SOFFLER, C.; STORCK, K.I.; HERMANSON, J.W.*: Muscle design features of the equine suspensory ligament.

The equine interosseus muscle (“suspensory ligament”) spans the fetlock joint of the forelimb and has been ascribed various functions and morphologic attributes. It stabilizes and prevents overextension of the fetlock. But, the muscle has been variously described as being void of muscle fibers or as being born with muscle fibers that are lost during ontogeny. If the muscle functions as a resting anti-gravity structure at the fetlock, we might hypothesize that it exhibit characteristics consistent with a postural muscle, including a predominance of type I, aerobic fibers. Alternatively, if the muscle is a critical element in the storage of elastic energy during high speed gaits, this muscle could be like comparable elastic storage elements of the equine forelimb (including superficial digital flexor) containing numerous type I fibers. We report observations of interosseus muscles from several ages and from several horse breeds. We provide evidence of high variance in the number of and the relative proportion of muscle fibers in transverse sections of interosseus. In addition, the muscle fibers are extremely short (2-3 mm). Short fibers are incapable of providing much work (length of shortening of the muscle-tendon unit) but occur in other muscles that store energy during locomotion. HIstochemical and immunocytochemical data suggest that the muscle is composed of about 95% type I (slow twitch, aerobic) fibers that appear both normal and highly functional. Thus, despite the preponderance of connective tissue in the equine interosseus, there appears to be a highly organized muscular component capable of adjusting the distribution of forces within the structure. Is this just another extreme form of muscle design that is typical in the horse? Supported in part by IBN-9819985.