Red Fast Twitch Fibers - Type II a
The red, fast twitch muscle fibers have a greater number of capillaries, and can, therefore,
outlast the white, fast twitch fibers. But they don’t have as much myosin-ATPase and, therefore,
generate less tension. These muscle fibers are able to use either oxidative (aerobic) or glycolytic
(anaerobic) metabolic pathways, and are called Fast Oxidative Glycolytic fibers. The long-distance
Kenyans and some middle distance runners have high proportions of these fiber types.
Red, Slow Twitch Muscle Fibers - Type I
The red, slow twitch muscle fibers have a tremendous number of capillaries which allow for
long-term sustained activity. However, there are fewer fibers in each motor unit, and each fiber
is thinner than its fast twitch cousins, so they produce less strength. To perform well, endurance
athletes must have a high proportion of these. Training can have significant effects on the
capacities of these fiber types, but in most cases cannot change the fiber types.
Coordination of Muscle Fiber Types to Motor units
All three muscle fibers occur within each muscle in genetically predetermined proportions.
The muscles themselves will vary in the proportions of these fibers. For example, a postural
muscle like the Soleus, which is used during standing to keep balance, has a high percentage of
slow twitch fibers; whereas the gastrocnemius, just above it, is used for explosive jumps and is
much higher in fast twitch fibers.
The white, fast twitch muscle fibers have very few mitochondria elements, thus restricting
the duration of its contractions. The red, fast twitch muscle fibers have more mitochondria
elements than the white fibers, meaning it can contract for longer periods prior to failing. The
red, slow twitch muscle fibers have a tremendous number of mitochondria elements, allowing for
sustained contractions for much longer periods than either of the other muscle fiber types.
Skeletal Muscle Tissue
Roughly 40% of body Structure of muscle cell
- Elongated: length of muscle
- Multiple flattened nuclei along outer portion near saracolemma
- Consists of bundles of fibers (myofibrils)
- Bundles of muscle fibers wrapped in connective tissue are called fasciculi
- Nerve connection to muscle belly called motor end plate.
- Myofibrils made of contractile proteins: actin (thin myofilaments) and myosin
(thick myofilaments) - Thin myofilaments also contain proteins troponin and tropomyosin which aid in contraction
- Actin connected to myosin through cross bridges
- Muscles that work together to produce a common movement: synergists
- Muscles that produce opposing movements: antagonists