| Neuroendocrine Responses to Resistance Exercise |
| - The type of resistance training workout used dictates the hormonal responses. - Tissue adaptations are influenced by changes in circulating hormonal concentrations following exercise |
| Basic Concepts Synthesis, Storage, and Secretion of Hormones - Hormones o Chemical messengers that are synthesized, stored in, and released by endocrine glands – body structures specialized for secretion – and certain other cells. - Neuroendocrinology o Close integration of chemical substances that have both neural & hormonal functions - Glands release hormones into the circulation, which carries the response information to hormone-specific receptors on target tissues and ultimately to the nucleus of the cell. - Many hormones affect multiple tissues in body. (Testosterone interacts with almost every tissue) Muscle Fiber Remodeling - The most prominent resistance training adaptation in muscle is an increase in the amount of a muscle’s contractile proteins, actin and myosin. - Stimulation of protein synthesis by heavy resistance training allows for both the quality and the quantity of muscle to be altered over a period of time. - The increases in protein are the first steps in muscle growth. - Anabolic hormones o Insulin, insulin-like growth factors, testosterone, & growth hormone promote tissue building - Catabolic hormones o Block the cell or attempt to degrade cell proteins such as cortisol and progesterone - The more muscle fibers involved with the performance of the exercise, the greater the extent of remodeling observed in the whole muscle. The Role of Receptors in Mediating Hormonal Changes - Lock-and-key theory o Given hormone interacts with a specific receptor - Cross-reactivity o Certain receptor accepts hormones that are not specifically designed for it. - Allosteric binding sites o Substances other than hormones can enhance or reduce the cellular response to the primary hormone. - It is usually the receptor or the hormone-receptor complex that transmits the message to the nucleus of the cell. - Down regulation of receptor function o When an adaptation is no longer possible, receptors become non-responsive to the specific hormone that is trying to stimulate that response from the cell Hormone-Receptor Interactions - Steroid Hormone Interactions o Adrenal cortex hormones and those secreted by the gonads, are fat soluble and diffuse across the sarcolemma of muscle fiber o Hormone-receptor complex (H-CR) causing a conformational shift in the receptor and activating it. - Polypeptide Hormone Interactions o Growth hormone and insulin o Not fat-soluble and cannot penetrate the sarcolemma, they rely on second messengers to get their message to the cell nucleus. Heavy Resistance Exercise and Hormonal Increases - Increase in anabolic hormone levels observed consequent to the performance of heavy resistance exercise protocols can result in increased interactions with various cellular mechanisms and enhance the development of muscle protein contractile units. - Acute hormonal secretions provide an abundant amount of information to the body regarding such things as the amount and type of physiological stress (epinephrine), the metabolic demands of the exercise (insulin response), and the need for subsequent changes in resting metabolism. - Among many different responses to force production stress, alterations in sarcolemma permeability to nutrients & sensitivity and synthesis of receptors in muscle cell membrane are affected. - If the stress is too great, catabolic actions in the muscle may occur as a result of the inability of anabolic hormones to bind receptors or by the down regulation of receptors in the muscle tissue - The magnitude of the hormonal response depends on the amount of tissue stimulated, the amount of tissue remodeling, and repair required consequent to the exercise. Thus, the characteristic of the exercise stimulus are again paramount to the response of the body to the exercise protocol - If specific program uses the same exercise, only specific set of muscle fibers in muscle unaffected and without any significant interactions with hormonal factors. Various load patterns and progression schemes. It is only the activated fiber that realizes the benefits of resistance exercise program and utilizes physiological mechanisms, including hormonal mechanisms, to adapt. Mechanisms of Hormonal Interactions - When exercise acutely increases the blood concentrations of hormones, a greater probability of interaction with receptors is possible. However, if physiological function to be affected is already close to a genetic maximum, the receptor will not be sensitive to the increase hormonal exposure. - Since adaptation to heavy resistance exercise typically are anabolic in nature, the recovery mechanisms involved are related to increases in the size of cells - Mistakes in exercise prescriptions can result in a greater catabolic effect or an ineffective exercise program. Hormonal mechanisms will either adversely affect cellular development or minimally activate mechanisms that augment the hypertrophy initiated by the neural recruitment and the force production demands of heavy- intensity resistance exercise. - Neural factors interact with this process too. The integration of the nervous system and the various hormonal mechanisms is different in trained and untrained people Hormonal Changes in Peripheral Blood - While one has to carefully interpret peripheral hormonal responses, they do provide an indication of status or responsibilities of glands or functional status of mechanisms controlled by hormone. - Many different physiological mechanisms may contribute in varying degrees to the observed changes in peripheral blood concentrations of hormones. o Fluid volume shifts § Body fluid tends to shift from the blood to the cells as a result of exercise. o Tissue, especially liver, clearance rates of a hormone § The clearance time of a tissue keeps the hormone out of the circulation and away from contact with target receptors in other parts of the body or can degrade it and make it nonfunctional. o Hormonal degradation o Venous pooling of blood o Interactions with binding proteins in the blood o Receptor interactions Adaptation of the neuroendocrine System - Amount of synthesis and storage of hormones - Transport of hormones via binding proteins - Time needed for the clearance of hormones through hepatic and extrahepatic tissues - Amount of hormonal degradation that takes place over a given period of time - How much of a blood-to-tissue fluid shift occurs with exercise stress - How tightly the hormone binds to its receptor (receptor affinity), which is an uncommon response to exercise training - How many receptors are in the tissue - The magnitude of the signal sent to the cell nucleus - The degree of interaction with the cell nucleus |
| The Primary Anabolic Hormones Testosterone - Used as a physiological marker to evaluate the anabolic status of the body - The effects of testosterone on development on strength and muscle size are related to the influence of testosterone on the nervous system o Testosterone can interact with receptors on neurons and increase the amount of neurotransmitters and influence structural protein changes leading to size changes of the neuromuscular junction - Hypertrophy does not typically take place with endurance training - Independently or in various combination several exercise variables can increase serum testosterone concentrations o Large-muscle group exercises o Heavy resistance (85-95% of 1 RM <repetition Maximum>) o Moderate to high volume of exercise, achieved with multiple sets and/or multiple exercises o Short rest intervals (30 s to 1 min) - Increases in serum total testosterone are evident when blood is sampled before and immediately after exercise protocols that utilize large-muscle group exercise - A recent report by Kraemer et al. suggests that increases may occur if the resistance training experience of high-school-aged males (14-18 years) is 2 years or more. Free Testosterone and Sex hormone-Binding Globuilin - It is only the free hormone that interacts with target tissues. Testosterone Responses in Women - The testosterone hormonal response patterns during credited as the responsible factors in differences in muscular development and strength between men and women. Training Adaptation of Testosterone - It appears that training time and experience may be very important factors in altering the resting and exercise- induced concentrations of this hormone Growth Hormone - The main physiological roles of growth hormone are: o Decreases glucose utilization o Decreases glycogen synthesis o Increases amino acid transport across cell membranes o Increases protein synthesis o Increases utilization of fatty acids o Increases lipolysis (fat breakdown) o Increases availability of glucose and amino acids o Collagen synthesis o Stimulates cartilage growth o Increases retention of nitrogen, sodium, potassium, and phosphorus o Increases renal plasma flow and glomerular filtration o Promotes compensatory renal hypertrophy - The secretion of growth hormone and thus the amount in the blood varies according to time of day, with the highest levels observed at night during sleep. - Secreting pulses also have different amplitudes throughout the day, and exercise appears to increase their amplitude. - Various external factors, such as age, gender, sleep, nutrition, alcohol consumption, and exercise, all alter growth hormone release patterns Release and Binding of Growth Hormone - Growth hormone is released into the peripheral circulation, where it attaches to specific binding proteins which represent the extracellular domain of the growth hormone receptor. Growth Hormone Responses to Stress - Growth hormone levels increase in response to breath holding and hyperventilation alone, as well as hypoxia. - It appears that the stimulus for growth hormone release is increased hydrogen ion concentractions - In resistance exercise a threshold probably exists for intensity when longer rest periods (>3 min) are used to elicit a signification stimulatory response of growth hormone to resistance exercise. - Lighter resistances, while increases in strength are observed, are typically directed toward improveing local musculuar endurance rather than maximal strength and power. - Serum increases in growth hormone are differentially sensitive to different resistance exercise protocols. Growth Hormone Responses in Women - Throughout the menstrual cycle women have higher blood levels of growth hormone compared with men due to greater frequency and amplitude of secretion. Training Adaptation of Growth Hormone - It appears that longer time period (2-24 hr) will need to be measured in order to see if changes in growth hormone levels exist with resistance exercise. Insulin-like Growth Factors - Many of the effects of growth hormone are mediated through small polypeptides called insulin-like growth factors (IGFs), or somatomedins. Exercise Responses of IGFs - It takes 8 to 29 hr for IGF to be produced following stimulation by growth hormone Training Adaptations of IGFs - Responses of IGF-I to heavy resistance training remain unclear |
| Other Hormonal Considerations |
| Adrenal Hormones Cortisol - The major catabolic effects of cortisol in muscle are: o Converts amino acids to carbohydrate o Increases levels of proteolytic enzymes (enzymes that break down proteins) o Inhibits protein synthesis - Cortisol has a greater catabolic effect in fast-twitch muscle fibers than slow-twitch fibers. - In the muscle, the anabolic effects of testosterone and insulin counter cortisol’s catabolic effects. - The acute increases in circulating cortisol following exercise implicate acute inflammatory response mechanisms in the tissue remodeling processes. Resistance Exercise Responses of Cortisol - Cortisol responds to resistance exercise protocols that create a dramatic stimulus to anaerobic metabolism. - As with growth hormone, resistance exercise protocols that utilize high volume, large muscle groups, and shorter rest periods result in elevations in serum cortisol values. - While chronic high levels of cortisol may have adverse effects, acute increases may be a part of a larger remodeling process in muscle tissue. Catecholamines - The catecholamines – epinephrine, norepinephrine, and dopamine – are probably more important for the acute expression of strength than any of the other hormones. - The physiological functions of epinephrine and norepinephrine in muscle: o Increases force production o Increases muscle contraction rate o Increases blood pressure o Increases energy availability o Augments secretion rates of other hormones, such as testosterone. Training Adaptations of Catecholamines - Heavy resistance training has been shown to increase the ability of an athlete to secrete greater amounts of epinephrine during maximal exercise. |