CSCS Study Guide Chapter 4: Endocrine System & Resistance Exercise

Edited by: Danielle Abel

As a strength & conditioning coach, you need to know how resistance training impacts the body from an endocrine perspective. These concepts will help you design periodized training programs promoting positive physiological exercise adaptations. 

General Adaptation Syndrome

Our body responds to stimuli, but this response, eventually fatigues. There are 3 different phases that you need to know as it relates to General Adaptation Syndrome. 

• Phase 1: Alarm phase, where the body mobilizes resources
• Phase 2: Resistance phase, where the body adapts to the stressor
• Phase 3: Exhaustion phase, where resources are depleted

Over time, the body develops general "fitness" when training is periodized, meaning times of higher and lower volume (sets & reps) & intensity (speed, load, etc.). However, if training volume &/or intensity is continuously high or is too high for the individuals based on their current fitness level, overtraining occurs and results in exhaustion and burnout. 

Managing the stress response

There are many variables that the strength & conditioning coach can manipulate to prevent overtraining; here are a few to be mindful of:

• Exercise selection
• Exercise order
• Sets
• Intensity
• Rest periods

Keep in mind, coaches can also leverage supercompensation, which is a short period of time following a peak and taper, where performance is improved.

What the endocrine system does

When you think about the endocrine system, you might think about hormones. Hormones are the communication cells of the body. Hormones travel through the bloodstream and send messages to tissues and organs of the body. 

The main hormones to know:

Anabolic: Promote tissue building

• Testosterone
• Growth Hormone
• IGF-1
• Insulin

Catabolic: Degrade cell proteins

• Epinephrine
• Norepinephrine
• Cortisol

From a timing perspective, testosterone and growth hormone peak within about 2 hours after training. Whereas insulin-like growth factor 1 (IGF-1) peaks about 8 hours following training and stays elevated up to 24 hours following training. 

The amount of hormones released after training depends on the amount of muscle tissue activated, the load (weight) used, and the rest interval between sets and exercises. 

For example, less muscle tissue will be activated in a bicep curl compared to a squat. 

Muscle protein synthesis | How muscle is built

Following disruption or damage to the muscle fibers, the inflammatory response is initiated, which results in the degradation of the damaged muscle tissue. This process is initiated and carried out by hormones. Following the "clean up" of the damaged muscle fibers, amino acids are brought in to be incorporated into the repaired muscle fibers.

As a result of the tissue repair process, more muscle tissue is created, resulting in a net increase in muscle size. 

Keep in mind that hormones can be upregulated and downregulated to either enhance or inhibit the effects of the hormone at binding sites on the tissue. 

• For example, if the maximal amount of protein has been added to a muscle fiber, hormones signaling muscle protein synthesis will be downregulated.  

Upregulation and downregulation can also be influenced acutely or chronically. As with Diabetes, overstimulation of insulin receptors can cause insulin resistance. 

Heavy Resistance Exercise and Hormonal Response 

Lifting heavy results in increases in anabolic hormone concentrations as well as increased membrane and receptor sensitivity to these anabolic hormones. Heavy resistance training can also increase the number of hormone receptors on the surfaces of tissues. 

However, overtraining or exhaustion, as is the case with Phase 3 of the General Adaptation Syndrome, may produce catabolic actions resulting in muscle degradation and downregulation of anabolic receptors. 

Essentially these hormones are communicating to the body the amount and type of stress that has occurred physiologically to the body. Receptors are less sensitive when the following are present:

• Physiological affected function is close to genetic potential for adaptation
• Resting hormone levels are chronically elevated (ie: disease, exogenous drug use, steriods, etc.)
• Mistakes in exercise prescription are made

Acute and Chronic Adaptations to Resistance Exercise

There are several ways we can impact endocrine function through the use of resistance exercise, including but not limited to the following:

• Hormone levels created & stored
• Transportation of binding proteins & hormones
• How much time is required for hormones to exit the liver as well as other tissues
• The amount of degradation of hormones over time
• The amount of receptors found in tissues

Testosterone

Testosterone is the primary androgenic hormone that binds with skeletal muscle. Testosterone promotes the release of growth hormone and also influences the nervous system by increasing neurotransmitters that help increase force production. 

Exercises that recruit large muscle groups have been shown to result in an acute increase in circulating levels of testosterone (at least in men from the research). Additionally, other factors also affect testosterone production:

• Rest periods of 30-60 seconds
• Heavy loads of 85-95% of 1RM
• Large muscle groups
• Moderate to high volume
• Years of chronic resistance training (2 years or more)

Keep in mind Sex Hormone Binding Globulin (SHBG) must not be bound to testosterone since bound proteins cannot interact with target tissues. For hormones like testosterone to interact with target tissues, the hormone must be free, not bound to a protein like SHBG. 

Women have 15-20 times less testosterone than men, so acute increases in testosterone following training are small. In adolescents who have not undergone puberty, testosterone levels are similar between males and females; as a result, strength is similar. 

Growth Hormone

Growth hormone is also an anabolic hormone like testosterone. Growth hormone stimulates insulin-like growth factor 1 (IGF-1), which is heavily involved in muscle protein synthesis. Growth hormone also has the following effects on the body:

• Decreases glucose utilization
• Increases lipolysis/fatty acid utilization
• Increases collagen synthesis

It also interacts with bone, fat cells, skeletal muscle, immune cells, and liver tissue. Growth hormone release changes continuously but is overall increased at night. 

Ways to increase the production & release of growth hormone:

• Using short rest periods of 1 to 3 minutes 
• Lifting moderate to heavy loads around 10RM
• Getting adequate sleep
• Women can leverage increased physiological levels of growth hormone in the 1st portion of their cycle

Insulin-Like Growth Factor-1 (IGF-1)

Another anabolic hormone is IGF-1. IGF-1 increases acutely in the 8 to 24 hours after increased growth hormone. Interestingly, in muscle tissue, IGF-1 is referred to as mechano-growth factor or MGF. 

IGF-1 has been shown to increase with supplementation of protein & carbohydrate timing before and after resistance training. 

Insulin

Insulin promotes the uptake of glucose and amino acids into the cell for energy. After food is consumed, the hormone insulin is upregulated to help move digested food substrates (like amino acids & glucose) from the blood into cells of the body to be used for energy or to be stored for later. 

Insulin actively plays a role in muscle protein synthesis by transporting amino acids into muscle fibers (cells). Additionally, due to insulin's anabolic effect, it also suppresses fat oxidation. 

Keep in mind with Type I Diabetes, the body does not produce insulin, so insulin must be administered for substrates like amino acids and glucose to be transported into the body's cells for energy utilization or to be stored for later. 

• In the case of Type II Diabetes, insulin is not as effective because the cells of the body become resistant to insulin resulting in increased circulating substrates, including glucose and fatty acids. 

Cortisol

Cortisol is produced by the adrenal glands that sit atop the kidneys. From a muscle protein synthesis perspective, cortisol breaks down proteins into amino acids. As a result, these amino acids are converted into glucose and can be used for energy inside the cell. 

Cortisol does the opposite of anabolic hormones and inhibits protein synthesis. It also plays a role in immunosuppression, resulting in a decreased immune system.

Resistance training triggers an acute increase in cortisol that begins the inflammatory cascade that initiates muscle protein synthesis. High-volume training with large muscle groups and short rest periods stimulates a greater cortisol response. 

However, chronically elevated cortisol of greater than 800nmol/L may indicate overtraining. 

Catecholamines

The adrenal glands also produce hormones such as dopamine, norepinephrine, and epinephrine. These hormones are collectively referred to as catecholamines. 

The catecholamines have the following effects on the body:

• Catabolic actions (breaking down of substrates or tissues)
• Increasing energy availability by increasing blood glucose
• Increasing force production 
• Dilating blood vessels within the body (vasodilation)
• Stimulating anabolic hormonal cascade 

The demand of the exercise impacts catecholamines, for example, high intensity of 10RM, short rest periods of 10-60 seconds, and/or high heart rates that correlate with lactate production. 

Using varied training protocols via training periodization allows the adrenal glands to recover and prevents chronically elevated cortisol levels that lead to overtraining symptoms and fatigue.

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