Sensory information is received
and processed in different areas in the brain. After the information is read,
impulses are sent via the spinal cord by motor nerves to the muscles. This
stimulates muscle contractions. The more motor units involved the greater
force produced by each muscle group. The muscles become more efficient through
repetition or practice.
The human body contains
over 215 pairs of skeletal muscles. The thumb alone is controlled
by nine separate muscles!
Muscle Fiber Types
A single skeletal muscle contains
two major fiber types: Slow twitch (Type I) and fast twitch (Type II). Slow
twitch muscle fibers take approximately 110 milliseconds to reach peak tension.
Fast twitch fibers can reach peak tension in about 50 milliseconds.
Slow twitch (Type I) muscle fibers
are highly aerobic. These fibers are very efficient at producing ATP from
the oxidation of carbohydrate and fat. ATP is a high energy compound made
up of adenosine triphosphate. The ATP provides t he cells with a means of
storing and conserving energy in a high energy compound. Type I fibers continually
produce ATP to maintain muscular activity. Because of this, these muscle fibers
are used mainly for endurance activities.
Fast twitch (Type II) muscle fibers
are not useful in endurance activities. These fibers are more useful for anaerobic
activities. Type II muscle fibers generate more force than Type I muscle fibers.
They are well suited for explosive bouts of exercise.
During the first two hours of cross
country skiing, skiers utilize aerobic and anaerobic muscle energy. As time
increases, the utilization of anaerobic muscle energy decreases. For example,
for the first minute of skiing, the skier uses 70% of anaerobic energy (Type
II muscle fiber) and at one hour the skier uses only 2% of anaerobic muscle
energy.
The opposite is true for the utilization
of aerobic energy use. For the first minute of skiing, the skier uses 30%
of aerobic energy (Type I muscle fibers) and at one hour, the skier is using
98% of aerobic energy. In the graph, you can see that for two hours of cross
country skiing almost 100% of the energy used is aerobic or Type I muscle
fibers.
