Kinesiology: Muscles

Muscle Structure and Function

Functions of Muscles

  • Ventilation:

    • Involves diaphragm and intercostals (involuntary movements).

  • Movement:

    • Skeletal muscles (voluntary, over 650 named muscles).

  • Digestion:

    • Smooth muscle present in organ walls (involuntary).

  • Blood Circulation:

    • Cardiac muscle (heart) and smooth muscle in arterial walls (involuntary).

Skeletal Muscle Structure

  • Muscle Fibers:

    • Composed of cylinder-shaped cells.

    • Contain myofilaments: actin (thin) and myosin (thick).

  • Bundles:

    • Muscle fibers are bundled to form the muscle belly.

  • Sarcolemma:

    • Connective tissue encasing each muscle fiber.

Contractile Machinery

  • Sarcomeres:

    • Basic units of contraction composed of myosin and actin.

  • Length-Tension Relationship:

    • Force generated by muscles depends on their length.

  • All-or-None Principle:

    • Muscle fibers contract completely or not at all.

Sliding Filament Theory

  • Sequence of Events:

    1. Action potential arrives at the neuromuscular junction.

    2. Acetylcholine (Ach) is released and binds to receptors on the sarcolemma (muscle fiber).

    3. Action potential travels along the sarcolemma and down T-tubules.

    4. Sarcoplasmic reticulum (SR) releases calcium ions.

    5. Calcium binds to troponin, allowing myosin head to attach to actin.

    6. Actin filaments are pulled inward (power stroke), causing muscle contraction.

Nerve-Muscle Interaction

  • Peripheral Nervous System (PNS):

    • Comprises motor & sensory neurons.

  • Motor Unit:

    • A motor nerve connected to muscle fibers it controls.

  • Motor Skill Types:

    • Fine Motor Skills: Small motor units (8-50 fibers).

    • Gross Motor Skills: Large motor units (up to 1,500 fibers).

  • Activation Threshold:

    • Stronger neural impulses engage more motor units.

Muscle Fiber Types

  • Fast Twitch (FT, Type II):

    • Quick force generation, fatigue rapidly.

  • Slow Twitch (ST, Type I):

    • Endurance-oriented, slower contraction.

Skeletal Muscle Coordination

  • Intramuscular Coordination:

    • Activation of more fibers within a single muscle.

  • Intermuscular Coordination:

    • Collaboration between multiple muscles.

  • Effects of Strength Training:

    • Trained individuals can activate 85% of muscle fibers.

    • Beginners activate about 60%.

Types of Muscle Contractions

  1. Static (Isometric):

    • No movement, generates force (e.g., planking).

  2. Dynamic Contractions:

    • Concentric: Muscle shortens to generate force (e.g., lifting a dumbbell).

    • Eccentric: Muscle lengthens under pressure (e.g., lowering a weight).

    • Isotonic: Muscle changes length (includes both concentric and eccentric).

    • Isokinetic: Constant speed contractions, often requires machinery.

    • Plyometric: Quick stretch followed by contraction (e.g., jumping exercises).

Factors Influencing Force Output

  • Joint Angle:

    • Optimal length leads to maximum force output.

  • Muscle Cross-Sectional Area:

    • Larger muscles can generate more force.

  • Absolute Strength:

    • Strength influenced by coordination and structure.

  • Relative Strength:

    • Strength compared to body weight.

  • Fiber Type:

    • Higher proportion of FT fibers correlates with greater force.

  • Age:

    • Aging leads to loss of FT fibers (sarcopenia).

  • Sex Differences:

    • Women generally have smaller muscle fibers and more ST fibers; men have higher testosterone levels promoting muscle growth.

Chemistry of Energy Production

Energy Sources

  • Types: Carbohydrates, Fats, Proteins → ATP (Adenosine Triphosphate).

  • ATP Breakdown:

    • Energy release occurs during ATP breakdown.

Three Energy Systems

  1. Phosphagen System (Immediate Energy):

    • Anaerobic

    • Utilizes stored ATP and creatine phosphate (CP).

    • 1 ATP per molecule of glucose

    • Provides energy for 7-12 seconds of high-intensity activity; no lactic acid production.

  2. Glycolytic System (Short-Term Energy):

    • Anaerobic

    • Uses glucose/glycogen

    • 2 ATP per glucose

    • Produces lactic acid (a fatigue factor), supports activity for 1-3 minutes.

    • Training enhances lactic acid clearance.

  3. Oxidative System (Long-Term Energy):

    • Aerobic

    • Involves aerobic metabolism (requires oxygen).

    • Utilizes carbohydrates, fats, proteins.

    • Produces 36 ATP per glucose, 147 ATP per fatty acid.

    • VO2 max indicates maximum oxygen consumption capability.

Energy System Interaction

  • All three energy systems cooperate based on activities' intensity and duration.

Cardiovascular System

Heart Structure

  • Layers of the Heart:

    1. Endocardium: Smooth lining.

    2. Myocardium: Muscular layer responsible for pumping blood.

    3. Epicardium: Protective outer layer.

  • Pericardium: Protective sac surrounding the heart.

Heart Function

  • Sinus Node:

    • Generates electrical impulses for heartbeat.

  • Blood Pressure:

    • Systolic: 120 mmHg during heart contraction.

    • Diastolic: 80 mmHg during relaxation.

Heart Rate & Stroke Volume

  • Stroke Volume:

    • Approximately 70 ml of blood pumped per heartbeat.

  • Heart Rate:

    • 40-70 beats per minute at rest.

  • Cardiac Output:

    • Calculated as Stroke Volume × Heart Rate.

Peripheral Circulatory System

  • Arteries:

    • Transports oxygenated blood away from the heart.

  • Veins:

    • Carries deoxygenated blood back to the heart.

  • Blood Components:

    • Plasma: Transports medium.

    • Red Blood Cells (RBCs): Responsible for oxygen transport.

    • Hemoglobin: Binds and transports oxygen.

    • White Blood Cells (WBCs): Defend against infection.

    • Platelets: Assist in clotting processes.

Oxygen Transport & Utilization

  • A-V O2 Difference:

    • Represents the difference in oxygen content in arteries versus veins.

  • CO2 Removal:

    • Transported as:

      1. Dissolved in plasma (10%).

      2. Bound to hemoglobin (20%).

      3. As bicarbonate in blood (70%).

Training Effects on the Cardiorespiratory System

  • Increases:

    • Heart size, stroke volume, capillary density, red blood cells, hemoglobin, mitochondrial function.

  • Lowers:

    • Resting heart rate, recovery time.

Respiratory System Function

  • Function: Supplies oxygen to blood and removes carbon dioxide.

  • Zones:

    • Conducting Zone: Comprises air passages (trachea, bronchi, bronchioles).

    • Respiratory Zone: Where gas exchange occurs (alveoli).

Ventilation Process

  • Inhalation:

    • Diaphragm contracts, chest expands, air enters lungs.

  • Exhalation:

    • Diaphragm relaxes, air is expelled from lungs.

Gas Exchange

  • Occurs in Alveoli:

    • Oxygen diffuses into blood; carbon dioxide diffuses out.

    • Oxygen transport relies on hemoglobin binding and partial pressure of oxygen (PO2) levels.

Effects of Exercise on the Body

Adaptations

  • Increases:

    • Oxygen transport efficiency, mitochondrial size, endurance capacity, blood volume, capillary density.

  • Reduces:

    • Heart rate, fatigue levels, recovery time.

  • Improves:

    • Fat utilization, lactic acid clearance.