W5-L4: Muscle Types

Question 1

Skeletal muscle contains two main types of fibers that differ in three characteristics. What are they?

Answer 1

1. Primary mechanisms used to produce ATP
2. Type of motor neuron innervation
3. Type of myosin heavy chain expressed
   * The proportions of each type of muscle fiber vary from muscle to muscle and from person to person

Question 2

Human Muscle Fiber Types

Answer 2

Question 3

What 4 factors do Fast-Twitch Fibers (Type II) Exhibit?

Answer 3

1. High capability for electrochemical transmission of AP’s
2. High myosin ATPase activity
3. Rapid Ca2+ release and uptake by efficient SR
4. High rate of crossbridge turnover

Question 4

How do Fast-Twitch Fibers (Type II) compare to slow-twitch fibers?

Answer 4

• Intrinsic speed of shortening and tension development ranges from 3 to 5 times faster than slow-twitch fibers
• Rely on well-developed, short-term glycolytic system for energy transfer

Question 5

When are Fast-Twitch Fibers (Type II) activated?

Answer 5

• Activation predominates in anaerobic-type sprint activities and other forceful muscle actions that rely entirely on anaerobic energy transfer
• Activation plays an important role in stop-and-go or change-of-pace sports such as basketball, soccer, lacrosse, or field hockey

Question 6

What 4 factors do Slow-Twitch Fibers (Type I) Exhibit?

Answer 6

1. Low myosin ATPase activity
2. Slow calcium handling ability and shortening speed
3. Less well-developed glycolytic capacity than fast-twitch fibers
4. Large and numerous mitochondria

Question 7

How do Slow-Twitch Fibers (Type I) compare to fast twitch?

Answer 7

• Generate energy for ATP resynthesis through aerobic system of energy transfer
• Highly fatigue resistant and ideally suited for prolonged aerobic exercise
• Slow-oxidative fibers with slow shortening speed; and rely on oxidative metabolism
• Both slow- and fast-twitch muscle fiber types contribute during near-maximum aerobic and anaerobic exercise

Question 8

Fiber Type Differences in athletes

Answer 8

• Endurance athletes more type I, power athletes more type II (nature vs nurture?)
• Sex differences?

Question 9

Answer 9

Trained twin has more Type 1

Question 10

Sex Differences in Fiber Type?

Answer 10

• A few studies have shown a higher proportions of the more fatigable type II fibers have been found in the vastus lateralis of men compared with women.
• But a sex-based fiber difference hasn’t been confirmed in other muscles, such as the biceps brachii.

Question 11

Summary

Answer 11

1. The number of sarcomeres in parallel is
   the primary determinant of force output
2. Actin and myosin bind in the presence of ATP and
   Ca+ . ‘power stroke’ shortens sarcomere
3. Motor nerve èAchè wave of depolarization è
   T- tubules è SR release Ca+ è Ca+ bind troponin
   è tropomyosin removed from actin binding cite
   ècrossbridge formation
4. Muscles are weaker above and below optimal
   length due to less actin/myosin overlap
5. Specific force: IIx > IIa > I
6. Oxidative capacity I > IIa > II

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