Muscle Fiber Types

Did my 45 minutes of low intensity cardio. Was really bored for the first 15 minutes, but found a few interesting magazine articles, and the time flew by. Afterwards, tried out a new recipe of sweet potato stew with lamb (from the Naked Chef), it was really tasty and healthy too! Tonight, I plan on getting to bed early so I'll be well rested for my leg day tomorrow.

15 Feb 07: Day 32
Had an interesting discussion about muscle fiber types and workouts based on them. It seems that so few people realize the importance of designing your workout around your fiber type.

TYPES OF MUSCLE FIBERS

Humans have basically three different types of muscle fibers.

Slow- twitch (ST or Type I) fibers are identified by a slow contraction time and a high resistance to fatigue. Structurally, they have a small motor neuron and fiber diameter, a high mitochondrial and capillary density, and a high myoglobin content, Energetically, they have a low supply of creatine phosphate (a high-energy substrate used for quick, explosive movements), a low glycogen content, and a wealthy store of triglycerides (the stored form of fat). They contain few of the enzymes involved in glycolysis, but contain many of the enzymes involved in the oxidative pathways (Krebs cycle, electron transport chain). Functionally, ST fibers are used for aerobic activities requiring low-level force production, such as walking and maintaining posture. Most activities of daily living use ST fibers.

Fast-twitch (FT or Type II) fibers are identified by a quick con- traction time and a low resistance to fatigue. The differences in the speeds of contraction that gives the fibers their names can be explained, in part, by the rates of release of calcium by the sarcoplasmic reticulum (the muscle's storage site for calcium) and by the activity of the enzyme (myosin-ATPase) that breaks down ATP inside the myosin head of the contractile proteins. Both of these characteristics are faster and greater in the FT fibers (Fitts & Widrick, 1996; Harigaya & Schwartz, 1969).

Fast-twitch fibers are further divided into fast-twitch A (FT -A or Type IIA) and fast- twitch B (FT -B or Type lIB) fibers.

FT -A fibers have a moderate resistance to fatigue and represent a transition between the two extremes of the ST and FT -B fibers. Structurally, FT -A fibers have a large motor neuron and fiber diameter, a high mitochondrial density, a medium capillary density, and a medium myoglobin content. They are high in creatine phosphate and glycogen and medium in triglyceride stores. They have both a high glycolytic and oxidative enzyme activity. Functionally, they are used for prolonged anaerobic activities with a relatively high force output, such as racing 400 meters.

Fast-twitch B fibers, on the other hand, are very sensitive to fatigue and are used for short anaerobic, high force production activities, such as sprinting, hurdling, jumping, and putting the shot. These fibers are also capable of producing more power than ST fibers. Like the FT -A fibers, FT -B fibers have a large motor neuron and fiber diameter, but a low mitochondrial and capillary density and myoglobin content. They also are high in creatine phosphate and glycogen, but low in triglycerides. They contain many glycolytic enzymes but few oxidative enzymes. Table 1 summarizes some major characteristics of the three fiber types.

Now, if you are really out of shape or very new to exercise and healthy eating, then learning proper resistance training technique, cardio and healthy eating will get you the most results early on.

When you have been exercising for awhile and want to take your progress to another level, you might want to re-design your workout program around your muscle type.

For more info on training right for your body type to maximize your goals; check out Mike Mahler's article of the same name for more info.
Fast vs Slow Twitch Muscle Fiber