Sedentary lifestyle is a medical neologism used to denote a type of lifestyle most commonly found in modern (particularly Western) cultures. It is characterized by sitting or remaining inactive for most of the day (for example, in an office), with little or no exercise. It is believed to be a factor in obesity, and, as such, may contribute to other diseases, such as type II diabetes, heart disease, depression and even hemorrhoids. Lack of exercise causes muscle atrophy, i.e. shrinking and weakening of the muscles and accordingly increases susceptibility to physical injury. Additionally, Physical fitness is correlated with immune system function; a reduction in physical fitness is generally accompanied by a weakening of the immune system.

Despite the well-known benefits of physical activity, many adults and many children lead a relatively sedentary lifestyle and are not active enough to achieve these health benefits. A sedentary lifestyle is defined as engaging in no leisure-time physical activity (exercises, sports, physically active hobbies) in a two-week period. Data from the National Health Interview Survey shows that in 1997-98 nearly four in 10 (38.3 percent) adults reported no participation in leisure-time physical activity. This study does not include any measure of physical activity that is performed as a job related duty so does not encompass the full range of possible exercise that adults may be getting.

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