Muscle Fiber
How muscles adapt to exercise depends largely on the inherent characteristics of the muscles themselves. Specifically, the types of fibers that make up individual muscles greatly influence the way athletes adapt to training programs. There is a reason why some athletes can sprint faster and get bigger muscles more easily than others, and why some athletes are able to run for much longer periods of time without fatigue. In order to train effectively, it is important to understand some of the complexity of skeletal muscles.
The
body has two basic types of skeletal muscle fibers, slow twitch and fast twitch.
Slow twitch fibers are characterized by a slow contraction time and a high
fatigue threshold. If you can stand a bit of science-speak, they have a small
motor neuron and fiber diameter, a high mitochondrial and capillary density,
high myoglobin content, low supply of creatine phosphate, low glycogen content,
and a wealthy store of triglycerides. All of simply means that slow twitch
fibers are recruited for aerobic athletic activity as well as most
activities in the course of daily living.
Fast twitch fibers are marked by a quick contraction time and a low resistance to fatigue. The difference in the speeds of contraction that gives the fibers their names can be partially explained by the rates of release of calcium and by the activity of the enzyme that breaks down ATP. Both of these processes are faster and greater in the fast twitch fibers.
Fast-twitch fibers are further divided into fast-twitch A and fast-twitch B. FT -A fibers have a moderate resistance to fatigue and represent a transition between the two extremes of the ST and FT -B fibers. They are used for prolonged anaerobic activities with a relatively high force output, such as racing 400 meters.
At any given speed of movement, the amount of force produced depends on the fiber type. During a dynamic contraction, when the fiber is either shortening or lengthening, it has been shown that a FT fiber produces more force than a ST fiber. Under isometric conditions, during which the length of the muscle does not change while it is contracting, ST fibers produce exactly the same amount of force as FT fibers. The difference in force is only observed during dynamic contractions. At any given speed, the force produced by the muscle increases with the percentage of FT fibers and, conversely, at any given force output, the speed increases with the percentage of FT fibers.
There is great variability in the percentage of fiber types among athletes. For example, it is well known that endurance athletes have a greater proportion of slow-twitch fibers, while sprinters and jumpers have more fast-twitch fibers. The greater percentage of FT fibers in sprinters enables them to produce greater muscle force and power than their ST -fibered counterparts
Differences in muscle fiber composition among
athletes have raised the question of whether muscle structure is an acquired
trait or is genetically determined. Studies performed on identical twins have
shown that muscle fiber composition is very much genetically determined, however
there is evidence that both the structure and metabolic capacity of individual
muscle fibers can adapt specifically to different types of training.
Training a FT -fibered muscle for endurance will not increase the number of ST fibers, nor will training a ST-fibered muscle for strength and power increase the number of FT fibers. With the proper training, FT -B fibers can take on some of the endurance characteristics of FT -A fibers and FT -A fibers can take on some of the strength and power qualities of FT-B fibers. However, there is no conversion of fibers. FT fibers cannot become ST fibers, or vice versa. What an athlete is born with is what he or she must live with.
Although the type of fiber cannot be changed from one to another, training can change the amount of area taken up by the fiber type in the muscle. In other words, there can be a selective growth of fibers based on the type of training.
The only way to directly determine the fiber-type composition in an athlete is to perform a muscle biopsy test. Some studies have tried to indirectly estimate the fiber-type composition within muscle groups of an individual by testing for a relationship between the different properties of fiber type and muscle fiber composition
A layman’s method used to determine fiber composition of a muscle-group is to initially establish the greatest weight one can move just once with that muscle-group, then perform as many repetitions at 80% of that weight as possible. Doing fewer than seven repetitions indicates that the muscle group is likely composed of more than 50% FT fibers. Doing 12 or more repetitions shows more than 50% ST fibers. If the athlete can do between 7 and 12 repetitions, then the muscle group probably has an equal proportion of fibers