Exercise Physiology: Structure and Function of Exercising Muscle

An Introduction to Exercise and Sport Physiology

• Definitions & Core Focus:

  • Anatomy: The study of an organism's physical structure.
  • Physiology: The study of body function through the interaction of organ systems to maintain homeostasis.
  • Exercise Physiology: The study of how the body's functions and structures are altered when exposed to physical activity, as exercise presents a significant disruption to homeostasis.
  • Environmental Physiology: A sub-discipline focused on the body's physiological responses to environmental stressors such as heat, cold, altitude, and humidity.
  • Sport Physiology: The application of exercise physiology principles to train athletes and optimize sports performance.

• Acute vs. Chronic Physiological Responses:

  • Acute Exercise: A single, discrete bout of physical activity.
  • Acute Responses: Immediate physiological adjustments made by the body to meet the increased metabolic demands of an acute exercise bout.
  • Chronic Adaptations (Training Effects): Long-term structural and functional changes in the body resulting from repeated stress over time through exercise training.

• Systemic Integration During Physical Stress:

  • Cardiovascular System: Delivers oxygen ($\text{O}_2$) and nutrient fuels to active skeletal muscles and peripheral tissues while removing metabolic waste products (e.g., carbon dioxide, lactate).
  • Respiratory System: Supplies oxygen to the circulatory system and removes carbon dioxide ($\text{CO}_2$) produced by active tissue metabolism.
  • Integumentary System: Maintains core body temperature by regulating skin blood flow and activating sweat glands to dissipate heat to the external environment.
  • Nervous and Endocrine Systems: Maintain fluid and electrolyte homeostasis, regulate systemic blood pressure, and coordinate metabolic fuel mobilization.

Classification and Macroanatomy of Muscle Tissue

• Three Types of Muscle Tissue:
  • Skeletal Muscle: Voluntarily controlled muscle attached to the skeleton; together with bones, it forms the musculoskeletal system. Skeletal muscle creates mechanical tension, shortens, and lengthens to produce human movement.
  • Smooth Muscle: Involuntary muscle located within the walls of hollow internal organs and blood vessels. Regulates blood vessel diameter (constriction and dilation) to direct blood flow and contracts to move contents through the digestive, urinary, and reproductive tracts.
  • Cardiac Muscle: Involuntary muscle located exclusively in the heart wall. Has inherent autorhythmicity (self-control), modulated by autonomic neural and endocrine inputs. Shares structural features (striations) with skeletal muscle.

• Macroanatomy and Structural Hierarchies of Skeletal Muscle:
  • Entire Muscle: Encased by a outer sheath of dense connective tissue called the epimysium, which binds the muscle together and establishes its shape. The whole muscle contains multiple internal tissue bundles called fascicles.
  • Fascicle (Fasciculus): A bundle of individual muscle fibers surrounded and held together by a connective tissue layer called the perimysium.
  • Muscle Fiber (Muscle Cell): Individual, multinucleated muscle cells housed within a fascicle, enclosed by a delicate connective tissue layer termed the endomysium.
  • Quantitative Scale: Muscle fibers are composed of sub-microscopic myofibrils divided longitudinally into functional units called sarcomeres. The longest human skeletal muscle fibers reach up to ~$12\,m$ ($4.7\,in$) in length, containing approximately $500{,}000$ end-to-end sarcomeres.

Microstructure and Internal Components of Muscle Fibers

• Plasmalemma (Cell Membrane):

  • Outer plasma membrane that fuses with connective tissue cords at the muscle ends to form tendons, which insert into bones to transmit contractile force.
  • Features junctional folds at the motor end plate within the innervation zone to receive neural action potentials.
  • Regulates cell membrane electrical potential, maintains intracellular acid-base balance ($\text{pH}$), and facilitates metabolite transport between capillary blood and the sarcoplasm.

• Satellite Cells:

  • Quiescent myogenic stem cells located between the plasmalemma and the basement membrane.
  • Activated during muscle growth, tissue hypertrophy, injury recovery, immobilization, and chronic strength or endurance adaptation.

• Sarcoplasm:

  • The gelatinous cytoplasm filling the intracellular space within and around myofibrils.
  • Contains high concentrations of stored glycogen, dissolved structural and enzymatic proteins, fats, minerals, intracellular organelles (mitochondria), and myoglobin.

• Transverse Tubules (T-tubules):

  • Lateral invaginations of the plasmalemma running perpendicularly across the muscle fiber.
  • Interconnected network that conducts electrical action potentials from the plasmalemma deep into the interior of the muscle fiber to activate all individual myofibrils.

• Sarcoplasmic Reticulum (SR):

  • A longitudinal network of fluid-filled tubules parallel to myofibrils that loop around each strand.
  • Includes enlarged terminal sacs called terminal cisternae positioned adjacent to T-tubules.
  • Functions as the primary storage and release site for intracellular calcium ions ($\text{Ca}^{2+}$), which are required to initiate muscle contraction.

Microscopic Sarcomere Structure and Myofilaments

• Sarcomere Functional Organization:
  • The basic functional contractile element of a myofibril, extending from one Z-disk to the adjacent Z-disk.
  • Striated appearance arises from overlapping dark and light protein bands:
  • I-band: Light band containing exclusively thin filaments (actin).
  • A-band: Dark band spanning the full length of thick filaments (myosin), including region of thick and thin filament overlap.
  • H-zone: Lighter region in the center of the A-band containing exclusively thick filaments (no thin filament overlap at rest).
  • M-line: Protein structure in the center of the H-zone that serves as the anchor point for thick filaments and stabilizes sarcomere architecture.
  • Z-disk: Dark structural boundary line composed of structural proteins that anchor thin filaments and titin.

• Thick Filaments (Myosin):

  • Represents ~$2/3$ of total skeletal muscle protein content.
  • Each thick filament is composed of roughly $200$ individual myosin molecules.
  • Each myosin molecule consists of two intertwined protein strands that fold at one end into globular heads (myosin heads).
  • Globular myosin heads protrude $360^\circ$ around the filament axis and contain binding sites for actin and adenosine triphosphatase ($\text{ATPase}$) enzymes.

• Thin Filaments (Actin, Tropomyosin, Troponin, Nebulin):

  • Actin: Globular proteins (G-actin) linked into polymer chains that form a double-helical strand backbone; possesses specific active binding sites for myosin heads.
  • Tropomyosin: A long, tube-like protein wrapping around the actin strand; physically covers myosin-binding sites on G-actin when the muscle is at rest.
  • Troponin: A complex of three regulatory protein subunits anchored to both actin and tropomyosin; binds calcium ions ($\text{Ca}^{2+}$) to shift tropomyosin off active sites.
  • Nebulin: An inelastic anchoring protein running alongside actin that regulates thin filament length and alignment.

• Third Myofilament (Titin):

  • A giant elastic protein filament extending from the Z-disk to the M-line (bound to myosin in the A-band, extending freely in the I-band).
  • Functions as an internal spring that increases stiffness upon activation and force production, increasing force when the active muscle is stretched.
  • Winding Filament Theory: Activation by $\text{Ca}^{2+}$ influx alters titin stiffness; titin then winds around thin filaments, rotating them to increase force development.
  • Maintains structural alignment, centers myosin within the sarcomere, and resists passive/active overstretching to prevent structural damage.

Excitation-Contraction Coupling and the Cross-Bridge Cycle

• Motor Units and the Neuromuscular Junction:
  • Motor Unit: A single $\alpha$-motor neuron and all individual muscle fibers it innervates.
  • Neuromuscular Junction (NMJ): The synapse or communication site between an $\alpha$-motor neuron axon terminal and the motor end plate of a muscle fiber.

• Steps of Excitation-Contraction Coupling:
  1. An action potential (AP) originates in the central nervous system and propagates down the axon of an $\alpha$-motor neuron.
  2. AP arrives at the axon terminal, triggering exocytosis of acetylcholine (ACh) into the synaptic cleft.
  3. ACh diffuses across the cleft and binds to ACh receptors on the plasmalemma at the motor end plate.
  4. Binding opens ligand-gated ion channels, causing sodium ($\text{Na}^+$) influx into the cell, depolarizing the membrane and generating a muscle action potential.
  5. AP travels along the plasmalemma and propagates down the T-tubules into the interior of the muscle fiber.
  6. Depolarization reaches the terminal cisternae of the sarcoplasmic reticulum (SR), triggering mass release of stored calcium ions ($\text{Ca}^{2+}$) into the sarcoplasm.
  7. $\text{Ca}^{2+}$ binds to troponin on the thin filament, causing a conformational structural shift that pulls tropomyosin off the active myosin-binding sites on G-actin.
  8. Uncovered active sites allow myosin heads to bind actin, forming cross-bridges.

• Sliding Filament Theory and Sarcomere Dynamics:
  • Relaxed State: Minimal overlapping between filaments; myosin-binding sites covered; no cross-bridge cycling occurring.
  • Contracted State: Myosin heads execute power strokes, pulling thin filaments toward the central M-line, causing filaments to slide past each other.
  • Sarcomere Dimensional Changes During Contraction:
  • A-band length: Remains constant.
  • I-band length: Shortens.
  • H-zone width: Shortens (or completely disappears).
  • Z-disks: Drawn closer together.

• Detailed 6-Step Cross-Bridge Cycle:
  1. Ready State: The myosin head is bound to actin at a tight $45^\circ$ angle relative to the filament axis.
  2. ATP Binding: Adenosine triphosphate ($\text{ATP}$) binds to the myosin head, causing a conformational change that releases myosin from the actin binding site.
  3. ATP Hydrolysis: Myosin $\text{ATPase}$ hydrolyzes bound $\text{ATP}$ into adenosine diphosphate ($\text{ADP}$), inorganic phosphate ($\text{P}_i$), and energy. Energy cocks the myosin head back to a $90^\circ$ angle ($\text{ADP}$ and $\text{P}_i$ remain bound).
  4. Re-attachment: The cocked myosin head binds to a new G-actin molecule further along the thin filament at a $90^\circ$ position.
  5. Power Stroke: Release of $\text{P}_i$ triggers the power stroke; the myosin head tilts back from $90^\circ$ to its original $45^\circ$ angle, sliding the thin filament toward the M-line.
  6. ADP Release: Myosin releases $\text{ADP}$ and remains attached in the $45^\circ$ ready state. Cycling continues as long as sarcoplasmic $\text{Ca}^{2+}$ and $\text{ATP}$ remain available.

• Mechanism of Muscle Relaxation:
  • Neural action potentials cease, ending electrical stimulation of the SR.
  • $\text{Ca}^{2+}$ is actively pumped out of the sarcoplasm back into the SR via SR $\text{Ca}^{2+}-\text{ATPase}$ pumps, requiring energy expenditure.
  • Sarcoplasmic $\text{Ca}^{2+}$ drops; troponin and tropomyosin revert to their resting conformation, re-covering active myosin-binding sites on actin.
  • Energy Dependency: $\text{ATP}$ is required for both contraction (detaching myosin heads and cocking) and relaxation (pumping $\text{Ca}^{2+}$ back into SR).

Skeletal Muscle Fiber Types and Characteristics

• Classification Systems and Fiber Nomenclature:

  • System 1 (Preferred): Type I | Type IIa | Type IIx
  • System 2: Slow twitch (ST) | Fast twitch a (FTa) | Fast twitch x (FTx)
  • System 3: Slow oxidative (SO) | Fast oxidative/glycolytic (FOG) | Fast glycolytic (FG)

• Subtype Proportions and Contraction Times:

  • Type I: Makes up ~$50\%$ of fibers in average human muscle; reaches peak force in ~$110\,ms$ (slow twitch).
  • Type II: Reaches peak force in ~$50\,ms$ (fast twitch).
  • Type IIa: Makes up ~$25\%$ of average muscle fibers.
  • Type IIx: Makes up ~$25\%$ of average muscle fibers.
  • Type IIc: Rare transitional subtype making up ~$1\% \text{--} 3\%$ of muscle fibers.

• Laboratory Methods for Fiber Type Identification:

  • Muscle Biopsy: A small tissue specimen ($10 \text{--} 100\,mg$) is removed via a specialized biopsy needle, frozen, sectioned, and evaluated.
  • Histochemical Staining: Tissue sections are incubated in solutions of varying $\text{pH}$ to evaluate myosin $\text{ATPase}$ activity; different fiber types exhibit distinct shading under light microscopy.
  • Gel Electrophoresis: Separates myosin heavy chain (MHC) protein isoforms by molecular weight.

• Structural and Functional Characteristics Comparison:

• Functional Performance During Exercise:

  • Type I Fibers: Possess high aerobic endurance; efficiently re-synthesize $\text{ATP}$ through oxidative phosphorylation (fat and carbohydrate breakdown); recruited during low-intensity aerobic endurance activities and daily posture/ambulation.
  • Type IIa Fibers: Generate greater contractile force than Type I but fatigue faster due to lower aerobic enzyme content; recruited for high-intensity short-duration endurance efforts (e.g., $1{,}600\,m$ run).
  • Type IIx Fibers: Poor aerobic capacity; rely on anaerobic energy systems; seldom recruited during everyday activities; active during highly explosive maximal power efforts (e.g., $100\,m$ sprint).

• Single-Fiber Peak Power Characteristics:

  • Single-fiber testing isolates individual fibers to measure shortening velocity ($V_0$) and force production.
  • All muscle fiber types achieve their individual peak power at approximately $20\%$ of their peak force output ($\text{Power} = \text{Force} \times \text{Velocity}$).
  • Peak Power Hierarchical Order: $\text{Type IIx} > \text{Type IIa} > \text{Type I}$.

• Distribution and Determinants of Fiber Types:
  • Distribution: Muscle fiber ratios vary widely between individuals, though upper and lower body ratios are generally similar within a single individual. The soleus muscle is composed predominantly of Type I fibers in all humans.
  • Athletic Specialization:
  • Endurance Athletes: Possess high proportions of Type I fibers (e.g., calf gastrocnemius of elite distance runners can exceed $90\%$ Type I).
  • Sprinters/Power Athletes: Possess high proportions of Type II fibers (gastrocnemius of world-class sprinters contains only ~$25\%$ Type I fibers).
  • Note: Fiber composition alone does not guarantee athletic success; cardiovascular capacity, motivation, training habits, and muscle size are critical co-determinants.
  • Determinants of Fiber Ratios:
  1. Genetics: Inherited genes dictate which $\alpha$-motor neurons innervate specific fiber populations, guiding their differentiation into Type I or Type II.
  2. Training Factors: Endurance or strength training can induce small shifts (~$10\%$) in myosin heavy chain isoform expression.
  3. Aging: Aging causes a progressive loss of Type II motor units, leading to a relative shift toward higher Type I fiber proportions.

Motor Unit Recruitment, Force Generation, and Muscle Dynamics

• Orderly Recruitment and the Size Principle:

  • Principle of Orderly Recruitment: Motor units are activated in a fixed, orderly sequence according to force demands.
  • Size Principle: The recruitment threshold is directly dictated by the anatomical size of the $\alpha$-motor neuron.
  1. Smallest motor neurons (Type I motor units) are recruited first at low force demands.
  2. Midsized motor neurons (Type IIa motor units) are recruited next as required force increases.
  3. Largest motor neurons (Type IIx motor units) are recruited last when maximal force or explosive power is required.

• Mechanisms for Controlling Force Production:

  • Total muscle force depends on five primary parameters:
  1. Number and Type of Motor Units Recruited: Activating more motor units or larger Type II units (which contain $\ge 300$ fibers per neuron) yields higher force.
  2. Muscle Size (Cross-Sectional Area): Muscles with a larger cross-sectional area contain more parallel sarcomeres, allowing greater overall force generation.
  3. Rate Coding (Frequency of Stimulation):
     • Rate Coding: The frequency at which neural action potentials are transmitted to individual motor units.
     • Twitch: The smallest contractile force response to a single electrical stimulus.
     • Summation: A series of three electrical stimuli delivered in rapid sequence before full muscle relaxation produces additive force generation.
     • Tetanus: Continual, high-frequency neural stimulation produces maximal sustained tension.

4. Sarcomere Length-Tension Relationship:
   • Optimal Sarcomere Length: Optimal force generation occurs when there is optimal overlap between thick myosin cross-bridges and thin actin binding sites.
   • Overly Shortened Sarcomere: Filaments overlap excessively and collide, reducing cross-bridge formation.
   • Overly Stretched Sarcomere: Filament overlap is reduced or lost, preventing myosin heads from binding actin.

5. Force-Velocity Relationship:
   • Concentric Contraction: Maximal force output decreases progressively as shortening velocity increases. Slower contraction velocities provide more time for cross-bridges to form and produce tension.
   • Maximal Isometric Force: Peak force produced when shortening velocity is zero ($V = 0$).
   • Eccentric Contraction: Maximal force output increases as lengthening velocity increases. Loads exceeding maximal isometric force stretch active cross-bridges, producing elevated mechanical tension.

• Types of Muscle Action:
  • Static (Isometric) Action: Muscle generates mechanical force without changing its overall length or joint angle. Myosin cross-bridges form and recycle without microfilament displacement.
  • Dynamic Action: Muscle generates force accompanied by changes in length and joint angle.
  • Concentric Contraction: Muscle shortens while producing force ($\text{Force} > \text{Resistance}$); sarcomeres shorten as filaments slide toward the M-line.
  • Eccentric Contraction: Muscle lengthens while producing force ($\text{Resistance} > \text{Force}$); cross-bridges remain bound while sarcomeres are pulled longer under tension (e.g., lowering a heavy weight).

Review Questions and Self-Assessment Knowledge Check

• Review Question 1: Exercise physiology emphasizes both acute and chronic responses to physical activity. During an intense bout of exercise performed in a hot environment, which of the following best integrates the primary physiological roles of the cardiovascular, respiratory, integumentary, and endocrine/nervous systems in maintaining homeostasis?

  • Correct Answer: B. The cardiovascular system delivers oxygen and removes waste, the respiratory system supplies oxygen and eliminates carbon dioxide, the integumentary system dissipates heat through skin blood flow and sweating, and the nervous/endocrine systems regulate fluid balance and blood pressure.

• Review Question 2: Which of the following statements most accurately distinguishes the three types of muscle tissue based on location, control, and function?

  • Correct Answer: D. Skeletal muscle is voluntarily controlled and works with bones to form the musculoskeletal system; smooth muscle is involuntary and regulates processes such as blood vessel diameter and movement through hollow organs; cardiac muscle is involuntary but shares some characteristics with skeletal muscle and is modulated by the nervous and endocrine systems.

• Review Question 3: Which statement best describes the structural organization and functional specialization of skeletal muscle in relation to contraction and repair?

  • Correct Answer: A. Skeletal muscle fibers are multinucleated, organized into fascicles surrounded by connective tissue layers, with sarcomeres as the functional unit. T-tubules rapidly conduct action potentials, and the sarcoplasmic reticulum stores $\text{Ca}^{2+}$ for contraction. Satellite cells reside between the plasmalemma and basement membrane, aiding growth and repair.

• Review Question 4: Which of the following correctly identifies the locations and filament composition of the major regions within a sarcomere?

  • Correct Answer: B. The I-band contains only thin filaments, the A-band contains both thick and thin filaments, the H-zone is located in the middle of the A-band and contains only thick filaments, and the M-line is located in the middle of the H-zone and helps stabilize the thick filaments.

• Review Question 5: Which of the following statements most accurately describes the structural and functional roles of the major proteins involved in skeletal muscle contraction?

  • Correct Answer: C. Myosin forms the thick filament and contains globular heads that interact with actin; actin contains myosin-binding sites; tropomyosin covers these sites at rest; troponin helps move tropomyosin; and titin extends from the Z-disk to the M-line, stabilizing myosin while contributing spring-like properties to the sarcomere.

• Review Question 6: Which sequence correctly describes excitation–contraction coupling in skeletal muscle?

  • Correct Answer: C. AP from brain $\rightarrow$ ACh release $\rightarrow$ binds plasmalemma receptors $\rightarrow$ depolarization down T-tubules $\rightarrow$ SR releases $\text{Ca}^{2+}$ $\rightarrow$ $\text{Ca}^{2+}$ binds troponin $\rightarrow$ tropomyosin moves $\rightarrow$ myosin binds actin $\rightarrow$ power stroke $\rightarrow$ $\text{ATP}$ detaches myosin $\rightarrow$ $\text{Ca}^{2+}$ pumped back into SR.