The muscular system includes the skeletal muscles and associated connective tissues. There are approximately 640 skeletal muscles within the typical human. However, the exact number is difficult to define because different sources group muscles differently. Sources vary total muscle count from 640 to 850. Skeletal muscle connects one bone to another and when the muscle contracts, one bone usually remains stationary and one bone moves about a joint axis. In some cases the muscle is connected to another muscle or skin and causes motion. Many muscles work together with other muscles to cause a coordinated motion of bone levers. The prime mover is the main muscle that causes a defined motion. Synergists work with the prime mover and may stabilize or assist the prime mover for smooth body motion. Antagonists are muscles that work in the opposite direction of the action of the prime mover. Muscles have stretch receptors and tendons have stretch receptors (Golgi Tendon Organs) that provide feedback on the state of muscle tension and proprioception. An antagonist that is too tight can hinder the prime mover’s performance in an activity. Optimal performance of muscles involves just the right combination of the feature set of prime mover’s, synergists, and antagonists.  The feature set includes muscle tension, muscle strength (force of contraction), muscle relaxation, muscle shortness, muscle length, reactivity to stretch, and weakness or strength due to the length-tension relationship of the muscle. The optimal muscle action also helps joint safety to prevent excessive compression, distraction and wear and tear of a joint.

The muscle belly lies between an origin and an insertion. The origin is attached to the stationary structure, usually a bone. The insertion is attached to the movable structure, usually a bone.

Muscle names originate from the following:

Action:
flexor, extensor, abductor, adductor

Number of origins:
Biceps, Triceps, Quadriceps

Location of origin or insertion:
brachioradialis, sternocleidomastoid

Location:
rectus femoris, brachialis, temporalis

Size:
palmaris longus, extensor pollicis brevis, gluteus maximus

Shape:
Deltoid, Trapezius, Rhomboideus Major, Serratus Anterior

Direction of Muscle Fibers:
Rectus femoris (parallel to femur), external oblique abdominal muscles, transverse abdominal muscles

Keywords:
How many muscles are in the Human Body? About 640.