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Physical exercise physiology

Exercise physiology is the study of how the body's structures and functions change when exposed to acute and chronic repetitions of exercise. Understanding these physiological mechanisms is essential for optimizing performance, preventing injury, and promoting overall health. This article explores how muscles function at the cellular level, which energy systems fuel physical activity, and how the cardiorespiratory system adapts during exercise.

Mechanisms of muscle contraction: the cellular basis of muscle function

Muscle contraction is a complex process involving the interaction of various cellular components within muscle fibers. The basic unit of muscle contraction is the sarcomere, which is composed of interlocking protein filaments—actin and myosin.

Skeletal muscle structure

  • Muscle fibers: Long, cylindrical cells with multiple nuclei and abundant mitochondria.
  • Myofibrils: The bundles of protein filaments in muscle fibers, made up of repeating units called sarcomeres.
  • Sarcomeres: The basic contractile units, defined by Z lines, contain thin (actin) and thicker (myosin) filaments.

Sliding beam theory

The sliding filament theory explains muscle contraction through actin sliding on myosin filaments, causing sarcomere contraction.

  • Calm state: Tropomyosin blocks myosin binding sites on actin filaments, preventing cross-link formation.
  • Excitation-contraction relationship:
    • Action potential: A nerve impulse triggers an action potential in the sarcolemma of a muscle fiber.
    • Calcium release: The action potential propagates through the T-tubules, stimulating the sarcoplasmic reticulum to release calcium ions.
  • Cross-volume formation:
    • Calcium binding: Calcium ions bind to troponin, causing tropomyosin to move and exposing myosin binding sites on actin.
    • Login: Energized myosin heads bind to actin, forming cross-sections.
  • Power stroke:
    • ADP and Pi release: Myosin heads rotate, pulling actin filaments toward the center of the sarcomere.
    • Muscle contraction: This action causes a muscle to contract.
  • Disconnecting the cross-capacity:
    • ATP binding: A new ATP molecule binds to the myosin head, causing it to detach from actin.
    • Reactivation: ATP hydrolysis recovers energy for the myosin heads for the next cycle.
  • Relaxation:
    • Calcium reuptake: Calcium ions are pumped back into the sarcoplasmic reticulum.
    • Blocking login locations: Tropomyosin covers the binding sites again, and the muscle relaxes.

The role of ATP in muscle contraction

  • Energy supply: ATP provides the energy needed for the cross-volume cycle.
  • ATP hydrolysis: The breakdown of ATP into ADP and Pi energizes the myosin heads.
  • ATP regeneration: Muscle fibers regenerate ATP through metabolic pathways to sustain contraction.

Energy systems: ATP-PCr, glycolytic and oxidative pathways

Muscle contractions require a constant supply of ATP.The body uses three main energy systems to regenerate ATP during exercise:

ATP-PCr system (phosphagen system)

  • Direct energy source: Provides energy for high-intensity, short-duration actions (e.g. sprinting).
  • Mechanism:
    • Phosphocreatine (PCr) donates a phosphate to ADP, forming ATP.
    • Ferment: Creatine kinase facilitates this rapid reaction.
  • Characteristics:
    • Anaerobic: No oxygen required.
    • Capacity: Limited by PCr stores, maintains activity for up to 10 seconds.

Glycolytic system (anaerobic glycolysis)

  • Short-term energy source: Fuels moderate to high intensity activity lasting 10 seconds to 2 minutes.
  • Mechanism:
    • Glucose breakdown: Glucose or glycogen is converted to pyruvate.
    • ATP yield: Net ATP content – ​​2 ATP molecules per glucose molecule.
  • Product:
    • Lactose formation: Under anaerobic conditions, pyruvate is converted to lactose.
    • Acidosis: Lactose accumulation lowers pH, contributing to fatigue.
  • Characteristics:
    • Anaerobic: Works without oxygen.
    • Speed: Faster in ATP production than the oxidative system, but less efficient.

Oxidative system (aerobic metabolism)

  • Long-term energy source: Supports activities lasting longer than 2 minutes (e.g., long-distance running).
  • Mechanism:
    • Aerobic glycolysis: Pyruvate enters the mitochondria and is converted to acetyl-CoA.
    • Krebs cycle: Acetyl-CoA is oxidized to produce NADH and FADH₂.
    • Electron transport chain: Electrons are transferred to oxygen, generating ATP.
  • Fuel sources:
    • Carbohydrates: Primary fuel during moderate to high intensity exercise.
    • Fat: Primary fuel during low-intensity, long-duration exercise.
    • Protein: Small contribution, mainly during long exercise.
  • Characteristics:
    • Aerobic: Requires oxygen.
    • Efficiency: Produces up to 36 ATP per glucose molecule.
    • Capacity: Virtually unlimited energy supply during long activities.

Cardiorespiratory and respiratory responses to exercise

Exercise induces significant adaptations in the cardiorespiratory systems to meet increased metabolic demands.

Cardiorespiratory responses

  • Heart rate (HR) increase
    • Mechanism: Sympathetic nervous system stimulation increases HR to improve cardiac output.
    • Impact: HR increases in proportion to exercise intensity.
  • Increasing the shot volume (SV)
    • Definition: The volume of blood pumped during each heartbeat.
    • Mechanisms:
      • To be filled: Increased venous return stretches the chambers (Frank-Starling mechanism).
      • Contractility: Sympathetic stimulation increases contractility of the striatum.
  • Cardiac output (Q) increase
    • Formula: Q = HR × SV.
    • Adaptation: Cardiac output can increase up to 5-6 times the resting level during intense exercise.
  • Redistribution of blood flow
    • Vasodilation: In active muscles, the arteriole dilates, increasing blood flow.
    • Vasoconstriction: Blood vessels in active regions narrow, redistributing blood.
  • Blood pressure changes
    • Systolic pressure: Increases due to higher cardiac output.
    • Diastolic pressure: Gradually does not stand out or decreases slightly.
    • Mean arterial pressure: Moderately increases, maintaining tissue perfusion.

Respiratory responses

  • Ventilation increase
    • Mechanism:
      • Tidal volume: The amount of air taken in during breathing increases.
      • Respiratory rate: The number of breaths per minute increases.
    • Stimuli:
      • Chemoreceptors: Detects increased levels of CO₂ and H⁺.
      • Neural input: Signals from the motor cortex and proprioceptors.
  • Increasing oxygen utilization (VO₂)
    • VO₂ max: Maximum oxygen consumption capacity.
    • Adaptation: Improves due to increased cardiac output and muscle oxygen extraction.
  • Optimization of gas exchange
    • Alveolar ventilation: Improves to facilitate the exchange of oxygen and carbon dioxide.
    • Diffusion capacity: Increases due to increased blood volume in pulmonary capillaries.

Integrated cardiorespiratory adaptations

  • Arteriovenous oxygen difference (a-vO₂ diff):
    • Definition: The difference in oxygen content between arterial and venous blood.
    • Adaptation: Increases during exercise as muscles extract more oxygen.
  • Oxygen supply: Coordinated cardiorespiratory responses ensure adequate oxygen supply to meet muscle needs.

Understanding the physiology of exercise provides insight into how the body responds to and adapts to physical activity. Muscle contraction at the cellular level involves complex processes fueled by ATP, which is regenerated through distinct energy pathways depending on the intensity and duration of the activity. The cardiorespiratory system undergoes significant changes to support the increased metabolic demands, highlighting the body's impressive ability to maintain homeostasis during exercise.

Links

McArdle, WD, Katch, FI, & Katch, VL (2015). Exercise Physiology: Nutrition, Energy, and Human Performance (8th ed.). Lippincott Williams & Wilkins.
Tortora, GJ, & Derrickson, B. (2017). Principles of Anatomy and Physiology (15th ed.). Wiley.
Alberts, B., et al. (2015). Molecular Biology of the Cell (6th ed.). Garland Science.
Hall, J. E. (2016). Guyton and Hall Textbook of Medical Physiology (13th ed.). Elsevier.
Marieb, EN, & Hoehn, K. (2018). Human Anatomy & Physiology (11th ed.). Pearson.
Brooks, GA, Fahey, TD, & Baldwin, KM (2005). Exercise Physiology: Human Bioenergetics and Its Applications (4th ed.). McGraw-Hill.
Hargreaves, M., & Spriet, LL (2006). Exercise MetabolismHuman Kinetics.
Kenney, WL, Wilmore, JH, & Costill, DL (2015). Physiology of Sport and Exercise (6th ed.). Human Kinetics.
Powers, SK, & Howley, ET (2012). Exercise Physiology: Theory and Application to Fitness and Performance (8th ed.). McGraw-Hill.
Berg, JM, Tymoczko, JL, & Stryer, L. (2015). Biochemistry (8th ed.). W. H. Freeman.
Fitts, RH (2008). The cross-bridge cycle and skeletal muscle fatigue. Journal of Applied Physiology, 104(2), 551-558.
Lehninger, AL, Nelson, DL, & Cox, MM (2017). Lehninger Principles of Biochemistry (7th ed.). W. H. Freeman.
Jeukendrup, A., & Gleeson, M. (2010). Sport Nutrition: An Introduction to Energy Production and Performance (2nd ed.). Human Kinetics.
Berne, R. M., & Levy, M. N. (2010). Cardiovascular Physiology (10th ed.). Mosby Elsevier.
Sherwood, L. (2015). Human Physiology: From Cells to Systems (9th ed.). Cengage Learning.
Guyton, AC, & Hall, JE (2015). Textbook of Medical Physiology (13th ed.). Elsevier.
Poole, DC, & Erickson, HH (2011). Cardiovascular function and oxygen transport: Responses to exercise and training. Comprehensive Physiology, 1(1), 675-704.
West, J. B. (2012). Respiratory Physiology: The Essentials (9th ed.). Lippincott Williams & Wilkins.
Forster, HV, & Pan, LG (1994). Contributions of central and peripheral chemoreceptors to the ventilatory response to CO₂/H⁺. Annual Review of Physiology, 56(1), 159-177.
Bassett, DR, & Howley, ET (2000). Limiting factors for maximum oxygen uptake and determinants of endurance performance. Medicine & Science in Sports & Exercise, 32(1), 70-84.

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