Molecular Muscle Physiology and Anti-Doping Regulations

Posted on Feb 6, 2026 in Biotechnology

Cellular Fate and the Cell Cycle

Cells can follow four major fates: proliferation (cell division), quiescence (reversible resting state), differentiation (specialization with reduced division), or apoptosis (programmed cell death). These outcomes depend on:

  • Internal factors: Gene expression patterns, regulatory proteins (e.g., p53, pRb), and metabolic status.
  • External factors: Growth factors, hormones, nutrient availability, and cell–cell contact.

Cell Cycle Mechanics

  • G1 phase: Cell growth, normal metabolism, and the decision point to continue the cycle or arrest.
  • S phase: DNA replication; fidelity is monitored by checkpoint machinery.
  • G2 phase: Preparation for mitosis and DNA integrity verification.
  • M phase: Mitosis resulting in two daughter cells.

Cyclins and CDKs

Cyclins rise and fall cyclically to activate specific Cyclin-Dependent Kinases (CDKs). These complexes phosphorylate proteins that drive progression into the next phase. DNA damage causes CDK inhibition to prevent the propagation of faulty DNA.

Cell Cycle Checkpoints

  • G1–S checkpoint: Checks nutrient status, growth factor stimulation, and the absence of DNA damage.
  • G2–M checkpoint: Verifies complete DNA replication and DNA integrity.
  • Spindle checkpoint: Ensures proper chromosome attachment prior to anaphase.

Apoptosis

A tightly regulated “self-destruct” program involving caspases. It is triggered by internal damage (e.g., DNA mutations, oxidative stress) or external signals (death receptors). This leads to chromatin condensation, membrane blebbing, and the formation of apoptotic bodies safely removed by phagocytes.

Myogenesis and Muscle Development

Skeletal muscle formation relies on Myogenic Regulatory Factors (MRFs): MyoD, Myf5, Myogenin, and MRF4.

  • All are bHLH transcription factors that bind to E-boxes (CANNTG).
  • MyoD is the “master regulator” committing cells to the muscle lineage.
  • MEF2 cooperates with MRFs to activate muscle-specific genes.
  • Id proteins inhibit MRF function by preventing proper dimer formation.
  • Differentiation requires cell-cycle exit via CDK inhibitors (p21, p18, p57).

Exercise-Induced Signaling and Adaptations

Insulin signaling: Promotes glucose uptake, glycogen storage, and protein synthesis.

IGF-1 and Muscle Hypertrophy

Mechanical load increases local IGF-1, which activates the PI3K–Akt–mTOR pathway, leading to increased protein synthesis and fiber growth.

Endurance Adaptations

  • AMPK: Activated by increased AMP during exercise; promotes mitochondrial biogenesis.
  • Calcium signaling: Activates CaMK, which stimulates PGC-1α.
  • PGC-1α: Increases oxidative enzymes, mitochondrial number/size, and stimulates VEGF for angiogenesis.

Fiber Type Transitions

Calcineurin dephosphorylates NFAT, promoting slow-twitch gene expression.

Hypoxia Responses

Low oxygen stabilizes HIF-1, which increases EPO (elevating RBC count) and stimulates angiogenesis.

Muscle Damage and DOMS

Eccentric exercise causes mechanical disruption to sarcomeres, mitochondrial swelling, and membrane instability. Delayed-Onset Muscle Soreness (DOMS) peaks at 24–72 hours. Key markers include:

  • Elevated creatine kinase (CK)
  • Reduced force production
  • Swelling and reduced range of motion

The Repeated Bout Effect

Exposure to eccentric work forms structural and neural adaptations that reduce future damage.

Antioxidants and Oxidative Stress

Exercise increases Reactive Oxygen Species (ROS) from the electron transport chain (ETC), immune activity, and calcium-induced mitochondrial stress. Antioxidants counter these effects:

  • Endogenous: SOD, catalase, and glutathione peroxidase.
  • Dietary: Vitamin C, Vitamin E, and carotenoids.

NSAIDs may reduce soreness but show inconsistent effects on CK levels. Estrogen can stabilize membranes and reduce permeability during muscle damage.

Doping Concepts and Strict Liability

SR2002 – Expanded Double-Sided Cheat Sheet (Side B)

What is Doping? Using prohibited substances or methods to unfairly enhance athletic performance. This includes intentional or unintentional ingestion and covers “methods” such as blood manipulation or gene doping.

Strict Liability Principle: Athletes are fully responsible for any banned substance in their body, regardless of intent, source, or knowledge. “Not knowing is not an excuse.”

The 11 Anti-Doping Rule Violations (ADRVs)

  1. Presence of a prohibited substance in a sample.
  2. Use or attempted use of a prohibited substance or method.
  3. Refusal or failure to submit a sample.
  4. Whereabouts failures (three strikes) or missed tests.
  5. Possession of prohibited substances or methods.
  6. Administration or attempted administration to an athlete.
  7. Tampering with doping control procedures.
  8. Trafficking of banned substances or methods.
  9. Complicity (assisting someone in doping).
  10. Prohibited association with banned support personnel.
  11. Retaliation against whistleblowers.

The WADA Prohibited List

Updated yearly by the World Anti-Doping Agency (WADA). Substances may be added at any time.

Substances Prohibited at All Times

  • Non-approved substances: Experimental, veterinary, or designer drugs.
  • Anabolic agents: Exogenous and endogenous steroids.
  • Peptide hormones and growth factors: EPO, GH, IGF-1, etc.
  • Beta-2 agonists: Except specific inhaled therapeutic doses.
  • Hormone and metabolic modulators: SERMs, myostatin inhibitors.
  • Diuretics and masking agents: Furosemide, spironolactone.

Methods Prohibited at All Times

  • Manipulation of blood and components: Blood transfusions and EPO boosting.
  • Chemical and physical manipulation: Tampering and IV infusions beyond limits.
  • Gene and cell doping: Gene transfer, editing, or altering expression.

In-Competition Only Prohibitions

  • Stimulants (amphetamine-like).
  • Narcotics (opioid analgesics).
  • Cannabinoids.
  • Glucocorticoids (systemic routes).

Prohibitions in Particular Sports

  • Beta-blockers: Prohibited in precision sports (e.g., shooting).

Psychological Drivers and Risks of Doping

Why Athletes Dope

Pressure to perform, the belief that others are doping, insecurity, body image concerns, financial gain, or misinformation. Coaches, peers, and the cultural environment may also influence behavior.

Health Risks of Doping

  • Anabolic steroids: Liver damage, infertility, cardiovascular disease, and psychological effects.
  • EPO/blood doping: Increased blood viscosity leading to stroke or heart failure.
  • Growth hormone: Acromegaly, organ growth, and metabolic dysfunction.
  • Diuretics: Severe dehydration, arrhythmias, and kidney failure.
  • Stimulants: Addiction, hypertension, and cardiac arrest.

Challenges in Doping Detection

  • Natural hormone fluctuations (testosterone, EPO, hGH).
  • Diet and training influences on biological markers.
  • Micro-dosing and designer drugs designed to evade detection.
  • Variability across countries and federations regarding threshold levels.