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Overview
  • Key topics of this chapter include
    1. Gross anatomy
    2. Muscle contraction
    3. Muscles type
    4. Muscle metabolism
    5. Types of contraction
    6. Muscle training
    7. Nutritional training
    8. Muscle injury
Gross Anatomy
  • Myotendinous junction
    • weak link in muscle and often site of tears (especially with eccentric contraction)
    • involution of muscles cells maximized surface area for attachment
  • Noncontractile elements
    • Epimysium surrounds muscle bundles
    • Perimysium surrounds muscle fascicles
    • Endomysium surrounds individual fibers
Muscle Contraction
  • Contractile elements
    • derived from myoblasts
    • the muscles fiber (an elongated cell) is the basic unit of contraction
    • a myofibril is a collection of sarcomeres
  • Sarcomere composition
    • filaments
      • thick myosin filaments
      • thin actin filaments
    • bands
      • H band is myosin only
      • I band is actin only
      • A band is both actin and myosin
      • Z line flanks each sarcomere and acts as site of attachment for actin filament
      • during muscle contraction
        • A band stays the same length
        • I band reduces in length
        • H zone reduces in length
  • Action stimulation
    • nerve cell body delivers electrical signal to motor endplate (junction between muscle and nerve)
      • nerve action potentials are started with passage of sodium ions through voltage gated channels 
    • Ach is released and diffuses across synaptic cleft to bind to Ach receptor
      • myasthenia gravis patient has shortage of Ach receptors
      • botox blocks release of Ach from end plate
    • Ach binding triggers depolarization of sarcoplasmic reticulum and release of calcium into muscles cytoplasm
    • excitation-contraction coupling  
      • in low calcium environment
        • tropomyosin blocks myosin-binding sites on actin 
      • in high calcium environment
        • calcium binds to troponin (on thin filaments) leading to a configuration change of tropomyosin (on thin filaments) 
        • exposing myosin-binding sites on actin filament
        • actin forms cross-bridges to myosin, and the ATP breakdown, the two fibers contract past one another
  • Types of muscle contraction
    • isometric
      • muscle contracts with constant length (e.g. pushing against an immovable object)
    • isokinetic
      • muscle contracts with constant speed (requires specific equipment like cybex machines)
    • plyometric
      • rapid lengthening followed by contraction of muscle groups (e.g. jumping up and down on boxes)
    • isotonic - muscle contract with constant tension
      • concentric
        • muscle shortens during contraction (e.g. biceps curl) 
      • eccentric
        • muscle lengthens during contraction (e.g. "negative" of a biceps curl)
  • Force generation
    • force generated is most dependent on muscle cross-sectional area  
    • muscle fiber size increases with strength conditioning 
  • Contraction speed
    • duration and speed of contraction is most dependent on fiber type
Muscle Types  

 
Type I muscle 
(slow twich - ST)
"slow red ox muscles"
Type II muscle
(fast twitch - FT)
Metabolism • aerobic / oxidative • anaerobic / glycolytic
Energy source
• Aerobic system (oxidative phosphorolation via Krebs cycle)
• ATP-CP system
Exercise duration
• endurance (distance running) 
• low strength of contraction 
• low speed of contraction 
• first to atrophy with deconditoning
• high strength of contraction 
• high speed of contraction (large force generation per cross sectional area) 
• fatigue rapidly 
• sprinting is example
Note

• high yield ATP
• requires O2 and thus more vascular 
• increase mitochondria in cells 

• high yield ATP (increased ATPase)
• low intramuscular triglycerine stor
es

Metabolic Systems
  • Three systems are used to generate energy for muscles
    • ATP-CP anaerobic system
      • (adenosine triphosphate-creatinine phosphate system, "phosphagen system") 
        • basis for creatine phosphate supplementation (main side effect: muscle cramping)
      • used for intense metabolic exercise lasting less than 20 seconds (e.g., 100 meter sprint)
      • converts carbohydrates stored within muscle into energy
      • anaerobic (does not use oxygen and does not produce lactate)
      • formulas
        • ATP –» ADP + P + energy
        • ADP –» AMP + P + energy
    • lactic anaerobic system (lactic acid metabolims)
      • intense muscle activity lasting 20 to 120 seconds (e.g., 400 meter sprint)
      • involves hydrolysis of one glucose molecule
      • formula
        • glucose –» lactic acid + energy
    • aerobic system
      • used in longer duration and lower intensity exercises
      • Krebs cycle generates ATP from glucose and fatty acids through oxidative phosphorylation
Muscle Injury
  • Muscles soreness
    • caused by edema and inflammation in the connective tissue
      • neutrophils are the most abundant cells early on after acute injury 
        • generates free radicals that possibly increase muscle damage
    • worse with unaccustomed eccentric exercise
    • peaks at 24-48 hours
    • elevated CK levels seen in serum
  • Muscles strain
    • occur at myotendinous junction (off during eccentric contraction which produces highest forces in skeletal muscle)
    • pathoanatomy in inflammation followed by fibrosis
  • Muscle atrophy
    • caused by disuse or nerve injury
    • leads to fatty infiltration and increased fatigability 
    • muscles crossing a single joint atrophy faster
    • loss of cross-sectional area leads to decreased force generation 
 

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