Two of the forearm flexors of the horse, the superficial and deep digital flexor muscles, are critical to support the digital and fetlock joints, exhibit differing insertions, and are passively supported by the proximal and distal check ligaments, respectively. These two muscles differ in histochemical composition and architecture. The differences are correlated with the different stress levels transmitted through their tendons, and the different frequencies of clinical breakdown that have been reported. Both muscles contain type I and type IIa fibers. A few type IIb fibers occurred in the deep digital flexor. The superficial digital flexor contained approximately 56% type I fibers, extremely short muscle fibers, and extensive connective tissue investment. In contrast, the deep digital flexor had three muscle heads: ulnar, radial, and "long" and "short" regions of the humeral head. The "long" and "short" regions of the humeral head contained 33% and 44% type I fibers, respectively, fiber lengths three to four times as long as those in the superficial digital flexor, and relatively less connective tissue investment. Flexor carpi radialis and flexor carpi ulnaris compared most closely with the humeral head of the deep digital flexor. These data suggest a correlation of the unique architecture of superficial digital flexor with its proposed elastic storage properties during locomotion in horses, and an explanation for the frequent breakdown of the superficial digital flexor in athletic horses.

Based on histochemical and immunohistochemical evidence, horse elbow extensor muscles are composed of two morphologically distinct muscle groups. The long and lateral heads of the triceps brachii are large, predominantly type II (presumed fast) muscles. The long and lateral heads of the triceps together account for 96% of the weight of the elbow extensors (long head of triceps is 81%). The long and lateral heads contain three histochemical fiber types: types I, IIa and IIb. Type I muscle fibers account for approximately 18 and 27% of the fibers in the long and lateral heads of the triceps, respectively. In the lateral head, type Πa and lib fibers account equally for the remaining 70% while in the long head of the triceps type lib fibers predominate (50%) over type Ha fibers (32%). In contrast, the much smaller medial head of the triceps (2% of triceps mass) and the anconeus (2% of mass) contain almost exclusively type I muscle fibers. It is hypothesized that the medial head and anconeus, with their slow fibers, contribute to the postural maintenance of the forelimb by preventing flexion at the elbow joint during passive stance. The larger long and lateral heads, with their generally fast fiber populations, are most likely important during dynamic activity.

The horse provides an interesting model for study of the structure and function of the mammalian diaphragm. Multiple regions of diaphragm from seven adult horses were prepared for histochemistry, immunocytochemistry, myosin heavy chain electrophoresis, and native myosin electrophoresis. Two additional adults were dissected to demonstrate myofiber and central tendon morphology and stained for acetylcholinesterase to demonstrate motor endplates. All regions of the adult diaphragm were histochemically characterized by a preponderance of type I fibers with some type IIa fibers. Type IIb fibers were absent in all adult specimens. Myosin heavy chain electrophoresis supported the histochemical study: two isoform bands were present on SDS gels that comigrated at the same rate as rat type I and IIa myosin heavy chain isoforms. No isoform was determined to comigrate with rat type IIb heavy chain isoforms. Native myosin isoform analysis revealed two isoforms that comigrated with rat FM-4 and FM-3 (FM = fast myosin) and two isoforms that comigrated with rat SM-1 and SM-2 (SM = slow myosin) isoforms. In some samples, a third slow native myosin isoform was observed that comigrated at the same rate as the SM-3 of the equine biceps brachii muscle. This doublet (or "triplet") of slow isoforms is unique to some horse muscles compared with other adult animals studied. It is not known if these multiple slow native myosin isoforms confer some functional advantage to the equine muscles. The adult equine diaphragm also differs in its morphology by having a large central tendon compared to that in other mammals, and is predominantly slow in fiber type and myosin isoform composition.

We identify a novel histochemical fibre type that is correlated with a unique myosin heavy chain isoform in the pectoralis muscle of the two bats Artibeus lituratus and Carollia perspicillata (Phyllostomidae). Pectoralis muscles sampled from four other species (Phyllostomus hastatus, Platyrrhinus helleri, Glossophaga soricina, and Diaemus youngi) exhibited myosin heavy chain isoforms with electrophoretic properties identical with those of A. lituratus andC. perspicillata. The pectoralis muscles of A. lituratus and C. perspicillata consisted mainly of the newly described type IIe fibre. Diaemus youngi differed from the other species studied by having an FM5 native myosin isoform, a feature shared with Desmodus rotundus. We hypothesize that the two-fibre-type composition of the pectoralis muscle in the phyllostomid bats may be related to a "two-gear" locomotor strategy and represents a synapomorphy. The distribution of fibre types and myosin isoforms correlated with family-level phylogenetic affinity rather than with functional characteristics. One of the fibre types was identical with type IIa of terrestrial mammals and a correlated IIa isoform pattern upon electrophoretic analysis. The IIa fibres had high oxidative and glycolytic potential. The second fibre type, which we call type IIe, had a histochemical and immunocytochemical pattern inconsistent with interpretation as a type IIb muscle fibre. Electrophoretic analysis confirmed that isoforms correlated with IIe fibres migrated at different rates from type I, IIa, IIb, and IIx isoforms. The IIe fibres had a fast oxidative metabolic pattern and were at least twice as numerous as IIa fibres. Thus, a novel type IIe fibre is predominant in the pectoralis of six phyllostomid species that exhibit highly divergent body sizes and (or) flight styles.

The vampire bat pectoralis muscle contains at least four fiber types distributed in a nonhomogeneous pattern. One of these fiber types, here termed IIe, can be elucidated only by adenosine triphosphatase (ATPase) histochemistry combined with reactions against antifast and antislow myosin antibodies. The histochemical and immunohistochemical observations indicate a well-developed specialization of function within specific regions of the muscle. In parallel, analyses of native myosin isoforms and myosin heavy chain isoforms indicate two points. First, the histochemical "type IIe" fiber is predominant in cranial portions of the muscle, and myosin extracted from these regions exhibits a unique electrophoretic mobility not observed in the myosin isoforms of more traditional laboratory mammals. Second, the type I fibers are confined to the pectoralis abdominalis muscle and a small adjacent region of the caudal part of the pectoralis. This pattern of type I fiber distribution is considered a derived character state compared to muscle histochemical phenotype and isoform composition in the pectoralis muscles of other phyllostomids we have studied (Artibeus jamaicensis, Artibeus lituratus, Carollia perspicillata). We relate this to the unique locomotory needs of the common vampire bat, Desmodus rotundus.