deleterious effect on bone mineral content. Osteoporosis, usually associated with the elderly (see Chap. 17), is common in athletes with amenorrhea (56). Interested readers are referred to Redman and Loucks's review of the possible causes of exercise induced menstrual. cycle irregularities (108). What is most interesting is that exercise itself may not suppress reproductive function, but rather the impact of the energy cost of the exercise on energy availability. For some first-hand insights into this issue, see Ask the Expert 5.1. Table 5.2 contains a summary of the information on each of the endocrine glands, their secretion (s), actions, controlling factors, the stimuli that elicit a response, and the effect of exercise on the hormonal response. This would be a good place to stop and review before proceeding to the discussion of the hormonal control of muscle glycogen mobilization and the maintenance of the plasma glucose concentration during exercise. However, before we leave this section, we need to introduce you to a new member of the endocrine gland family-muscle itself (see A Closer Look 5.4)