Hormones and their role in exercise

Testosterone

Testosterone is the primary androgenic hormone that interacts with skeletal muscle to promote growth and remodeling. It is crucial for muscle protein synthesis, neuromuscular function, and recovery following resistance training. The testosterone response to exercise is acute and transient, and long-term adaptations depend as much on androgen receptor sensitivity as on circulating hormone levels.

Key functions of testosterone in exercise:

• Enhances protein synthesis and muscle fiber repair.
• Increases neurotransmitter release, promoting greater motor unit activation.
• Regulates anabolic processes that support hypertrophy and performance.
• Binds to androgen receptors on muscle cells to activate muscle growth.

Factors affecting testosterone response:

1. Exercise selection: Large muscle group exercises (e.g., squats, deadlifts) elicit greater testosterone responses.
2. Training intensity: Moderate to high volume resistance training (85-95% of 1RM) stimulates higher testosterone levels.
3. Rest intervals: Shorter rest periods (30 seconds to 1 minute) can enhance hormonal response.
4. Training experience: Individuals with more training years exhibit more robust hormonal adaptations.

Free vs. bound testosterone

Testosterone in circulation exists in both bound and free states. Free testosterone, which is not attached to sex hormone-binding globulin (SHBG) or albumin, is available to interact with muscle cells. Resistance training has been shown to increase free testosterone levels temporarily, enhancing its anabolic effects.

Testosterone responses in women

Women have significantly lower circulating testosterone (15- to 20-fold less than men), yet resistance training can still elicit small but meaningful increases. While most studies show no acute rise in testosterone in women post-exercise, some data suggest increases in free testosterone with high-volume, multi-set resistance exercises.

Total Testosterone (nmol/L) and Free Testosterone (pmol/L) responses to six sets of squats at 80% of 1RM with 2 minutes of rest between sets. The Midpoint (Mid) sample taken after three sets.

Growth hormone

Growth hormone (GH) plays a pivotal role in muscle growth, metabolism, and overall physiological adaptations to resistance training. It is primarily secreted by the anterior pituitary gland and exists in various molecular forms, with the 22 kDa variant being the most commonly studied.

Physiological functions of growth hormone:

GH exerts a wide range of effects on multiple target tissues, including:

• Decreases glucose utilization, promoting greater reliance on fatty acids for energy.
• Increases amino acid transport across cell membranes, enhancing protein synthesis.
• Promotes lipolysis, breaking down fat stores for energy.
• Increases collagen synthesis, supporting joint and connective tissue health.
• Enhances immune cell function, aiding recovery and adaptation.
• Stimulates cartilage and bone growth, which is crucial for long-term training adaptations.
• Increases renal plasma flow, assisting in fluid balance and recovery.

Regulation of GH secretion

GH secretion is tightly regulated by neuroendocrine feedback mechanisms involving:

• Hypothalamus: Releases growth hormone-releasing hormone (GHRH), which stimulates GH secretion.
• Somatostatin: Inhibits GH release to prevent excessive levels.
• Peripheral feedback: Insulin-like growth factors (IGFs) released by the liver provide negative feedback to regulate GH output.

Exercise variables influencing GH release:

1. Exercise intensity and volume: Higher intensity (70-85% of 1RM) and multiple sets elicit greater GH responses.
2. Rest intervals: Shorter rest periods (e.g., ≤1 minute) result in higher GH concentrations compared to longer rest intervals.
3. Metabolic stress: GH secretion correlates with lactate production and the degree of metabolic challenge induced by exercise.
4. Training status: Trained individuals may experience more efficient GH responses due to improved receptor sensitivity.
5. Age: GH secretion declines with age, impacting muscle hypertrophy potential.

Functions of growth hormone in exercise:

• Stimulates protein synthesis and muscle repair.
• Mobilizes free fatty acids for energy during exercise.
• Enhances collagen synthesis, supporting connective tissue integrity.
• Works synergistically with other anabolic hormones like insulin-like growth factor-1 (IGF-1).

Gender differences in GH response:

Women typically exhibit higher baseline GH levels compared to men due to hormonal fluctuations associated with the menstrual cycle. Studies indicate:

• During the follicular phase, women experience higher GH concentrations than in the luteal phase.
• Exercise-induced GH responses are more pronounced with higher exercise intensity and shorter rest periods.

Hormonal adaptations over time

Long-term resistance training leads to both acute and chronic hormonal adaptations, improving the body’s ability to handle exercise-induced stress. Some adaptations include:

• Increased baseline hormone levels: Regular training enhances resting concentrations of anabolic hormones such as testosterone and GH.
• Enhanced receptor sensitivity: Muscle cells become more responsive to hormone signaling.
• Reduced catabolic effects: Training can suppress excessive cortisol release, which helps prevent muscle breakdown.
• Greater hormone efficiency: The endocrine system becomes more adept at managing hormonal fluctuations.

Insulin-like growth factors (IGFs)

IGFs play a critical role in mediating the anabolic effects of GH by promoting muscle protein synthesis and cellular growth. The liver primarily produces IGF-1 in response to GH stimulation, but local muscle production also occurs independently.

Roles of IGFs in exercise adaptations:

• Stimulates muscle hypertrophy by increasing protein synthesis.
• Facilitates recovery by promoting satellite cell activation.
• Enhances glucose uptake, improving energy availability for training.
• Regulates autocrine and paracrine signaling in muscle tissue, driving localized muscle growth.

Binding proteins and IGF availability

IGFs in circulation are bound to specific binding proteins (e.g., IGFBP-3), which regulate their bioavailability and transport to target tissues. Key factors affecting IGF function include:

• Training intensity and volume: Higher workloads can upregulate IGF production.
• Nutritional status: Adequate protein and carbohydrate intake optimize IGF responses.
• Hormonal interactions: GH, insulin, and thyroid hormones influence IGF activity.

Exercise responses of IGF-1:

1. Acute response: Immediate post-exercise increases are often observed with resistance training.
2. Chronic adaptation: Long-term training leads to sustained elevations in IGF-1, contributing to greater muscle hypertrophy.

Training adaptations of GH and IGFs

Over time, consistent resistance training leads to adaptations in GH and IGF-1 secretion, enhancing muscle growth and metabolic efficiency.

Key adaptations observed:

• Increased baseline levels: GH and IGF-1 levels rise at rest with chronic training.
• Improved muscle sensitivity: Target tissues become more responsive to hormonal signaling.
• Reduction in catabolic hormones: Lower cortisol levels help preserve muscle mass.

Training recommendations for optimizing GH and IGF-1 response:

• Utilize high-intensity resistance training with short rest intervals.
• Incorporate progressive overload to maintain hormonal stimulation.
• Ensure adequate recovery to prevent hormonal desensitization.

Mechanisms of hormonal transport and clearance

Hormones exert their effects through complex transport and clearance mechanisms:

1. Synthesis and storage: Hormones are synthesized and stored in glands until triggered by exercise stimuli.
2. Transport via binding proteins: Many hormones, like testosterone, are transported in the bloodstream bound to proteins like SHBG.
3. Receptor affinity: Hormones must bind to specific receptors to exert their effects; receptor availability and affinity can change with training.
4. Clearance and degradation: Hormones are metabolized and cleared through the liver and kidneys, impacting their duration of action.

Cortisol: The primary catabolic hormone

Cortisol is a glucocorticoid hormone released by the adrenal cortex in response to stress, including resistance training. While often viewed as purely catabolic, cortisol is necessary acutely for energy mobilization and stress management but harmful if chronically elevated as it can impair recovery and adaptation.

Functions of cortisol:

• Promotes gluconeogenesis, converting amino acids into glucose for energy.
• Inhibits protein synthesis, leading to muscle breakdown if chronically elevated.
• Suppresses immune function, helping to manage inflammation.
• Regulates fat metabolism, promoting lipolysis during prolonged exercise.

Exercise responses of cortisol:

1. Acute increases: Resistance training, particularly with high volume and short rest intervals, results in transient spikes in cortisol.
2. Chronic adaptations: Over time, well-structured training can reduce baseline cortisol levels and enhance anabolic-to-catabolic balance.
3. Sex differences: Women typically experience greater cortisol responses than men, but their receptor sensitivity and metabolic adaptations may differ.

Cortisol and resistance training adaptations:

• The testosterone-to-cortisol ratio (T:C ratio) is often used as an indicator of an athlete’s anabolic or catabolic state.
• Training-induced cortisol responses should be monitored to avoid overtraining and excessive muscle breakdown.
• Proper nutrition (adequate protein and carbohydrate intake) can help mitigate cortisol’s negative effects on muscle tissue.

Catecholamines and their role in exercise

The adrenal medulla secretes the catecholamines epinephrine, norepinephrine, and dopamine, which play a central role in the body’s acute responses to exercise. These hormones prepare the body for high-intensity effort by rapidly mobilizing energy and enhancing performance capacities. Catecholamines also stimulate other anabolic hormones, including testosterone and growth hormone, which further support training adaptations.

Key functions of catecholamines:

• Increase force production by enhancing neural drive.
• Elevate energy availability by mobilizing glycogen and fat stores.
• Increase cardiac output, improving oxygen and nutrient delivery to muscles.
• Enhance metabolic enzyme activity, supporting ATP production.
• Stimulate other anabolic hormones, such as testosterone and growth hormone.

Training adaptations of catecholamines:

1. Increased secretory capacity: Repeated exposure to high-intensity training improves the adrenal gland’s ability to produce catecholamines.
2. Faster recovery: Well-conditioned athletes clear catecholamines more efficiently, reducing stress and improving readiness for the next session.
3. Reduced stress response: Chronic training leads to more efficient responses to physical stressors, reducing excessive hormonal spikes.

Optimal training strategies for catecholamine release:

• High-intensity training with short rest intervals enhances catecholamine release.
• Multi-joint compound exercises stimulate greater hormonal responses.
• Gradual exposure to higher training loads improves the efficiency of catecholamine function.

Practical considerations

To maximize endocrine adaptations from resistance training, consider the following strategies:

Increasing testosterone levels:

• Utilize compound exercises (squats, deadlifts, power cleans).
• Train at 85-95% of 1RM with multiple sets and short rest periods (30-60 seconds).
• Maintain consistent high-volume training over at least two years.

Enhancing growth hormone secretion:

• Incorporate higher lactate-producing exercises, such as high-rep sets with short rest.
• Ensure a protein-carbohydrate intake pre- and post-workout to support GH release.
• Use progressive overload, increasing intensity systematically over time.

Optimizing adrenal hormone function:

• Vary training protocols to avoid adrenal exhaustion and non-functional overtraining.
• Implement periodization with alternating periods of high and low intensity to allow hormonal recovery.
• Monitor fatigue levels and adjust workload to maintain endocrine balance.

Hormonal regulation of muscle hypertrophy:

Hormonal adaptations to resistance training contribute significantly to muscle hypertrophy. Several key processes are involved:

• Muscle fiber recruitment: Hormones facilitate greater motor unit activation, leading to improved force production.
• Protein turnover: Anabolic hormones like testosterone and GH increase protein synthesis while reducing protein breakdown.
• Satellite cell activation: Hormonal signaling plays a crucial role in muscle regeneration by activating satellite cells.
• Energy mobilization: Hormones such as cortisol and epinephrine provide energy during exercise but must be managed to prevent excessive catabolism.

Considerations for strength coaches

To optimize the endocrine responses to resistance training, strength coaches should:

1. Tailor exercise selection: Prioritize multi-joint movements that engage large muscle groups.
2. Manipulate rest intervals: Use shorter rest periods to maximize hormonal responses.
3. Optimize nutritional strategies: Adequate protein and carbohydrate intake to support anabolic hormone production.
4. Monitor training load: Avoid overtraining, which can negatively impact hormonal balance.
5. Manipulating training variables: Adjusting intensity, volume, and rest to optimize hormonal responses.
6. Recovery optimization: Ensuring sufficient sleep and stress management to regulate cortisol levels.
7. Monitoring biomarkers: Tracking hormone levels as indicators of overtraining or recovery status.

Other hormonal considerations

Several other hormones interact with the endocrine system and play important roles in exercise adaptations:

1. Insulin: Facilitates glucose uptake and glycogen storage post-exercise, promoting recovery and muscle growth.
2. Thyroid hormones (T3, T4): Regulate metabolic rate and energy expenditure.
3. Beta-Endorphins: Help manage pain and stress during prolonged exercise.

Adrenal hormones and their role in exercise:

The adrenal glands play a vital role in the body’s response to exercise stress. They are divided into two main parts:

1. Adrenal medulla: Secretes catecholamines such as epinephrine, norepinephrine, and dopamine in response to immediate stressors (fight-or-flight response).
2. Adrenal cortex: Produces corticosteroids like cortisol, which regulate metabolism and inflammation.