you have a barbell in your hands and you press it up over your head.
Several things happen at once: First, the muscles of the shoulder (the
deltoids) lift your arms upward; then the muscles at the back of the
upper arm (the triceps) contract and cause the arms to straighten. Any
workout you make, whether pressing a weight overhead, walking, or
simply talking, is the result of any number of complex combinations of
The action of
individual muscle fibers, on the other hand, is quite simple - a fiber
contracts when stimulated and relaxes when the stimulation ceases.
Contraction of an entire muscle is the result of the contraction of many
tiny, individual muscle fibers. Fibers contact on an all-or-nothing
basis. That is, they always contract as hard as they can, or they don't
contract at all. However, after a series of contractions a fiber begins
to get tired and the amount of effort it can generate diminishes. When
you lift a maximum amount of weight one time, you use only a fraction of
the total amount of fiber in the muscle. The amount of weight you can
lift is determined by three things: 1) how much fiber you are able to
recruit; 2) how strong the individual fibers are; 3) your lifting
When you do
only one or two repetitions of a lift, your body never gets a chance to
recruit fresh fiber to replace what is getting weak and tired.
Weightlifters learn to recruit an unusually large number of fibers in
one maximal lift. But they put such an immense strain on those fibers
that the body adapts and protects itself by making those fibers bigger
and thicker. This is called fiber hypertrophy.
No matter how
many fibers the weightlifter involves in one maximal lift, he still uses
fewer than he would if he used less weight and did more repetitions.
Therefore, he trains and strengthens only part of the muscle structure.
Also, the weightlifter does a limited number of different kinds of
lifts, so there are many angles at which the muscle is never trained at
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