Chapter 6 The Muscular System 179 Copyright Goodheart-Willcox Co., Inc. the period during which the motor units must rest before responding to another nerve stimulus becomes longer. A muscle fiber reaches absolute fatigue once it is unable to develop tension when stimulated by its motor axon. SELF CHECK 1. What is measured to determine muscular strength? 2. What is measured to determine muscular power? 3. What is muscular endurance? 4. What influences muscular endurance? sirtravelalot/Shutterstock.com Figure 6.11 Success in the hurdles, combining running speed and jumping, requires muscular power. Supplements You Do Not Want to Take Some athletes and bodybuilders have succumbed to the temptation to take drugs to increase their muscle size, strength, and power production. The results have included devastating physiological and psychological side effects, as well as ruined professional careers and public disgrace. Famous athletes such as Lance Armstrong, Mark McGwire, Alex Rodriguez, José Canseco, Ben Johnson, and Marion Jones have been exposed for using banned drugs. The World Anti-Doping Code prohibits supplementing with all forms of anabolic agents at all times for athletes. Anabolic means “building up,” and refers to increasing bone and muscle mass. Different types of anabolic drugs have been developed for medical treatment of certain conditions. They are not intended for use by athletes and bodybuilders to build muscle. For example, anabolic androgenic steroids are synthetic versions of the male hormone testosterone. These drugs are designed for medical treatment of delayed puberty, some forms of impotence, and diseases that cause wasting of muscle tissue. When used inappropriately, however, these drugs can cause severe, long-lasting, and sometimes irreversible negative effects on health, including: • liver damage, including blood-filled cysts and tumors • kidney failure • elevated blood pressure and other cardiovascular effects contributing to early heart attack and stroke • stunted growth due to premature closure of epiphyses in long bones • abnormally increased aggressiveness • depression and suicide In children, these steroids can disrupt puberty. In adolescents, stunted growth and accelerated puberty changes can result from steroid use. Males may experience breast development, shrinking of the testicles, impotence, reduced sperm production, and an increased risk of prostate cancer. Females may experience irreversible deepening of the voice, cessation of breast development, male pattern hair growth, enlarged clitoris, and abnormal menstrual cycles. Another banned anabolic agent is human growth hormone (HGH). Synthetic human growth hormone is approved by the FDA for specific uses in children and adults. In children, HGH injections are approved for treating medical conditions that impair normal growth. In adults, HGH is used to treat conditions that prevent normal absorption of nutrients in the intestines or cause muscle wasting. Negative side effects of HGH include severe headaches, vision loss, acromegaly, high blood pressure, heart failure, diabetes, tumors, and crippling arthritis. Diuretics are also banned for use as anabolic agents. However, they are medically used for treatment of hyper- tension, kidney disease, and congestive heart failure. These drugs are sometimes taken by athletes trying to reduce body weight, notably in sports that classify competition by weight. The side effects of diuretics are dehydration, muscle cramps, dizziness or fainting, low blood pressure, and a loss of coordination and balance. Use of all of these drugs for purposes other than medical treatment is not only inadvisable, but also dangerous. Many of the identified side effects can also result in death. The intelligent, safe way to increase muscle size and strength is through a program of progressive resistance training. Clinical Application