Muscular endurance is the ability of a muscle to repeatedly exert force against resistance. Performing multiple repetitions of an exercise is a form of muscular endurance, as is running or swimming. If your muscles have to contract in a similar pattern more than one time, you are using muscular endurance.
Many factors contribute to muscular endurance, including genetics. If you're not genetically predisposed to muscular endurance you can train to improve it.
Muscle Fiber Types
Your muscles are made up of different types of muscle fibers. The two main types are fast-twitch and slow-twitch. Slow-twitch fibers play the greatest role in muscular endurance. They do not generate much force, but they are far more resistant to fatigue than fast-twitch muscles. Fast twitch muscles go to work when the force is too great to handle for the slow-twitch muscles. They activate to perform short-duration powerful movements.
People are naturally slow or fast-twitch dominant. If you're slow-twitch dominant, you're likely better at endurance sports. If you're fast-twitch dominant, you're probably better at Olympic weightlifting or football. Genetics aside, you can tweak your training program to increase your proportion of slow-twitch muscle fibers.
The Role of Strength
It used to be thought that endurance athletes should stay out of the gym to prevent putting on bulk. Endurance runners just ran more in the hopes of improving performance. We know now that strength training is important for muscular endurance.
The stronger a muscle is the easier it is to complete a given task -- for example, propelling a runner forward. The less work the muscle has to do, the more energy it has to go the distance. Strong, efficient muscles also don't require as much blood and oxygen, so they put less strain on the heart which resulting in greater endurance.
Training for Endurance
Any training program should be periodized, meaning it has different phases. Because you need strength for endurance, you should include a strength phase in your program. For building strength use a heavier weight for lower reps -- six maximum -- and lift at a higher intensity. Take longer rest breaks of 2 to 4 minutes between sets to allow for muscle recovery.
To train your slow-twitch muscles, lift lighter weight for a higher number of reps -- eight or more. Also use a slower tempo, such as two seconds up, two seconds down. Take shorter rest breaks of 30 seconds or less in between sets to accustom your muscles to work in a state of fatigue.
The fibers in your muscles that fatigue can fail because of a lack of energy. Glycogen, or sugar, is required for both peak and sustained muscular effort. A diet low in carbohydrates can make it difficult to sustain muscular endurance.
If your goal is optimal muscular endurance, you need to eat plenty of fruits and vegetables and complex carbs from whole grains. You also need lean protein and healthy fats. Additional carbohydrates and protein following a workout can help you recover faster and promote muscular endurance.
Proper hydration is also key to optimal muscular endurance. Make sure you are drinking enough water based on your body size, activity level, sweat output and the climate where you live.