Treppe and Muscle Contraction

• Treppe

• Repeated stimulations immediately after relaxation phase:

  • Stimulus frequency < 50/second
  • Causes a series of contractions with increasing tension
  • Likely due to an increase in Ca²⁺ ions in the sarcoplasm

• Wave Summation

• Repeated stimulations before the end of the relaxation phase:

  • Stimulus frequency > 50/second
  • This increase in tension is called summation of twitches or wave summation
  • Causes a second, more powerful contraction

• Incomplete Tetanus

• Rapid cycles of contraction and relaxation while the muscle is not allowed to completely relax

• Muscle produces almost peak level of tension

• Complete Tetanus

• If stimulation frequency is high enough, muscle never begins to relax and is in continuous contraction

• High levels of Ca²⁺; relaxation phase is eliminated

• Tension Production in Whole Skeletal Muscles

• Depends on:

  • Tension produced by stimulated muscle fibers
  • Total number of muscle fibers stimulated
  • Frequency of stimulation

• Motor Units in Skeletal Muscle

• Motor Unit: all muscle fibers controlled by one motor neuron

  • Contains few to hundreds of muscle fibers contracting simultaneously
  • Fibers of one motor unit intermingled with those of other motor units

• Recruitment (Multiple Motor Unit Summation)

• In whole muscle, smooth motion and increasing tension achieved by slowly increasing the size or number of motor units stimulated

• Maximum tension when all motor units reach tetanus, sustained for a short time

• Allows for rotation and recovery of motor units

Muscle Tone and Contractions

• Muscle Tone

• Normal tension and firmness at rest

• Actively maintains body position without motion

• Increased muscle tone increases metabolic energy used even at rest

• Types of Skeletal Muscle Contractions

1. Isotonic Contraction
   • Muscle changes length with rising tension, resulting in motion
   • If muscle tension > resistance, muscle shortens (concentric contraction)
   • If muscle tension < resistance, muscle lengthens (eccentric contraction)
   • Example: Biceps Brachii
2. Isometric Contraction
   • Muscle develops tension but does not change length
   • Cannot overcome load of the weight

Resistance, Muscle Relaxation, and Muscle Action

• Resistance (Load) and Speed of Contraction

• Heavier resistance:

  • Longer time for shortening to begin
  • Slower contraction speed (e.g., biking uphill)

• Muscle Relaxation

• After contraction, muscle returns to resting length by:

  • Elastic forces (tendons and ligaments pulling on sarcomeres)
  • Opposing muscle contractions (reverse motion)
  • Gravity (assists or replaces opposing muscle contraction)

• Muscle Action

• Prime Mover (Agonist): main muscle responsible for movement

• Synergist: assists the prime mover

• Antagonist: opposes the action of the prime mover

ATP and Muscle Contraction

• ATP in Muscle Contraction

• Sustained contraction requires significant ATP

• Muscle fibers store energy to start contraction

• Must produce more ATP as needed

• ATP and CP Reserves

• ATP: active energy molecule

• Creatine Phosphate (CP): storage molecule for excess ATP

  • ATP + Creatine → ADP + Creatine Phosphate
  • Recharging ATP from ADP:
  • ADP + creatine phosphate → ATP + creatine

• ATP Generation

• Aerobic Metabolism: primary energy source at rest, producing 34 ATP per glucose via Krebs Cycle

• Anaerobic Glycolysis: primary source during peak activity when O₂ is low, producing 2 ATP from glucose

Muscle Fatigue and Recovery

• Muscle Fatigue

• Can no longer perform required activity

• Resulting in:

  1. Depletion of metabolic reserves
  2. Damage to sarcolemma and SR
  3. Low pH (lactic acid buildup)
  4. Muscle exhaustion and pain

• Recovery Period

• Time required after exertion for muscles to return to normal

• Oxygen availability resumes mitochondrial activity; lactic acid is recycled

• Cori Cycle

• Removal and recycling of lactic acid by the liver

• Liver converts lactic acid to pyruvic acid, releasing glucose for muscle glycogen

• Oxygen Debt (EPOC)

• Body’s need for more O₂ post-exercise to restore normal metabolic activities

Hormones and Muscle Metabolism

• Influencing Hormones
• Growth Hormone, Testosterone, Thyroid Hormones: elevate energy consumption
• Epinephrine: stimulates muscle metabolism and increases contraction force

Muscle Performance

• Key Components of Muscle Performance
• Force: maximum tension produced
• Endurance: duration an activity can be sustained
• Strength: ability to produce enough force to move an external load
• Power: strength x speed
• Performance depends on muscle fiber types, conditioning, and genetics

Types of Skeletal Muscle Fibers

1. Fast Fibers

• Contract quickly, large diameter, few mitochondria
• Fatigue quickly, supported by anaerobic metabolism
• Known as white muscle fibers, fast-twitch glycolytic fibers (Type II-B)

1. Slow Fibers

• Contract slowly, fatigue slowly, richer in mitochondria
• Surrounded by capillaries, contain myoglobin (bonds O₂)
• Known as red muscle fibers, slow-twitch oxidative fibers (Type I)

1. Intermediate Fibers

• Mid-sized, less myoglobin than slow fibers
• Less fatigue than fast fibers, known as fast-twitch oxidative fibers (Type II-A)

Hypertrophy vs. Atrophy

• Hypertrophy

• Increase in diameter and number of myofibrils, more mitochondria, larger glycogen reserves

• Atrophy

• Decrease in muscle size and strength due to disuse

Physical Conditioning

• Improvement in Power and Endurance

• Differentiate between aerobic (endurance) and anaerobic (strength) activities

• Warm-up Activities

• Essential for flexibility and injury prevention

• Anaerobic Endurance

• Use fast fibers, improve through brief, intense workouts and hypertrophy

• Aerobic Endurance

• Supported by mitochondria, improved through repetitive and cardiovascular training

• Purpose of Warm-up

• Prepares muscles for exertion and enhances flexibility