Which energy system is predominantly used by Type I muscle fibers during low-intensity, long-duration activity?

Which energy system is predominantly used by Type I muscle fibers during low-intensity, long-duration activity?

• A. Oxidative energy system
• B. Phosphagen system
• C. Lactic acid system
• D. None.

Answer: (A)

Type I fibers are slow-twitch and built for endurance, with many mitochondria, high capillary density, and a lot of myoglobin to deliver and use oxygen efficiently. Because the body needs a steady, sustainable supply of ATP during long, easy activities, these fibers predominantly rely on aerobic metabolism. Oxidative phosphorylation uses oxygen to produce ATP from fatty acids and glucose, which fits well with the slow, continuous energy demand of low-intensity, long-duration work. While other systems can contribute briefly—the phosphagen system for very rapid energy at the start and the lactic acid system if intensity rises—the oxidative energy system is the main source of ATP for Type I fibers in sustained, low-intensity activity. This is why endurance-trained muscles depend so heavily on aerobic pathways.

Type I fibers are slow-twitch and built for endurance, with many mitochondria, high capillary density, and a lot of myoglobin to deliver and use oxygen efficiently. Because the body needs a steady, sustainable supply of ATP during long, easy activities, these fibers predominantly rely on aerobic metabolism. Oxidative phosphorylation uses oxygen to produce ATP from fatty acids and glucose, which fits well with the slow, continuous energy demand of low-intensity, long-duration work.

While other systems can contribute briefly—the phosphagen system for very rapid energy at the start and the lactic acid system if intensity rises—the oxidative energy system is the main source of ATP for Type I fibers in sustained, low-intensity activity. This is why endurance-trained muscles depend so heavily on aerobic pathways.