Muscular endurance is the ability of a muscle to lift weight repeatedly over time. Muscular endurance differs from muscular strength which is a measure of how much force you can exert in one repetition such as maximum weight, one-repetition bench press. You exhibit muscular endurance when you lift a light weight for dozens of repetitions, run for miles, row a kayak or carry a small child through the store.
If your intention is to become strong and larger in size, you should train muscular strength by lifting heavy weights for six to 10 repetitions. If you are an athlete who requires agility and stamina, opt for lighter weights and 20 or more repetitions to build muscular endurance. Training for muscular endurance doesn't noticeably change the size of your muscles or the amount of weight you can heave. If your focus is to improve your health, ability to do daily activities and ward off osteoporosis, go for eight to 12 repetitions of a weight that fatigues you in the last couple of repetitions, according to the Centers for Disease Control and Prevention.
While a one-rep maximal test is an effective way to measure muscular strength, testing for muscular endurance is trickier. One way to test endurance is through exercises that ask you to lift a certain percentage of your body weight. For example, the pushup test, in which you see how many pushups you can complete in a set amount of time, is a common test of muscular endurance. Another way is to lift a fixed percentage of your one repetition max for a certain exercise - such as measuring how many weighted squats or bench presses you can complete in 60 seconds.
You can't totally isolate muscular strength from muscular endurance. A person's strength affects the measure of their endurance. For example, if you can barely perform one pushup because you are weak in the upper body - you can't adequately measure your endurance because you cannot complete multiple repetitions. Building strength can help you develop better endurance.
Your muscles have two types of muscle fibers: Short twitch and fast twitch. Short-twitch muscle fibers contribute to muscular endurance. Fast twitch fibers come in types A and B. Type A fast twitch fibers exhibit some endurance characteristics and help you with long, anaerobic activity such as a long sprint or carrying heavy kettlebells across the gym floor. Type B fast twitch fibers activate during short, power activities such as jumping, short sprints or your one-rep max lift. Your genetics determine your composition of fibers - athletes that excel at anaerobic sprinting and power lifting are likely to have a greater preponderance of fast-twitch fibers. Marathon runners and other endurance athletes are likely to have a greater amount of slow-twitch fibers.