The obvious goal of resistance training is to change the composition of the skeletal muscles. That is, the goal is to increase strength and endurance of the muscles. Just how this happens in the body is rather complex, and knowledge of this process can aid in the construction of an effective training program.47
Changes to muscle composition generally are of two types: 1) Hypertrophy, and 2) changes in muscle subtype.
Muscle Hypertrophy
Muscle hypertrophy is the increase in the cross sectional dimension of individual muscle fibers. In humans, training does not increase the number of fibers, but rather the size of each individual fiber.
Sattelite cells are largely responsible for the beneficial changes. Usually sattelite cells in each muscle fiber are dormant, but are activated in resonse to muscle overload. The sattelite cells then proliferate and multiply, fusing to the existing muscle fibers, donate their nuclei to the fibers, and increase the number of actin and myosin fibers in each muscle fiber. Theis proliferation period can last up to 48 hours after the training stimuli.
A number of growth factors are instrumental in muscle hypertrophy. This includes:
Insulin Like growth factors.
Fibroblast growth factor
Hepatocyte Growth factor
Several hormones also influence skeletal muscle hypertrophy. Growth hormone stimulates Insulin Like Growth Factor and leads to sattelite cell proliferation. In addition testesterone increases protein synthesis, and induces muscle hypertrophy. In contrast, cortisol produced by the adrenal cortex, initiates the breakdown of protein and inhibits muscle hypertrophy.107
Muscle Fiber Sub-Types
In general, muscle fibers in the body are one of three types:
Fiber Type |
Type I |
Type IIb |
Type IIa |
|
Name |
Slow, oxidative, red |
Fast, glycolytic, white |
Fast, oxidative, red |
|
ATPase |
Slow |
Fast |
Fast |
|
Fiber Diameter |
Moderate |
Large |
Small |
|
Glycolytic capacity |
Moderate |
High |
High |
|
Oxidative Capacity |
High |
Low |
High |
|
Example |
Postural muscles of back |
Hand, eye muscles |
Legs in well trained athlete |
Of the muscle types above, it is obvious that for endurance athletes the goal is to maximize type Iia muscle fibers, the question is, how is that done.
To begin, genetics play a large role in the distribution of Type I vs Type II muscle. Unfortunately, conversion of type I fibers to Type II is either difficult or impossible for most athletes. In laboratory animals it involves many days (up to 60) or CONTINUOUS muscle stimulation to convert Type I fibers to Type II.
In
cotrast, it appears that athletes have much more control over the
distribution of Type Iia and Type Iib fibers. It appears that
strenuous exercise of any type, both resistance and endurance
training favors the conversion to Type Iia fibers. Thus, regardless
of genetic factors, the fast muscle fibers in well trained athletes
will be largely Type IIA.
Resistance training changes mostly the diameter of individual muscle fibers, and thus increases the force available with each contraction. This process affects mostly Type II fibers. Increases in size will begin to be noticed after 6 to 8 weeks of strength training. However, changes of Type IIB fibers to Type IIA occure more quickly. Only 2 weeks of resistance training are needed to induce change of fiber sub-type. With time, a resistance training program will cause a near dissapppearance of Type IIB muscle fibers, which are completely replaced by Type IIA. This increase in Type IIA fibers will lead to increased muscle endurance. However, resistance training does not lead to changes in the muscle ability to extract oxygen from the bloodstream.
Conversely, endurance training tends to promote the development of oxygen delivery and use. Changes include increased cappillaries, mitochondria, and oxidative enzymes. Endurance training tends to reduce the size of muscle fibers; this allows greater oxygen uptake by the fiber, even though the strength of each contraction is smaller. Endurance training too increases the proportion of Type IIA fibers.
In conclusion, it is obvious that the athlete should ideally possess a muscle structure in which the maximum number of Type II fibers are Type IIA. A combination of Resistance and Endurance training is the ideal method to attain this.
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