Muscular fitness is a general term that describes the general health, strength, and endurance of one's muscles. Muscular fitness is related to other fitness conditions, such as muscular strength, which describes the amount of resistance that one's muscles can counter, and muscular endurance, which is the amount of muscular work one can do over time. People work to improve their muscles to achieve a number of goals; common goals involve improving one's appearance, improving one's overall health, and reducing stress. To accomplish these goals, people often engage in a rigorous weight training regimen. To accomplish these ambitions, people must often exercise for several hours per week.

There are many possible benefits to improving muscular fitness. One's muscles tend to grow stronger and larger as they gradually become able to lift heavier loads for longer periods of time. Various proteins and other substances that are involved in muscle work are produced in greater quantities, improving muscular strength and endurance. The nervous system can become more efficient, allowing the brain the send instructions to the muscles more clearly and efficiently and removing nervous inhibitions to muscular fitness. There may even be strong psychological benefits; a muscular fitness regimen often leaves an individual less stressed and more focused.

Muscular fitness encompasses many factors of muscular performance relating to strength, endurance, and overall health. Muscular strength is measured in two primary ways: Dynamic strength is a measure of the maximum weight that can be lifted once, while static strength is a measure of the maximum force that one can apply to an unmoving object, such as a wall. Muscular endurance is measured through multiple lifting repetitions using weights that are below one's maximum capacity. Isometric endurance is simply a measure of how long one can hold a certain amount of weight with no movement or repetitions. Muscular strength and endurance improve when muscle fibers grow stronger and new muscles form and when the supply of oxygen and energy to the muscles becomes more efficient.

The ways of improving muscular fitness are closely related to the ways of measuring it. To improve dynamic strength, for example, one must generally lift weights repeatedly. One should also gradually increase the weight he is lifting. Over time, this strengthens and increases the number of muscle fibers, making muscles stronger. Lifting weights is also an important part of developing muscular endurance. With endurance improvement, however, it is important to use lighter weights that can be lifted with more repetitions.