Muscular endurance is the ability of a skeletal muscle or group of muscles to continue contracting over a long period. When you have good muscular endurance, you have the ability to resist fatigue while holding a position, carrying something for a long period of time, or repeating a movement without getting tired. As you train for muscular endurance through a process called endurance conditioning, your muscles adapt as a result of changes in the slow-twitch or red fibers. These fibers have a high capacity to use oxygen due to the large amount of blood supply found in them. They are slow to contract but have the ability to continue to contract over long periods of time.