I was under the impression that both ends of a muscle contract. For instance, the fibers of the biceps run parallel to the humerus so I thought they pulled toward the middle.
But now I'm confused because it sounds like the contraction of the fibers doesn't necessarily have to parallel the motion of the muscle itself. In other words, just because the fibers contract doesn't mean that the muscle lengthens or shortens in a particular direction.
Can someone clarify if muscles contract in both directions?
Or perhaps a better question is do muscle contract in a particular direction and is this the same direction that the fibers contract or does the muscle as a whole move in a different direction in comparison to the direction the fibers are contracting?
Answer
The muscular fibers (or more exactly the actin and myosin filaments) are contracted towards each other. This makes them "move to the middle" and you build up a force on both sides of the muscle. See this image (from the Wikipedia), which illustrates this process:
There is also an animated picture available, which illustrates this process even better (from here):
The process of the contraction applies a equal force to both endpoints of the muscle.
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