supported metabolically. In fact, the easy fatigability of atrophied muscle is due to the requirement that more motor units be recruited for identical external force,- the fatigability per unit cross-sectional area is normal. The magnitude of the skeletal muscle endurance training response is limited by factors outside the muscle, since cross-innervation or chronic stimulation of muscles in animals can produce adaptations 5 times larger than those created by the most intense and prolonged exercise.
Local adaptations of skeletal muscle to endurance activity reduce reliance on carbohydrate as a fuel and allow more metabolism of fat, prolonging endurance and decreasing lactic acid accumulation. Decreased circulating lactate, in turn, reduces the ventilatory demands of heavier work. Because metabolites accumulate less rapidly inside trained muscle, there is reduced chemosensory feedback to the central nervous system at any absolute workload. This reduces sympathetic outflow to the heart and blood vessels, reducing cardiac oxygen demands at a fixed exercise level.
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