Updated: 6/17/2021

Muscle Biology & Physiology

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  • Overview
    • Key topics of this chapter include
      • Gross anatomy
      • Muscle contraction
      • Muscles type
      • Muscle metabolism
      • Types of contraction
      • Muscle training 
      • Nutritional training
      • 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 vs. Type II muscles
      Type I muscle
      (slow twitch - ST)
      "slow red ox muscles"
      Type II muscle
      (fast twitch - FT)
      Metabolism
      • Aerobic/oxidative
      Anerobic/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 stores
  • 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
      • use of Angiotensin-II receptor blockade increases muscle regeneration after contusion which decreases fibrosis
        • proposed mechanism is IGF-1 blockade decreasing apoptosis cascade

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(OBQ13.206) A 46-year-old bodybuilder experiences a distal biceps tendon rupture while performing a dumbbell biceps curl. This type of injury most commonly occurs in which phase of muscle contraction?

QID: 4841
1

The eccentric phase of this isometric exercise

11%

(347/3204)

2

The concentric phase of this isometric exercise

4%

(114/3204)

3

The eccentric phase of this isokinetic exercise

19%

(593/3204)

4

The concentric phase of this isotonic exercise

11%

(357/3204)

5

The eccentric phase of this isotonic exercise

55%

(1760/3204)

L 3 B

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(OBQ12.109) Progressive overloading of muscles in adults during exercise leads to which of the following?

QID: 4469
1

Increased muscle fiber length

3%

(134/3920)

2

Decreased musculotendinous junction length

1%

(43/3920)

3

Slowed peak contraction velocity

4%

(170/3920)

4

Muscle fiber hypertrophy

89%

(3497/3920)

5

Decreased sarcomere length

1%

(53/3920)

L 1 B

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(OBQ12.181) The cross-sectional area of a muscle is the factor most responsible for which of the following?

QID: 4541
1

Amount of maximal tension

86%

(3449/3998)

2

Speed of contraction

3%

(137/3998)

3

Duration of contraction

3%

(110/3998)

4

Type of contraction

3%

(110/3998)

5

Fatigability

4%

(167/3998)

L 1 B

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(OBQ09.127) A collegiate football player is hit in the thigh by an opposing player's helmet. Radiographs are unremarkable and a clinical image of the injury is shown in Figure A. Which of the following cells appear first at the site of injury?

QID: 2940
FIGURES:
1

Osteoblasts

0%

(5/2079)

2

Neutrophils

90%

(1873/2079)

3

Myoblasts

4%

(85/2079)

4

Lymphoblasts

3%

(66/2079)

5

Eosinophils

2%

(41/2079)

L 1 D

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(SAE07SM.4) The force generated by a muscle is most highly dependent on its

QID: 8666
1

cross-sectional area.

82%

(281/342)

2

fiber type.

6%

(22/342)

3

length.

9%

(31/342)

4

morphology.

1%

(2/342)

5

level of conditioning.

1%

(4/342)

L 2 E

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(SBQ07SM.4) Regarding skeletal muscles, which of the following is true?

QID: 1389
1

Force generated is most dependent on muscle length

5%

(112/2187)

2

Force generated is most dependent on muscle fiber type

2%

(38/2187)

3

Type I muscle is comprised of fast twitch fibrils

6%

(121/2187)

4

Duration and speed of contraction are most dependent on cross-sectional area

13%

(281/2187)

5

Duration and speed of contraction are most dependent on muscle fiber type

74%

(1623/2187)

L 2 C

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(OBQ05.163) While flexing the elbow to perform a biceps curl, what type of muscle contraction is occuring?

QID: 1049
1

Isometric

4%

(35/846)

2

Isokinetic

4%

(37/846)

3

Plyometric

1%

(10/846)

4

Eccentric

5%

(43/846)

5

Concentric

85%

(716/846)

L 1 D

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EXPERT COMMENTS (5)
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