps brachii’s long and lateral heads are largely type II, known as fast muscles. These two heads jointly contribute to 96% of the elbow extensors’ total weight, with the long head of the triceps accounting for an impressive 81% of this mass.
• These heads consist of three distinct histochemical fiber types: I, IIa, and IIb. From the total, the slow type I fibers account for about 18% and 27% of the fibers in the long and lateral heads respectively.
• In the lateral head, the remaining 70% of fibers are split equally between type IIa and IIb. However, in the long head of the triceps, the type IIb fibers – known as fast glycolytic fibers – make up 50%, outweighing type IIa fibers which account for 32%.

Comparisons to the Smaller Muscles

Contrarily to the larger muscles, the smaller muscles including the medial head of triceps brachii and anconeus, which only represent 2% of the triceps mass each, are considerably different:

• These smaller muscles comprised almost exclusively slow, type I fibers.

Hypothesized Role of These Muscles

Based on the observed fiber types and muscle distribution, this research hypothesizes the role these muscles play:

• The medial head and anconeus, characterized by the prevalence of slow fibers, presumably contribute to the horse’s forelimb postural upkeep by forestalling flexion at the elbow joint while in a passive stance.
• On the other hand, the more sizable long and lateral heads – with predominantly fast fiber populations – are posited to be essential during dynamic activities, such as running.