2/5 exercise phys

Muscle Fiber Types Overview

• Discussion on muscle fiber types focuses on the nomenclature and descriptions in Table 2.1 from the textbook.

• Importance of understanding human expression of muscle fiber genes, specifically the Type IIx gene instead of Type IIb.

Contraction Thresholds

• The stimulus threshold for contraction varies among muscle fiber types.

  • SO (Slow Oxidative) Fibers: Lowest stimulus threshold; require less electrical current to contract.

  • Fast Twitch Fibers: Higher threshold, recruited with increasing intensity rather than speed.

• Recruitment of muscle fibers is intensity-related; as intensity increases, more fast twitch motor units are activated until maximal effort is achieved.

Myosin ATPase

• Myosin ATPase levels are crucial in determining contraction speed.

  • High ATPase: Fast twitch fibers contract faster.

  • Low ATPase: Slow twitch fibers contract slower.

• Differences in myosin isoforms influence the contraction speed and affinity.

Structural Differences Among Fiber Types

• Size of muscle fibers can differ significantly.

  • Notable example: Highly trained endurance athletes may possess larger slow twitch fibers compared to fast twitch fibers.

Mitochondrial and Capillary Presence

• Mitochondria in muscle fibers are vital for aerobic ATP production.

  • SO Fibers: High mitochondrial presence.

  • FOG (Fast Oxidative Glycolytic) and FG (Fast Glycolytic) Fibers: Lower mitochondrial levels.

• Capillary supply is greater in slower twitch fibers, which aids in aerobic energy production.

Glycogen Storage

• Muscle fibers store glucose in the form of glycogen.

  • Both fast and slow twitch fibers have glycogen stores, but the distribution can vary.

  • Mixed fiber types allow for various activities (e.g., walking vs. sprinting).

Neuron Characteristics

• Neurons innervating slow twitch fibers are generally smaller compared to those for fast twitch fibers.

  • Smaller neurons yield slower conduction velocities.

  • Larger neurons associated with fast twitch fibers allow for faster electrical conduction.

Biochemical Differences

• Biochemical factors differentiate fiber types:

  • ATPase Levels: Fast twitch fibers have more, reinforcing explosive actions.

  • Glycolytic Enzymes: Present in fibers favoring anaerobic metabolism.

  • Krebs Cycle Enzymes: Determined presence leads to being labeled as oxidative.

• Staining techniques help identify fiber types based on metabolic characteristics.

Functional Differences

• Force Production: Smaller fibers produce less force than larger fibers.

• Fatigability: Fast twitch fibers fatigue quickly while slow twitch fibers can work longer due to greater fatigue resistance.

Muscle Fiber Classification Techniques

• Muscle Biopsy Technique: Used to determine fiber types in a laboratory setting.

  • Involves a simple surgical procedure to collect a sample of muscle fibers.

• Bergstrom Biopsy Technique: Utilizes staining methods to identify fibers based on myosin ATPase and SDH (Succinate Dehydrogenase) levels.

Cross-Innervation Studies

• Research indicates that the characteristics of muscle fibers can change based on the neuron they are innervated by:

  • Use/Disuse Theory: Suggests that the neuron's characteristics influence the threshold for muscle contraction.

  • Axoplasmic Flow Theory: Proposes that chemical signals from the neurons influence muscle fiber traits.

Conclusion

• Understanding muscle fiber types is crucial for applications in exercise science and rehabilitation.

• Further research is needed to clarify the mechanisms of muscle fiber characteristics as influenced by innervation.