 In this video, I will describe the arrangement of muscle fascicles and various muscle types, parallel, convergent, penne, and circular fascicle arrangements, and describe how skeletal muscles are named, and define the prime mover, synergist, and antagonist. This illustration shows us seven different general shapes of muscles, resulting from the alignment of fibers and fascicles within those muscle organs. A parallel fascicle arrangement example shown here is the sartorius, a muscle found running on the anterior of the thigh, diagonally from the origin at the anterior superior iliac spine, down to the insertion on the tibia near the medial condyle. Having a parallel fascicle arrangement enables a wide range of motion as this muscle contracts, and notice its parallel non-fusiform, meaning that it's roughly the same width near the tendons as it is in the center of the muscle. In contrast, biceps brachii found on the anterior brachial region has a parallel fusiform arrangement where the muscle is wider in the center of the belly and tapered towards the tendons. This enables a larger number of muscle fibers to be packed into this muscle organ to produce a larger amount of force. An example of a convergent fascicle arrangement is pectoralis major. This muscle has a broad origin all along the clavicle and sternum, and comes to a narrow insertion onto the humerus at the greater tubercle. This broad origin all coming together to converge onto a narrow insertion enables the muscle to be very forceful, but this arrangement can also enable a variety of movements depending on which muscle fascicles are activated. When the more superior regions of pectoralis major contract, the muscle can produce the action of flexion at the glenohumeral joint. Whereas when the more inferior fascicles contract, the muscle can produce the action of adduction at the glenohumeral joint. An example of a muscle that has a circular fascicle arrangement is orbicularis aureus, found surrounding the lips. Orbicularis aureus closes the lips in order to guard the entrance to the mouth. A pennate fascicle arrangement means that the fascicles are oriented diagonally onto a tendon. A unipennate fascicle arrangement has all of the fascicles on one side of a long tendon. The example of a unipennate fascicle arrangement is in the muscle extensor digitorum, which is found on the posterior interbracial region. Having this unipennate arrangement enables this muscle to be very forceful, even though it doesn't need to produce a wide range of movement to extend the digits or extend the metacarpal phalangeal joints. A bipennate fascicle arrangement is where the fascicles are attaching obliquely on both sides of a long tendon, and the rectus femoris muscle on the anterior of the thigh, the anterior femoral region. One of the quadriceps femoris muscles is an example of a muscle with a bipennate fascicle arrangement. This muscle can produce a very forceful contraction to extend the tibiofemoral joint. An example that is multipennate is the deltoid muscle found superficially in the acromial region. A multipennate muscle is a muscle in which the tendon branches and fascicles are attaching obliquely from a variety of directions onto the branching tendon. The deltoid muscle has that multipennate arrangement, which will enable the muscle to be very forceful, and also to have a wide variety of different actions that are possible, as the anterior region of the deltoid can contract producing flexion of the glenohumeral joint, the posterior region of the deltoid could contract producing extension of the glenohumeral joint, and the middle region of the deltoid could contract producing abduction of the glenohumeral joint. One of the ways that muscles have been named is based on their shape. For example, the deltoid is named after the triangular shape because the Greek letter delta is a triangle, and the deltoid has a roughly triangular shape. Another example here is quadratus femoris, so this has a rectangular shape, and quadratus means rectangular. Another way that muscles are named is based on their origin and insertion. For example, sternocleidomastoid is a muscle that has its origin from the sternum and clavicle, and has its insertion at the mastoid process of the temporal bone, so the mastoid and sternocleidomastoid referring to the sternum and clavicle, and mastoid referring to mastoid process of the temporal bone. Another example is the styloglossus. Stylo referring to the styloid process here of the temporal bone, and glossis referring to the tongue. The insertion of styloglossus is into the tongue. The Greek word for tongue is glossis, so styloglossus is the origin and insertion. Similarly, hyoglossus has its origin from the hyoid bone here, and insertion into the tongue. Another way that muscles are named is based on the fiber direction and location. For example, rectus abdominis has its fibers running straight up and down on the anterior of the abdominal region, and the external abdominal oblique has its fibers running obliquely, roughly 45 degree angle relative to rectus abdominis, in the lateral abdominal walls. Another way that muscles are named is based on the location and number of origins. For example, biceps brachii has two origins. Biceps literally translates to two heads. There are two origins, the short head and the long head of biceps brachii. Similarly, there are three heads of the triceps brachii, three different origins, and both biceps brachii and triceps brachii get the ending word brachii referring to the brachial region. Quadriceps femoris has four origins and is located on the anterior of the femoral region. Muscles are also named based on their relative size and based on their action. For example, extensor carpi radialis longus, located on the posterior anti-brachial region, is slightly longer than extensor carpi radialis brevis, and both of these muscles are named after the action of extending the radiocarpal joint. The primed mover is the main muscle responsible for producing an action. For example, the biceps brachii is the prime mover for the action of flexion at the humeral ulnar joint, commonly referred to as the elbow joint. The prime mover is also known as the agonist, so agonist and prime mover are synonyms, and the muscle that resists the action or slows down the action is known as the antagonist. So the triceps brachii is the antagonist for the action of flexion at the humeral ulnar joint. Typically, the antagonist of one action will be the agonist or prime mover for the opposite action, and so the triceps brachii would be the prime mover for extension of the humeral ulnar joint, whereas biceps brachii would become the antagonist during extension of the humeral ulnar joint. A synergist is a muscle that assists the prime mover in an action. For example, with flexion of the humeral ulnar joint, biceps brachii is the prime mover, and brachioradialis and brachialis are both synergists that can assist with biceps brachii's flexion of the humeral ulnar joint.