Eccentric training is another type of physical fitness training centered on improving eccentric muscle contraction. As a backgrounder, this type of muscle contraction occurs when the muscle elongates or lengthens from the influence of an opposing force. The contraction happens explicitly when a muscle gives in or goes with the flow or direction of an opposing force.
Fitness is defined as the quality or state of being fit.[5] Around 1950, perhaps consistent with the Industrial Revolution and the treatise of World War II, the term "fitness" increased in western vernacular by a factor of ten.[6] The modern definition of fitness describes either a person or machine's ability to perform a specific function or a holistic definition of human adaptability to cope with various situations. This has led to an interrelation of human fitness and attractiveness that has mobilized global fitness and fitness equipment industries. Regarding specific function, fitness is attributed to persons who possess significant aerobic or anaerobic ability, i.e. endurance or strength. A well-rounded fitness program improves a person in all aspects of fitness compared to practicing only one, such as only cardio/respiratory endurance or only weight training. 

Possibly one of the most important parts of fitness training is flexibility and mobility. Flexibility is the ability of your muscles to stretch, and mobility is being able to move your joints and tissues through their full range of motion. Both are important to athletic performance and for avoiding injury. Muscles and joints that are flexible and mobile — as well as strong — are much less susceptible to sprains and other injuries.
Many of the activities you do to build strength will also help your balance. Balance is partly a matter of developing your small stabilizer muscles that provide support and keep you steady. Free weight exercises, such as lunges and deadlifts, will help strengthen the stabilizer muscles. Even better, include some single-leg exercises, such as single-leg deadlifts and pistol squats, in your strength-training routine.
Physical activity boosts the immune system. This is dependent on the concentration of endogenous factors (such as sex hormones, metabolic hormones and growth hormones), body temperature, blood flow, hydration status and body position.[37] Physical activity has shown to increase the levels of natural killer (NK) cells, NK T cells, macrophages, neutrophils and eosinophils, complements, cytokines, antibodies and T cytotoxic cells.[38][39] However, the mechanism linking physical activity to immune system is not fully understood.
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