Voluntary muscles can be flexed.
The flex
triceps brachii
This muscle is also used to flex the thigh. The rectus femoris is the only muscle that can flex the hip. Injury to the rectus femoris muscle may result from overuse, as seen with kicking- or sprinting-related injuries. Inflammation of the muscle causes pain in the groin during physical exercises that use this muscle.
gluteal muscle goup
The elbow is a hinge type joint, so it will flex and extend with minimal rotation.
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
triceps brachii
The Biceps flex the elbow The Triceps extend the elbow