ARCHITECTURE AND THE DIVISION-OF-LABOR IN THE EXTENSOR CARPI RADIALISMUSCLE OF HORSES

The extensor carpi radialis muscle of the horse is deceptive at first appearance. It has a fusiform shape similar to other forearm extensor muscles. The fiber arrangement also appears long and relatively parall el. However, it may contain two or more compartments that correlate wi th differing functional roles. Histochemical and immunocytochemical an alysis of proximal and distal regions of the muscle (n=9) demonstrate that the proximal portion of the muscle is composed of a mean of 13% t ype I, presumed slow twitch, and 61% type IIb, presumed fast twitch fi bers. In contrast, the distal compartment is composed of a mean of abo ut 43% type I and only 22% type IIb fibers. The type I and IIa fibers are all highly aerobic based on nicotinamide dinucleotide tetrazolium reductase reactions. Correlative data regarding the myosin isoforms ha s been obtained with 4% SDS-PAGE analysis of myosin heavy chain isofor ms which demonstrate isoforms migrating at rates similar to rat type I , IIa, and IIx. The latter has been referred to as type IIB/X in a stu dy of the horse's gluteus medius muscle. We propose that the in-series 'compartmentalization' of the muscle, while not conforming strictly t o the definitions of neuromuscular compartments, relates to the insert ion of the lacertus fibrosus, a distal slip of the biceps brachii, upo n the extensor carpi radialis. Earlier studies demonstrated a high pro portion of type I fibers in the equine lateral biceps brachii which we re thought to stabilize the shoulder during long periods of quiet stan ding. Because of action imposed on the distal compartment by the bicep s brachii, slow and fatigue-resistant functions are part of the limb's passive stay apparatus to effect long-term standing by the horse. Thu s, the fatigue-resistant compartments of biceps brachii and extensor c arpi radialis may constitute an in-series arrangement of the two muscl es. The proximal compartment is suited to provide powerful, more fatig able contractions during locomotion and likely affects stress or strai n within the distal postural compartment.