Equine skeletal muscle
Equine skeletal muscles have a considerable potential to adapt during training and these adaptations have important physiological implications that influence stamina, strength and speed.
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Skeletal muscle consists of bundles of long spindle shaped cells called muscle fibers that attach to bone by tendinous insertions. The blood vessels and nerves that nourish and control muscle function run in thin sheets of connective tissue that surround bundles of muscle fibers. Each nerve branch communicates with one muscle fiber at the motor end. The nerve and all the muscle fibers that it supplies are together termed a motor unit. Each time that a nerve is stimulated all of the muscle fibers under its control will contract.
A muscle’s unique ability to contract is conferred by the highly organized parallel, overlapping arrangement of actin and myosin filaments. These repeating contractile units or sarcomeres extend from one end of the cell to another in the form of a myofibril. Each muscle fiber is packed with myofibrils that are arranged in register giving skeletal muscle a striated appearance under the microscope. Muscle contraction occurs when the overlapping actin and myosin filaments slide over each other, serving to shorten the length of the muscle cell from end to end and mechanically pulling the limb in the desired direction. The sliding of the filaments requires chemical energy in the form of ATP (S.J. Valberg and J.M. MacLeay).
When a muscle contracts during exercise, it does so in response to a predetermined recruitment of particular muscle fibers. This orderly recruitment of muscle fibers leads to smooth, coordinated movement. Each motor contains fibers of the same type. As exercise begins, a select number of motor units are recruited to provide the power to advance the limb. At slow exercise intensities, type 1 muscle fibers and a small number of type 2A muscle fibers are stimulated. The force produced by any muscle is proportional to the cross-sectional area that is active. As the speed or duration of exercise increases, more muscle fibers will be recruited and this occurs in the order of their contractile speed. Only at near-maximal exercise intensities or after several hours of submaximal exercise are type 2B fibers recruited (S.J. Valberg and J.M. MacLeay).
When a nerve is stimulated, a wave of electrical depolarization occurs that quickly reaches the neuromuscular junction. In response, the nerve terminal releases acetylcholine which binds to the motor-end plate of the muscle fiber and initiates electrical depolarization of the muscle cell membrane. Electrical depolarization of the muscle cell membrane triggers the release of calcium that is sequestered in intracellular membranous storage sites (sarcoplasmic reticulum) into the myoplasm via the calcium release channel. Increased concentrations of intracellular calcium allows the interaction of actin and myosin filaments which then slide over each other in a rachet like fashion to produce a contraction (cross-bridge cycling). Tension is generated as the shortening filaments tug both ends of the myofiber toward the middle. Muscle must relax after each contraction by actively pumping calcium back into the storage sites preventing actin-myosin interaction. In addition, ion pumps in the cell membrane actively repolarize the muscle cell membranes. All of the processes necessary for relaxation are active, meaning that they require energy in order for them to occur (S.J. Valberg and J.M. MacLeay).
Muscular Responses to Training
Muscle training is essential to increase or maintain the athletic performance of sports horses and to reduce the incidence of exercise-induced injuries in the musculoskeletal system of these athletes. Muscle conditioning is a more restricted term which means improving athletic performance by inducing muscular changes that can be evaluated by using objective and scientific methods.
What can be modified in skeletal muscle with training?
Equine skeletal muscle has considerable potential to adapt during training, largely mediated by the structural and functional plasticity of muscle fibres. These long-term (weeks to months) adaptations occur independently from the immediate or short-term muscular metabolic responses to either a single bout of sub-maximal or near-maximal exercise. Depend- ing on the basal muscle status (i.e. breed, age, sex, level of fitness and history of the horse training) and characteristics of the stimulus (i.e. nature, intensity, duration, and frequency of exercise bouts and total length of the conditioning programme), the adaptive response to training can take two different forms. First, the quantitative response, when myofibres increase (hypertrophy) in size but otherwise retain their basal structural, physiological and biochemical properties.
And secondly, the qualitative responses or remodelling, where myofibres do not change in size but acquire markedly different enzymatic and structural characteristics (i.e. fibre type transitions). In practice, the most common adaptive response of equine skeletal muscle to training takes a mixed form of remodelling with discrete or substantive hypertrophy and often accompanied by increases in the number of capillaries (J.-L. L. Rivero 2007).