Thus, the FT muscle fibers can produce energy and high power very rapidly anaerobically, and thus high power, but not for very long before fatigue occurs, since lactic acid is produced and little energy can be produced from glycogen as compared to fat. Because of their large size, oxygen is less able to diffuse into the FT fibers, but they are low in fat, which requires oxygen for its use, and are high in glycogen, which doesn’t require oxygen for its use. The reverse is the case for the small slow-twitch (ST) muscle fibers. The fast-twitch (FT) muscle fibers are large fibers that have high strength but low oxidative capacity because the fiber is high in myofibrils rather than mitochondria, where oxygen is utilized. Thus, a horse may have the genetic ability to be good at a particular type of activity, and appropriate training will increase this ability. Increased muscle size for strength is provided by an increase in both the number and size of muscle fibers, both of which are increased by genetics and strength (speed and weight) training whereas increased oxygen-utilizing capacity for endurance is influenced by genetics and cardiovascular training. Strength requires large muscles, whereas prolonged activity requires oxygen-utilizing ability, which requires small muscle fibers-not the strength provided by big muscle fibers. ![]() Speed, like pulling or lifting a heavy load, requires strength.
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