Hormones and Muscle Adaptations

Hormone Overview

Cortisol is catabolic, promoting proteolysis, inhibiting amino acid anabolism, suppressing the immune system, and promoting fat oxidation. It is elevated during physiological stress like exercise. Testosterone is anabolic, elevating muscle protein synthesis (MPS) and inhibiting proteolysis, and is also elevated post-exercise. Epinephrine (adrenaline) increases heart rate, cardiac output, promotes blood glucose/glycogen breakdown for ATP, and fat metabolism.

Growth Hormone (GH) signals Insulin-like Growth Factor-1 (IGF-1) release, both promoting Muscle Protein Synthesis (MPS). Estradiol (type of estrogen) is potentially anabolic for women, while progesterone is catabolic, promoting proteolysis after ovulation. Thyroid hormones (T<em>3T<em>3 and T</em>4T</em>4) are permissive, allowing other hormones to function.

Hormone Categories

  • Anabolic Hormones: Promote MPS. Examples include Testosterone, Growth Hormone, IGF-1, and Insulin.

  • Catabolic Hormones: Break down muscle tissue. Examples include Cortisol and Progesterone.

  • Permissive Hormones: Enable other hormones to perform their functions. Thyroid hormones are an example.

Muscle Remodeling and Damage

Resistance training causes muscle remodeling by inducing Exercise-Induced Muscle Damage (EIMD), which leads to Delayed Onset Muscle Soreness (DOMS), typically peaking around 4848 hours post-workout. This damage triggers an inflammatory response and activates immune cells, signaling anabolic hormones to initiate repair and growth. The mTOR pathway plays a crucial role in leading to MPS after this damage.

Hormone Action and Receptors

Hormones operate via the Lock and Key Theory, where specific hormones (keys) bind to specific receptors (locks). The number and sensitivity of these receptors can change. Downregulation refers to the decreased number or sensitivity of receptors, reducing a hormone's ability to bind. Conversely, upregulation increases receptor number or sensitivity.

Steroid Hormones

Steroid hormones (e.g., Testosterone, Estrogen, Cortisol) are typically fat-soluble and derived from adrenal glands or sex glands. Their mechanism of action involves diffusion across the sarcolemma, binding to a specific intracellular receptor to form a hormone-receptor complex. This complex then translocates to the nucleus, binding to DNA to influence gene expression, regulating processes like protein synthesis and the development of sex characteristics.