Exercise can be defined as an increase in skeletal muscle activity; however, the physiological response to exercise involves much more than the muscles themselves and depends on coordinated changes in the other systems covered in previous chapters. Exercise is associated with sports, and everyone can picture a well-trained athlete, with superb physical conditioning that involves the entire body and mind. Of course, exercise is also necessary for the basic activities of daily life, and one of the most serious consequences of disease can be a limitation of exercise capacity. For example, a simple activity such as walking up stairs can be as difficult as running a marathon for someone with chronic lung disease.
The approach to exercise physiology in this chapter is based on the integrative ''fight or flight'' response of the autonomic nervous system (Chapter 9). When we are frightened, for example, the sympathetic nervous system is activated to produce a well-orchestrated suite of physiological changes that prepare us to exercise (i.e., fight our attacker or take flight). The systems that support muscular activity (e.g., respiratory and cardiovascular) are activated, while vegetative and restorative systems (e.g., gastrointestinal and renal) are inhibited. This chapter will not consider the specifics of muscular movement, which is a discipline in its own right (kinesiology), or muscle physiology per se. For example, the different effects of isometric (constant muscle length) versus isotonic (constant force) exercise on the cardiovascular system are described here, but the differences in isometric and isotonic muscle function are described in Chapter 7.
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