Fast and Slow Muscle Fibers

What changes take place in the characteristics of muscle fibers with conditioning?

Aerobic conditioning and anaerobic conditioning can convert more muscle fibers into intermediate fibers. In aerobic conditoning more slow twitch fibers are produced, in anaerobic conditioning more fast twitch fibers are produced .

Fast fibers (white fibers, fast twitch)

- Contract (reach peak twitch tension) very quickly, strong contractions, they can reach their max tension faster because they can form cross-bridges faster, and their peak tension is higher - Large in diameter - Densely packed myofibrils - Large glycogen stores - Few mitochondria - Fatigue rapidly, use a lot of ATP (go through ATP faster than the body can produce) - Anaerobic metabolism

Muscle atrophy

- Lack of muscle activity - Reduces muscle size, tone and power - Fibers smaller and weaker: proteins broken down and replaced with fibrous tissue (connective tissue) - Initially reversible: If muscle dies then losses are permanent

Intermediate fibers

- Mid-sized Have characteristics of both fast and slow twitch Most closely resemble fast fibers - Few myoglobin (compared to slow twitch) Extensive capillary network, pinkish colors More resistant to fatigue than fast

Muscle hypertrophy

- Muscle growth from heavy training - Increase diameter of muscle fibers - Increase number of myofibrils - Increase mitochondria, glycogen reserves - Occurs in muscles stimulated to near maximal tension

Slow fibers (red fiber, slow twitch)

- ½ diameter of fast - 3X longer to reach peak tension, form less cross-bridges - More mitochondria, less glycogen (make their ATP primarily aerobically, whether contracting or at rest) - Specialized to contract for extended periods, slow to fatigue - Surrounded by extensive capillaries: O2 - Contain red pigment: myoglobin (Binds oxygen)

What are the 3 major types of skeletal muscle fibers?

Fast fibers, slow fibers, intermediate fibers. All long and striated and multinucleated