Muscular fitness is commonly defined as a combination of muscular strength and muscular endurance. Muscular strength refers to your ability to exert maximal force during a limited number of repetitions while muscular endurance refers to your ability to perform repeated repetitions over an extended period of time. While muscular endurance has several health and fitness benefits, there are also disadvantages relating to maximal strength, power and speed.
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Skeletal muscles contain two main types of muscle fibers that are responsible for different contractions and activities. Fast-twitch muscle fibers generate high amounts of force in a short period while slow-twitch fibers produce lower amounts of force and are resistant to fatigue. As a result, if you have high amounts of muscular endurance, your muscles consist primarily of slow-twitch muscle fibers. With muscular endurance training, the fast-twitch muscle fibers slowly atrophy while the slow-twitch fibers hypertrophy.
Strength is defined as the ability of a muscular unit or units to apply maximal amounts of force. With slow-twitch muscle fibers and muscular endurance, however, your ability to produce maximal strength decreases. For example, with muscular strength, you can perform seven to 12 repetitions of an exercise using 80 percent of your one-rep maximum. If you have more muscular endurance, however, you won’t be able to produce more than seven repetitions.
Speed is commonly defined as the ability to minimize the time between repeated movements. With slow-twitch muscle fibers and muscular endurance, the speed of the muscle contractions becomes slower, which ultimately decreases your maximum speed potential. For example, world-class sprinters have primarily fast-twitch muscle fibers, while world-class marathoners have primarily slow-twitch muscle fibers. During running workouts, muscular endurance reduces your ability to run intervals faster than an athlete with muscular strength.
Three metabolic pathways provide energy to the working muscles during physical activity. Muscular endurance uses the oxidative pathway and neglects the glycolytic and phosphagen pathway. The phosphagen system is used during high-powered activities, while the glycolytic system is used during moderate-powered activities. For total fitness and all-around conditioning, you need to have a balance of all three pathways.