CSCS Study Guide Chapter 4: Endocrine Responses to Resistance Exercise

Chapter 4 of the Essentials of Strength Training and Conditioning covers a basic introduction to the body's hormonal response to exercise. Some of the resulting changes are short-term, some are long. Learning about this process is important to ultimately improve performance.

Chapter 4 of Essentials of Strength Training and Conditioning is about the bodies endocrine response to resistance training. Many of the responses to resistance training occur both in the short term and long run. The body is able to specifically control it's response to exercises in a dosed relationship.

Key Items:

  • Only recruited muscle fibers activated during resistance exercise are subject to adapt.
  • Testosterone is increase the most with large muscle group exercise, heavy resistance (85-95% of 1RM), moderate to high volumes of exercise and/or short rest intervals. These methods can be used independently or in combination.
  • Vary exercise stress to avoid continually taxing the adrenal glands. That will result in nonfuncitonal overreaching or eventually overtraining.

Endocrine Responses to Resistance Exercise

  • The endocrine system supports and helps sustain the bodies normal homeostatic functions.
  • The significance of the endocrine system in resistance training is reflected in the notion of periodization which is simply the organization of a training program.
  • General Adaptation Syndrome - How the adrenal gland responds to stress. There is an initial alarm phase where function is impaired then adaptation above the baseline level.
  • General adaptation syndrome plays out in resistance training when the body responds to the stress of training and as long as the stress is removed, adaptation occurs. It will now take more stress to produce further gains.
  • The most important part of adaptation is removing the stress.
  • Changes in circulating hormones are only one of the observable changes in the body from metabolic adaptation. It is possible in some programs to see no changes in blood concentrations.
  • Resistance training is the only natural stimulus that causes considerable increases in lean mass. There are significant differences in a programs ability to produce lean mass depending on program variables.

Synthesis, Storage, and Secretion of Hormones

  • Hormones - chemical messengers, stored in the bodies endocrine glands and released in blood.
  • Endocrine Glands - hormone storage centers. Stimulated by a chemical signal or direct neural stimulation.
  • The main endocrine glands are the Hypothalamus, Pituitary gland, Thyroid gland, parathyroid gland, heart, adrenal glands, liver, pancreas, kidneys, ovaries (female) and/or testes (males).
  • Neuroendocrinology - the study of how the nervous system and endocrine system interact.
  • Steroid Hormones - a steroid that acts as a hormone. Helps control metabolism, the function of the immune system, the development of secondary sexual characteristics and salt and water balance. Fat soluble and pass through cell membranes.
  • Thyroid Hormones - hormones responsible for regulating metabolism.
  • Target Tissue Cells - the specific cells that the hormone signal is targeting.
  • Hormones can be released in blood, interact within cells themselves or on adjacent cells.
  • Intracrine and autocrine are on the cell itself and never hit the blood. Paracrine also never hits the blood but works on adjacent cells.
  • Many hormones affect many cells within the body like testosterone which interacts with almost every tissue in the body.
  • The amount of muscle activated dictates the hormonal response.

Muscle as the Target for Hormone Interactions

  • Neuroendocrine Immunology - the study of the relationship between the neural, endocrine and immune systems.
  • Anabolic Hormones - hormones that promote tissue building.
  • Catabolic Hormones - hormones that promote tissue breakdown.

Roles of Receptors in Mediating Hormonal Changes

  • Many hormones cannot cross cell membranes so they require secondary messengers inside of cells to relay messages. The hormone interacts with these messengers via receptors in cell membranes.
  • Lock-and-key theory - the theory that the signal from a hormone only interacts with specific cells that have receptors for those hormones. It is now known to be more complex than this.
  • Cross-reactivity - when a hormone partially bonds and interacts with a cell.
  • Allosteric binding sites - sites where substances other than hormones can bond and enhance or reduce the cell's response to the primary hormone.
  • Downregulation - when a hormone can not interact with a cell due to an "over-stimulation".
  • A cell can change the number of receptors it has or their sensitivity.

Categories of Hormones

  • There are three main categories of hormones; steroids, peptides and amines. Each interacts with muscle differently.

Steroid Hormone Interactions

  • The events that lead up to the biological effect is the same for any steroid hormone.
  • Hormone-receptor complex (H-RC) - the combination of a hormone and it's receptor.
  • The H-RC combines with another and moves towards the cells DNA so that it can be opened for transcription of specific proteins.

Polypeptide Hormone Interactions

  • Polypeptide Hormones - hormones made up of chains of amino acids. Water soluble and cannot cross cell membranes.
  • Secondary Messengers - inside the cell, carry the external message from hormones that reach the outside of the cell membrane but cannot cross over.

Amine Hormone Interactions

  • Amine hormones are made from either tyrosine or tryptophan. They also rely on secondary messengers like peptide hormones but, they do not have a negative feedback loop regulation.

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Heavy Resistance Exercise and Hormonal Increases

  • Long term consistent heavy resistance training (months or years) brings about significant changes in the size, strength, and power of the trained musculature.
  • A key concept is that physiological systems are sensitive to the type of exercise and will respond using different exercise systems based on their involvement.
  • Hormones are present before, during and after exercise. Short term they provide feedback to the body on the type and amount of training and the need for ensuing changes.

Mechanisms of Hormonal Interactions

  • The system of hormonal interaction with muscle depends on several factors including whether that muscle has reached it's maximum size and lost sensitivity, sensitivity is lost due to chronically high levels of exposure to a hormone, or if the exercise program is designed incorrectly and causes too much of a catabolic effect.

Hormonal Changes in Peripheral Blood

  • Hormone concentration changes in blood can tell us a lot about the physical stress of a workout. It's best viewed in the context of all the other processes that stimulate muscle and protein synthesis.
  • Interpretation of the blood hormone concentrations can be tricky but provides an indication of the bodies response.
  • Just because blood concentrations of a a hormone are higher, does not mean that they are higher within a cell. It's just more likely that the hormone is interacting with a cells receptors.

Adaptations in the Endocrine System

  • When you train muscle you're also training endocrine glands.
  • Adaptation is related to changes in the target organs and how the exercise stress is tolerated.
  • Hormones represent an attempt by the body to return to normal homeostatic levels.Changes can be acute or long term.

Primary Anabolic Hormones

Testosterone, growth hormone and insulin-like growth factors.


  • The primary hormone that interacts with skeletal muscle. Binding with the cell receptor is key.
  • The book states change in circulating testosterone concentrations is a dramatic anabolic signal for target tissues in the body.
  • Testosterone can influence the body through changes in the nervous system.
  • Even though there is increased circular testosterone following aerobic endurance exercise, there is not typically any hypertrophy. Oxidative stress may in fact promote decreasing muscle fiber size.
  • Things that cause the highest serum testosterone level increases in boys and younger men include:
    • Large muscle group exercises
    • Heavy resistance (85-95% of 1RM)
    • Moderate to high volumes of exercise by using either multiple sets, exercises or both
    • Short rest times (30 sec-1 min)
    • Two years or more of resistance training exercise
  • Diurnal Variations - normal fluctuation in hormone levels.
  • Testosterone follows a circadian rhythm and is highest in the morning and lowest in the evening.
  • Higher in young men than older men.
  • Women have a much higher sensitivity to increases in serum testosterone levels despite the smaller absolute levels compared to men.
  • There's still much research to be done but, it appears that resistance training increases muscle androgen receptor content.

Growth Hormone

  • There's confusion about growth hormone and what it's responsible for. Advances in endocrinology have exposed the need for further study.
  • There is a much higher concentration of previously non-researched aggregates of GH than the main isoform 22kDa that was initially studied.
  • Growth hormone is very important in exercise and you should know some of the role in which it and it's super family affect exercise.
  • Release is affected by the type of training, volume and rest periods.
  • Highest growth hormone increase in research came from 10RM and 1 min rest periods.
  • Periodizing resistance training over the menstrual cycle remains to be examined.
  • We aren't close to understanding how this changes over the long term.

Insulin-Like Growth Factors

  • Insulin-like growth factors or IGFs - small polypeptides. It's been assumed that an IGF super family might be an important biomarker for health and performance according to the text.
  • IGF-I - has primarily been studied in exercise because it plays an important role in protein anabolism. Much is unknown about the release of IGFs in response to resistance training.

Adrenal Hormones

  • Plays an important role in the fight or flight response.
  • Is separated into two major divisions; the medulla and the cortex.
  • The medulla is stimulated by the nervous system and responds quickly.
  • The cortex is stimulated by adrenocorticotrophic hormone released from the anterior pituitary.


  • The primary signaling hormone for the metabolism of carbohydrate. Tied to the amount of available muscle glycogen.
  • When glycogen is low the body uses carbs and protein for fuel.
  • Follows a circadian rhythm, highest in the morning and lowest in the evenings.
  • Proteolytic Enzymes - Enzymes that break down proteins.
  • Chronic elevation of cortisol is detrimental but, short term elevation helps the muscle building process by aiding in the removal of damaged proteins.


  • These hormones act as central motor stimulators, peripheral vascular dilators and help enhance enzyme and calcium release in muscle.
  • Training has to be varied in order to give the adrenal glands a break. If it is not, recovery will be delayed and exhaustion can occur-diminishing the ability of the body to release catecholamines.

Other Hormonal Considerations

There are a number of different hormones also responsible for maintaining the bodies normal function and responding to resistance training. They help maintain an optimal environment for other reactions to take place.

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Steven Mack is founder and a Certified Strength and Conditioning Specialist at the private training studio, Simple Solutions Fitness. He consults for Stronger by Science, a leader in fitness research dissemination, and is a former Mizzou football walk-on. Steven dedicates his professional life to helping people through his writing, speaking, and role as a personal trainer.