Four forearm flexor muscles of the horse, Equus caballus: Anatomy and histochemistry

Abstract

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.

The effect of gait and digital flexor muscle activation on limb compliance in the forelimb of the horseEquus caballus

“…The SL, an evolutionary modification of the interosseus muscle, is completely fibrous, with in young animals, at most, only remnants of muscle fibres. The superficial digital flexor muscle is almost completely fibrous in the hind limb and in the forelimb has fibres of length 2-6·mm (Biewener, 1998b;Dimery et al, 1986;Hermanson and Cobb, 1992). The deep digital flexor muscle has three heads, humeral, radial and ulnar.…”

“…The fibres are 7±1·mm in the short-fibre compartment, 18±1·mm in the intermediate-fibre compartment and 112±13·mm in the long-fibre compartment (M. P. McGuigan and R. Hagan, unpublished data). The radial and ulnar heads are much smaller with fibres of approximately 17·mm (Hermanson and Cobb, 1992). The SDF and DDF muscles are force-protected by accessory ligaments (Dyce et al, 1987) that link the tendon distal to the muscle belly to the bone (Fig.·1).…”

Contractile properties of muscle fibers from the deep and superficial digital flexors of horses

“…Although their tendons have essentially equivalent relationships to the main joint they act upon, the metacarpophalangeal joint (fetlock) of the distal limb, they show remarkable diversity in their muscle architecture. The "short" (humeral) compartment of the deep digital flexor (DDF) has long, unipennate fibers, whereas the superficial digital flexor (SDF) has short, multipennate fibers (8,20,54). Muscle architecture is an important component of musculoskeletal structure and function.…”

Horses damp the spring in their step

“…Animals (including humans) that store elastic energy in their tendons have compliant tendons 5 and minimize length change in the associated muscle [7][8][9] . The muscle instead preloads 10,11 and/or powers the system by shortening at low tendon forces 7 . Further effciency gains may be achieved by shortening or removing the muscle fbres, because the energetic cost of generating isometric force is a function of muscle fbre length over activated volume [3][4][5] .…”

“…However, the equine forelimb superfcial and deep digital fexor muscles remain well developed (mass 600-1,000 g), and 980 (see Methods) of the physiological cross-sectional area comprises extremely short (superfcial, 2-6 mm; deep, 6-17 mm) muscle fbres 1,6,10,11 . Similar short-fbred muscles exist in the hindlimb.…”