• Introduction
  • The basics of hormonal adaptations to resistance training
  • Testosterone
  • Cortisol
  • Growth hormone
  • Insulin-like growth factor-1
  • Final thoughts
  • Bibliography
  • Amino acid-derived hormones: small molecules derived from the amino acids tyrosine and tryptophan.
  • Cortisol: a steroid hormone in the glucocorticoid class of hormones. Also known as the body’s main stress hormone.
  • Cytoprotection: protection to cells against harmful substances.
  • Endocrine system: the hormone system. Consists of glands and their chemical messengers - hormones.
  • Gonads: primary reproductive organs.
  • Growth hormone: a polypeptide hormone that promotes growth and regulates the metabolism of carbohydrates, proteins, and lipids.
  • Homeostasis: a self-regulating process by which living organisms can maintain internal stability while adjusting to changing external conditions.
  • Hypothalamus: a region of the brain that produces hormones that control several bodily functions.
  • Lipid-derived hormone: hormones mainly derived from cholesterol, with a structure similar to it. The primary class of lipid hormones in humans is the steroid hormones.
  • Negative feedback loop: a type of self-regulating system where the product of a reaction leads to a decrease in another.
  • Peptide hormone: hormones made of short polypeptide chains.
  • Pituitary gland: a pea-sized endocrine gland located at the base of the brain below your hypothalamus.
  • Satellite cell: a type of stem cell found in skeletal muscle.
  • Somatostatin: a polypeptide hormone secreted by the pancreas that inhibits the production of certain other hormones.
  • Testosterone: the primary male sex hormone produced in the gonads.

Introduction

Consistent resistance training offers a variety of structural and functional changes in the body. These adaptations can be divided into two distinct categories; acute (short-term) and chronic (long-term) adaptations. Acute responses mostly take place in the hormonal, neural, and muscular systems. Chronic adaptations, on the other hand, can be observed in the muscular, skeletal, hormonal, cardiovascular, and neurological systems. These aforementioned adaptations are often proportional to the volume, intensity, and frequency of training. When presented with a greater training stimulus than the body is accustomed to, and given sufficient time to recover, the body adapts to the physical demands by becoming stronger. 

Hormones are chemical messengers that regulate the functions of cells, tissues, organs, and systems within the human body. They are secreted by various glands (i.e. hypothalamus, pineal, pituitary, thymus, thyroid, adrenal, etc.) and organs (i.e. ovaries, tested, pancreas) and travel via the blood stream to their target cells. This complex system of glands and organs that regulate functions such as mood, metabolism, homeostasis (a self-regulating process by which living organisms maintain internal stability), growth and development, reproduction, and sleep, is known as the endocrine system.

This post focuses on the hormonal adaptations to resistance training, as well as its significance on strength development.

The basics of hormonal adaptations to resistance training

The hormones of the human body can be divided into three categories according to their chemical structure; 1) lipid-derived, 2) amino acid-derived, and 3) peptide hormones (including peptides and proteins). Most lipid hormones are derived from cholesterol, and have a similar structure to it. The primary class of lipid hormones in humans are steroids, which include reproductive hormones like estrogens and testosterone, as well as cortisol, which plays a part in metabolism. Like cholesterol, steroids are hydrophobic (repelled by water). Because blood is primarily water, steroid hormones must be transported to their target cells via carrier proteins. Steroid hormones can also readily diffuse through the the cell membrane to reach the intracellular hormone receptors, and ultimately allowing them to complete their functions (i.e. regulating protein synthesis). Due to their construction and function, steroid hormones also remain in circulation longer than peptide hormones (i.e. cortisol half-life ~60-90mins vs epinephrine half-life ~1min). 

Amine hormones (e.g. melatonin, dopamine, epinephrine, norepinephrine, etc.) are derived from a single amino acid. Peptide hormones, on the other hand, consist of short amino acid chains, whereas protein hormones are comprised of longer polypeptides. Since amine, peptide, and protein hormones are hydrophilic (water-soluble), they are unable to diffuse through the lipid bilayer of the cell membrane. To reach the cell, these hormones bind to specialized receptors on the outside of the cell membranes, altering their structure and/or charge distribution within the receptor. This activates proteins inside the cell that ultimately carry out the actions specified by the hormone. It is also important to note that few hormones only have a single function, and they often need to interact with other molecules to carry out their function. For example, testosterone, a steroid hormone, requires the presence of insulin-like growth factors to stimulate protein synthesis. This interdependence leads to increased muscle hypertrophy when sufficient training stimulus is present.

There are several hormones that help improve muscular strength and muscular remodelling. These include 1) testosterone, 2) human growth hormones (HGH), 3) insulin-like growth factor-1 (IGH-1), and 4) cortisol. The first three aid in both muscle growth and muscle repair by improving protein synthesis and inhibiting protein degeneration. Cortisol, on the other hand, has a catabolic effect on myofibrillar proteins and suppresses protein synthesis. Thus, promoting protein degeneration. 

Resistance training has been shown to elicit a substantial acute hormonal response, which is considered more important for tissue growth and remodelling than chronic adaptations in resting hormonal concentrations. These anabolic hormones remain elevated for ~15-30mins after exercise provided that training stimulus is sufficient. Several studies have demonstrated improvements in both lean muscle mass and strength despite having little change in hormonal concentrations. However, some studies have also found resting concentrations of testosterone and cortisol, and their ratio, to correlate well with changes in strength and muscle size. Hormonal changes also play a significant role in energy substrate metabolism during recovery.

Resistance training induced hormonal elevations occur in four distinct ways; 1) acute adaptations during and after training, 2) acute and chronic changes to training stimulus, 3) improvements in muscle receptors, and 4) chronic changes in resting hormone concentrations. Additionally, nutritional intake, sex, age, maturity, and training modality affect the endocrine responses to resistance exercise. Volume (total amount of work performed) is considered the most important training variable for muscle growth. The optimal training volume should elicit the highest anabolic (muscle building) stimulus, or the best ratio between anabolic and catabolic hormones. Thus, creating the most favorable conditions for physical adaptations. According to studies, multi-set programs (3-4 sets per muscle group) stressing large muscle groups result in higher acute elevations in testosterone, hGH, and cortisol than single-set programs. Although the acute hormonal response can be varied with training volume, the point at which increasing volume stops inducing a higher hormonal response is yet to be determined. Additionally, studies examining the relationship between training volume and hormonal responses have mostly focused on hypertrophy and not on maximal strength or endurance.

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Hormonal Adaptations To Resistance Training


Heavy resistance training acutely increases secretion of various catabolic and anabolic hormones.Anabolic hormones remain elevated for ~15-30mins after exercise provided that training stimulus is sufficient.Testosterone is an anabolic hormone that stimulates protein synthesis and inhibits protein degradation.Cortisol is a catabolic hormone released in proportion to the intensity of the activity.Growth hormone stimulates stimulates growth of nearly all cells of the body, especially during childhood.Insulin-like growth factor-1 (IGF-1) manages the effects of growth hormone in the body.

Testosterone

Testosterone the strongest naturally secreted steroid androgenic hormone (i.e. a hormone that stimulates male characteristics). Testosterone is mainly produced in the Leydig cells of the testes in males, and ovaries of females, while small amounts of testosterone is also produced by the adrenal glands of both sexes. In muscle, testosterone stimulates amino acid uptake and promotes protein synthesis, while simultaneously inhibiting protein degradation by counteracting cortisol signaling. The amount of testosterone produced by the gonads (reproductive glands) is regulated by the hypothalamus and the pituitary gland. The hypothalamus releases gonadotrophon-releasing hormone (GnRH), which triggers the pituitary gland to release luteinizing hormone (LH), which, in turn, travels to the gonads and stimulates the production and release of testosterone. On the other hand, a higher testosterone concentration suppresses the production of GnHR, and helps maintain normal testosterone levels. 

Testosterone is often considered the main promoter of muscle growth, and subsequent increases in muscle strength. This makes it an essential component in resistance training related physical adaptations. Serum free testosterone has been shown to increase acutely after a bout of resistance training. These findings have been consistent in both males and females, albeit significantly higher among males. Several resistance training variables (volume, intensity, choice or exercises, etc.) can affect acute responses in testosterone concentration. It seems that high-volume, moderate-intensity exercises with short rest periods, and ones that include large muscle groups (i.e. Olympic lifts and compound exercises) result in the largest acute elevations in testosterone. This is thought to be more important for hypertrophy than chronic elevations in testosterone. In summary, a resistance training session should consist of high-volume exercises to increase metabolic demand, and subsequently increase acute testosterone response.

In addition to sex and acute training variables, a person’s age also affects circulating testosterone concentrations. For example, pre-pubescent children do not experience acute increases in testosterone as a result of resistance exercise, whereas after puberty some acute testosterone increases can be found in boys but not in girls. In men, testosterone concentration seems to decline at a yearly rate of 1-3% after the age of 35-40 (andropause), which also leads to diminished acute testosterone response following a resistance training bout. Similarly, testosterone concentration gradually declines in women until menopause (age-related end of monthly menstruation), after which testosterone production is drastically reduced. 

Currently, there is little, or inconclusive evidence showing that resistance training increases blood testosterone concentration in the long term. 

Cortisol

Cortisol is a steroid hormone from the glucocorticoid family produced by two adrenal glands located on top of each kidney. The outer part of each gland is known as the adrenal cortex and the inner part is named the adrenal medulla. Cortisol secretion is mainly controlled by the hypothalamus (an almond-sized gland located below the thalamus and above the pituitary gland) and the pituitary gland (a pea-sized gland located in the brain). This so-called the hypothalamic–pituitary–adrenal (HPA) axis is a major neuroendocrine system, and a key pathway in the communication between the central nervous system and the immune system. The HPA axis is often referred to as the body’s main stress response system. When cortisol levels are low, the hypothalamus releases corticotropin-releasing hormone (CRH), causing the pituitary gland to secrete adrenocorticotropic hormone (ACTH) into the bloodstream. This, in turn, stimulates the production and release of cortisol by the adrenal glands. As cortisol levels rise, the secretion of CRH and ACTH are blocked, leading to reduced cortisol levels. This is known as a negative feedback loop (biological responses that regulates homeostasis).

Although cortisol is widely known as a stress hormone that triggers the ‘fight or flight’ response, it also affects several metabolic processes such as increasing the availability of glucose, free fatty acids, and amino acids from endogenous (internal) sources. Although the metabolic influence of cortisol involves the activation of catabolic processes and suppression of anabolic actions in various cells in the body, these actions are critical for promoting protein synthesis. This is because free amino acids can be ‘recycled’ as building blocks for the synthesis of new proteins, which is an essential step in the process of adapting to external stress. 

Studies have found that resistance training leads to significant increases in acute cortisol levels in both men and women. Regardless of performing resistance training or aerobic exercise, cortisol is released in proportion to the intensity of the activity. As the intensity increases, the concentration of other hormones such as glucagon, adrenaline, noradrenaline, and growth hormone increase, while others decrease (i.e. insulin). Interestingly, cortisol production is significantly higher during prolonged aerobic exercise in comparison to resistance training. This is because cortisol increases the use of alternate fuels during physical exertion (protein and fatty acid breakdown, stimulation of gluconeogenesis, inhibition of glucose uptake, etc.).

For resistance training, it seems that high intensity, high volume training with short recovery periods results in the greatest acute increase in serum cortisol levels. This is primarily due to greater sympathetic activation correlated with increases in heart rate. High-intensity, low-repetition strength protocols with longer rest periods display diminished acute cortisol concentrations. Similarly, the acute increase in cortisol tends to diminish when lower repetitions or longer rest periods are used. Overall, higher intensities tend to produce higher increases in cortisol in comparison to low and moderate training protocols. Interestingly, resistance training protocols showcasing the highest acute increases in cortisol also elicit the highest acute levels of testosterone and growth hormone. 

Growth hormone

Human growth hormone (hGH), also known as a somatotropin, is a polypeptide chain consisting of 191 amino acids produced by somatotropic cells found in the anterior pituitary gland. Human growth hormone is released in an intermittent pulsatile fashion (up to 10 growth hormone bursts per day, each lasting about 90 minutes and separated by 120 minutes) according to circadian rhythm, with the maximal release in the second half of the night. This makes sleep an essential physiological factor in growth hormone release.

Growth hormone production is regulated by two hypothalamic peptides; 1) GH-releasing hormone (GHRH), which stimulates growth hormone synthesis and secretion, and 2) somatostatin, which inhibits growth hormone release. Ghrelin, a peptide mainly produced in the stomach, also helps in the regulation of growth hormone secretion, although its role is not yet fully understood. The amplitude and frequency of growth hormone secreting pulses are dependent on various eternal and internal stimuli, such as age, pubertal status, gender, menstrual cycle phase, nutrition, sleep, body composition, and physical exercise. It is also likely that gonadal hormones (i.e. estrogen, progesterone, and testosterone), insulin, and insulin-like growth factor-I have a mediating effect on the aforementioned factors.

Human growth hormone performs several vital functions in the body. These include regulation of physical growth (turnover of muscle, bone, and collagen), metabolism (insulin secretion, glucose synthesis, lipid metabolism), sleep, body fluid homeostasis, and body composition. Growth hormone also stimulates the production of insulin-like growth factor-I (IGF-1) from the liver. IGF-1 is a hormone that stimulates the uptake of amino acids from the blood. This allows for the formation of new proteins, especially in skeletal muscle cells, cartilage cells, and other target cells. This is especially important after a meal when glucose and amino acid concentrations are elevated in the blood.

Resistance training is the most potent physiological stimulus for growth hormone release in both men and women. GH concentration begins to rise approximately 10-20 minutes into the exercise, and peaks around the end of the training bout before returning to baseline values around 60 minutes after the exercise. It seems that resistance training increases the amplitude of each growth hormone pulse rather than its frequency. The degree of elevation depends on several training variables, such as training intensity, total work performed, and the amount of muscle mass recruited during training. Increasing the volume and intensity of the exercise seems to elevate human growth hormone the most. Similarly, shorter rest periods (30-60s) and multi-set exercises show greater elevation of acute levels of growth hormone when compared to long rest periods and single-set exercises. High-intensity, high-volume exercises with short recovery periods also elicit significant blood lactate elevations in the blood, which has been suggested to be the primary factor in growth hormone secretion.

Insulin-like growth factor-1 

Insulin-like growth factor-1 (IGF-1), also known as somatomedin C, is a polypeptide hormone with a structure similar to insulin. It is typically produced in the liver as a response to growth hormone stimulation. IGF-1 and growth hormone interact in a bidirectional fashion, where growth hormone promotes the synthesis and secretion of IGF-1, which subsequently inhibits the secretion of growth hormone from the pituitary gland through a negative feedback loop. Together, the GH/IGF-1 axis creates a system of mediators, receptors, and binding proteins that regulate glucose homeostasis, tissue growth, and hypertrophy. IGF-1 plays an integral part in promoting cell growth and differentiation in childhood, whilst maintaining an anabolic effect in adults. In addition to its anabolic functions, IGF-1 also has its own antioxidant, anti-inflammatory, and cytoprotective (protection to cells against harmful substances) effects. It is also notable that age-related decline of IGF-1 is associated with health effects such as sarcopenia (age-related loss of muscle mass), cardiovascular disease, and cognitive decline. 

The amount of free serum IGF-1 depends on both the production of growth hormone, as well as on the degree of binding to its carrier proteins. To date, six IGF-1 binding proteins (IGFBP-1 to IGFBP-6) have been identified. These IGFPBs prolong the half-life of IGFs, carries them into circulation, and act as autocrine/paracrine (types of cellular signalling) regulators of their biological actions. Of all binding proteins, IGFBP-3 is by far the most abundant, and therefore considered the major IGF-1 carrier. IGFBP-3 can either promote or inhibit IGF-1 activity. Interestingly, IGFBP-3 can also bind to cells that regulate gene transcription in the nucleus. 

Studies have shown that resistance training results in significant elevations in IGF-1 and IGFBP-3 in both men and women. The increases in IGF-1 concentration usually return to baseline values ~30 minutes after exercise. Participating in regular physical exercise generally increases IGF-1 serum concentrations. This may play an integral part in neurogenesis (formation of new neurons), hypertrophy as well as increasing maximal strength via improved translational efficiency (the rate of mRNA translation into proteins within cells) and satellite cell proliferation in muscle and in the central nervous system. Interestingly, it seems that high-intensity training and prolonged endurance activities can result in long-term decreases in IGF-1 and IGFPB-3. In particular, lower levels of IGFPB-3 seem to be linked to tiredness and indicate excess training. It may therefore be used as a reliable marker for overtraining. 

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Final thoughts

Hormonal adaptations to resistance training occurs in several ways. These responses and adaptations can be divided into four categories; 1) acute adaptations during and after training, 2) acute and chronic changes to training stimulus, 3) improvements in muscle receptors, and 4) chronic changes in resting hormone concentrations. It is well established that resistance exercise acutely increases both anabolic and catabolic hormones. This also seems to play a more important role in protein synthesis and cell remodelling in comparison to chronic changes in hormone levels.

Moderate intensity training with high volume and short rest periods, and exercises utilizing large muscle groups seem to elicit the greatest acute elevations in testosterone, cortisol, growth hormone, and insulin-like growth factor-1. These, in turn, ultimately result in greater increases in hypertrophy and muscular strength.

Did you learn anything new about hormonal adaptations to resistance training? Let us know in the comments.

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