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. Muscular endurance, however, is 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.
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. As a result, you might be good at doing lots of reps or running super far, but you'll lose some of the quick power required to sprint or lift really heavy weights.
Speed and Strength Potential Limited
When you've developed high levels of muscular endurance, you may sacrifice some strength. Although you can do an exercise for multiple reps, you may not have as high of a one-repetition maximum as someone who has more fast twitch fibers and well developed muscular strength.
Speed is commonly defined as the ability to minimize the time between repeated movements. When you have an abundance of 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. Your body uses the phosphagen system during high-powered activities, while it taps into the glycolytic system during moderate-powered activities. For total fitness and all-around conditioning, you need to create a balance between all three pathways.
When you've developed muscular endurance at the expense of power and strength, you may not perform as well at certain sports. You might be able to run a marathon or bike a century ride, but you may struggle with achieving a fast 5K pace or riding sprints in the velodrome.
Focusing on muscular endurance too much can also make you more vulnerable to injury because you train endurance at the expense of strength and power. This can lead to doing the same activity repetitively, which taxes certain muscles more than others and leads to imbalances.