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How do hormones affect muscles?

This is an excerpt from Practical Guide to Exercise Physiology 3rd Edition With Web Resource by Robert Murray,W. Larry Kenney.

Whenever you are physically active, signals from your brain and from active muscle cells activate the endocrine system to secrete hormones to help support and sustain exercise. The endocrine system includes glands such as the hypothalamus, pituitary, pancreas, thyroid, testes and ovaries, adrenal glands, and even the gastrointestinal tract, all of which secrete hormones that help muscle cells respond to the demands of exercise and adapt to the stress of training. Muscles themselves are now considered to be the largest endocrine organ in the body because of the hundreds of chemicals they secrete during exercise. These chemical messengers, referred to as myokines, travel throughout the body and influence the function and response of all other tissues.

Every cell in your body is constantly exposed to hundreds of different chemical messengers—including hormones—that provide information about the current state of the body and possible future requirements, which could involve adaptations requiring the production of new proteins. Steroid and nonsteroid hormones such as testosterone, insulin, insulin-like growth factor 1 (IGF-1), and growth hormone (GH) are among the signals that promote the production of increased functional proteins in muscle cells. Figure 1.8 shows how testosterone affects protein production in a muscle cell. Illegitimate use of large doses of testosterone and other anabolic steroids—including prohormones and designer steroids—can result in large increases in muscle mass and strength because hormones such as testosterone continuously stimulate the production of contractile proteins.

Whenever you exercise, your body is flooded with a variety of hormones such as the ones mentioned already, along with epinephrine and norepinephrine (the catecholamines), cortisol, glucagon, neurotransmitters such as dopamine, and many myokines such as lactate and interleukin-6 (IL-6). It is interesting to note that the hormones secreted during and after exercise appear to have little to do with gains in muscle mass and strength. Individuals with a higher density of androgen receptors in their muscle cells are more likely to experience muscle hypertrophy with training, because increased receptor density results in greater sensitivity to anabolic hormones that support muscle growth. Use of anabolic steroids in adolescents and young adults upregulates androgen receptor content, giving users a lasting—and unfair—advantage provided they continue training.

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Although many coaches and athletes refer to lactic acid as a by-product of muscle metabolism, the scientifically correct term is lactate. However, in nonscientific settings, the two terms are interchangeable.

More Excerpts From Practical Guide to Exercise Physiology 3rd Edition With Web Resource

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