The two components of muscle fitness are strength and muscular endurance. Strength is the amount of force produced with a single maximal effort of a muscle group. Muscular endurance is the capacity of skeletal muscles or group of muscles to contradict over a long period of time. Both the strength and muscular endurance are needed to increase the capacity for work and also decreases the chance of injury to prevent low back pain, poor posture and also other hypokinetic conditions. Muscle fitness training increases the fitness of the bones, tendons, ligaments, and muscles. It has been found to be therapeutic for patients with chronic pain. Progressive resistance training is a type of physical activity for improving the muscle fitness. There are many types of progressive resistance exercises designed to promote or maintain the muscle fitness.

The three types of muscle tissue are smooth, cardiac, and skeletal. They have different structures and functions. Smooth tissue consists of long, spindle-shaped fibers with each fiber contains only one nucleus. These fibers are involuntary and are located in the walls of stomach, esophagus, and intestine to move food and waste products through the digestive tract. Cardiac muscle tissue is also involuntary and it contracts in response to the demands of the cardiovascular system. The heart muscles contracts at a slow steady rate at rest but contracts more frequently and forcefully during physical activity. Skeletal muscle tissue consists of long, cylindrical, and multinucleated fibers. They provide the force needed to move the muscle system and can be controlled voluntarily. Skeletal muscles are made up of slow (red), intermediate, and fast (white) twitch fibers. Fast-twitch muscle fibers generate greater tension than slow-twitch fibers. These fibers are suited to fast, high-force activities such as jumping, sprinting, and weight-lifting movements. Strength training primarily increases the size of fast-twitch fibers and intermediate fibers. Slow-twitch fibers generate less tension but are more resistant to fatigue because they rely on aerobic metabolism. If you train specifically for muscular endurance, the muscle adapt primarily to the slow-twitch fibers including increased activity of aerobic enzymes in the muscle.

An example of fast-twitch muscle fibers in animals is the white meat in the flying muscles of a chicken. The chicken exerts a powerful force to fly a few feet up to a perch as it is heavy. A wild duck that flies for hundreds of miles has dark meat (slow – twitch fibers) in the flying muscles for better endurance. People who want large muscles will use progressive resistance exercises designed to build strength (fast- twitch fibers). People who want to be able to persist in activities for a long period of time without fatigue will want to use progressive resistance training programs designed to build muscular endurance (slow- twitch fibers). Each person inherits a certain percentage of fast twitch and slow twitch muscle fibers. This allocation influences the potential of a person for muscle fitness activities. Individuals with a larger percentage of fast-twitch will generally increase muscle size and strength more readily than individuals endowed with a larger percentage of slow-twitch fibers. People with a larger percentage of slow-twitch fibers have greater potential for muscular endurance performance. Regardless of genetics, all people can improve their strength and muscular endurance with proper training. Women have smaller amounts of the anabolic hormone testosterone and therefore have less muscle mass than men. Because of this, women typically have 60 percent to 85 percent of the absolute strength of men. Absolute muscular endurance also favors men over women, though not as dramatically as for strength. When differences in size and muscle mass are taken into consideration, women have relative strength and relative muscular endurance similar to men. Maximum strength is usually reached in the twenties and typically declines with age. Though muscular endurance declines with age, it is not as dramatic as a decrease in absolute strength. As people grow older, regardless of gender, strength and muscular endurance is better among people who train than people who do not. This suggests that progressive resistance training is one antidote to premature aging.

A stronger person has an advantage when tested on absolute endurance (the number of times you can move a designated number of pounds). But the stronger person has a disadvantage when tested on relative muscular endurance (the number of times you can move a designated percentage of your maximum strength). For this reason, men and women can compete more evenly in relative muscular endurance activities. In fact, on some endurance tasks women have done as well or better than men. For example, the women at the United States Military Academy do as well as the men on tests of abdominal muscular endurance. Cardiovascular endurance depends primarily upon the efficiency of the heart muscle, respiratory system, and circulatory system. It is developed with activities that stress these systems, such as cycling, running, and swimming. Muscular endurance depends upon the efficiency of the local skeletal muscle and the nerves that control them. Most forms of cardiovascular exercises such as running require good cardiovascular and muscular endurance. For example, if your legs lack the muscular endurance to continue contracting for a sustained period of time it will be difficult to perform well in running or other aerobic activities.