Categories of hormones

Hormones are categorized into three major types based on their molecular structure and mode of action:

1. Steroid hormones

Steroid hormones, such as cortisol and testosterone, are fat-soluble. Because of this, they can diffuse across cell membranes and act inside the cell. Their mechanism involves:

1. Diffusion into the target cell.
2. Binding to intracellular receptors to form a hormone-receptor complex (H-RC).
3. The H-RC binds to DNA, triggering gene transcription.
4. Protein synthesis occurs, leading to muscle adaptation.

Examples:

• Testosterone: Stimulates protein synthesis and muscle growth.
• Cortisol: Regulates metabolism and modulates immune responses.

2. Polypeptide hormones

Unlike steroid hormones, which act more slowly but have longer-lasting effects, polypeptide hormones act rapidly through membrane signaling. These hormones, including growth hormone and insulin, are made up of amino acids and cannot cross cell membranes.

Instead, they bind to membrane receptors and initiate intracellular signaling cascades, such as the JAK/STAT pathway. This leads to:

• Activation of signaling pathways inside the cell.
• Initiation of protein synthesis and metabolic changes.

Examples:

● Growth Hormone (GH): Promotes muscle hypertrophy and repair.

● Insulin: Regulates glucose uptake and storage.

3. Amine hormones

Amine hormones are derived from amino acids (e.g., epinephrine, norepinephrine). Like polypeptide hormones, they act more quickly than steroid hormones, which are slower but produce longer-lasting effects.

Amine hormones bind to membrane receptors and act via secondary messengers, leading to rapid physiological changes. They play a crucial role in the body’s acute stress response.

Examples:

• Epinephrine: Increases heart rate and energy mobilization.
• Dopamine: Modulates motor control and motivation.

Hormonal responses to resistance exercise

Heavy resistance training induces significant endocrine responses that contribute to muscle hypertrophy, strength gains, and recovery. The extent of hormonal changes depends on:

1. Exercise volume and intensity: Higher loads and longer durations stimulate greater hormonal responses.
2. Rest intervals: Shorter rest periods often result in higher acute hormonal elevations.
3. Muscle mass activated: Larger muscle groups elicit a more significant endocrine response.
4. Training experience: Adaptations are more pronounced in trained individuals.

Key hormonal responses

• Anabolic hormones: Testosterone, GH, and insulin-like growth factors (IGFs) promote muscle building.
• Catabolic hormones: Cortisol and catecholamines help with energy mobilization but can lead to muscle breakdown if excessive.
• Acute vs. chronic adaptations: Acute responses occur immediately post-exercise, while chronic adaptations develop over weeks or months of consistent training.

Hormonal changes in peripheral blood

Monitoring hormone levels in the bloodstream provides insights into the physiological stress imposed by exercise. Factors influencing peripheral hormone concentrations include:

• Fluid shifts: Exercise redistributes fluids, altering hormone concentration.
• Plasma volume changes: Lower plasma volume can artificially elevate hormone levels.
• Hormone clearance rates: The rate at which hormones are removed from circulation.

While peripheral blood measures are useful, they must be interpreted with caution. Local tissue responses to exercise can differ from circulating hormone levels, meaning blood concentrations do not always reflect the actual hormonal activity at the target tissues.

Accurate interpretation requires considering the total physiological context, such as neural activation, nutrient availability, and metabolic demands.

Adaptations in the endocrine system

Long-term resistance training leads to adaptations in the endocrine system that improve the body’s ability to manage stress and recover. These adaptations include:

1. Increased hormone sensitivity: Target tissues become more responsive to hormonal signals.
2. Enhanced hormone production: Regular training leads to higher baseline levels of key anabolic hormones.
3. Receptor upregulation: Greater receptor density and affinity improve hormone binding efficiency.
4. Reduced catabolic effects: Training mitigates the negative impact of cortisol and other stress hormones.