Skeletal muscles are usually from myotome except the branchiomeric muscles in the head which are from hypomere mesoderm. They a re voluntary muscles and the muscle is striated, the many nuclei are peripheral and the fibres are long, cylindrical and unbranched. Cardiac muscles are striated but involutary. They have branching strands and come from mesenchyme. Muscles can only contract.
Relaxation is the lack of contraction.
They often work in pairs called agonists and antagonists . Some examples are abductors and aductors, flexors and extensors, constrictors and dilators. Cut through the skin of the cat or rat from the pelvic region to the sternum and the limbs. Skin this area by inserting the handle of the scalpel between the skin and the muscles. The muscles are connected to the skin by faschia and muscles attached to the faschia may twitch the skin.