The muscle pushes against the other muscle that does the opposite action. Take the bicep for example the bicep pulls while the tricep also pulls. the two muscles cancel each other out, but while still pulling making the muscle more showing.
supraspinatus muscle
All Muscle with Funkmaster Flex - 2006 was released on: USA: 2006
Flex Your Love Muscle - 2012 was released on: USA: 8 January 2012
If you are asking what does someone mean when they say "I have muscle," it is slang for having a muscle "stick out" when you flex it.
hamstrings
Bicep.
The Quadriceps.
triceps brachii
The flex
the muscle is getting bigger because its pushing on the muscle behind it
The Biceps flex the elbow The Triceps extend the elbow
triceps brachii