Muscles of Arm and Hand Posterior
The arm is divided by a fascial layer (known as lateral and medial intermuscular septa) separating the muscles into two osteofascial compartments: the anterior and the posterior compartments of the arm. The fascia merges with the periosteum (outer bone layer) of the humerus. The compartments contain muscles which are innervated by the same nerve and perform the same action. Two other muscles are considered to be partially in the arm. The large deltoid muscle is considered to have part of its body in the anterior compartment. This muscle is the main abductor muscle of the upper limb and extends over the shoulder. The brachioradialis muscle originates in the arm but inserts into the forearm. This muscle is responsible for rotating the hand so its palm faces forward (supination). Deltoid Muscles: In human anatomy, the deltoid muscle is the muscle forming the rounded contour of the shoulder. Anatomically, it appears to be made up of three distinct sets of fibers though electromyography suggests that it consists of at least seven groups that can be independently coordinated by the central nervous system. It was previously called the Deltoideus and the name is still used by some anatomists. It is called so because it is in the shape of the Greek letter Delta (triangle). It is also known as the common shoulder muscle, particularly in lower animals (e.g., in domestic cats). Deltoid is also further shortened in slang as "delt". The plural forms of all three incarnations are deltoidei, deltoids and delts. A study of 30 shoulders revealed an average weight of 191.9 grams (6.77 oz) (range 84 grams (3.0 oz)–366 grams (12.9 oz)) in humans. Brachioradialis Muscle: Brachioradialis is a muscle of the forearm that acts to flex the forearm at the elbow. It is also capable of both pronation and supination, depending on the position of the forearm. It is attached to the distal styloid process of the radius by way of the brachioradialis tendon, and to the lateral supracondylar ridge of the humerus. Brachioradialis flexes the forearm at the elbow. When the forearm is pronated, the brachioradialis tends to supinate as it flexes. In a supinated position, it tends to pronate as it flexes. The brachioradialis is a stronger elbow flexor when the forearm is in a midposition between supination and pronation at the radioulnar joint. When pronated, the brachioradialis is more active during elbow flexion since the biceps brachii is in a mechanical disadvantage. With the insertion of the muscle so far from the fulcrum of the elbow, the brachioradialis does not generate as much force as the brachialis or the biceps. It is effective mainly when those muscles have already partially flexed at the elbow. The brachioradialis flexes the forearm at the elbow, especially when quick movement is required and when a weight is lifted during slow flexion of the forearm. The muscle is used to stabilize the elbow during rapid flexion and extension while in a midposition, such as in hammering. The brachioradialis is synergistic with the brachialis and biceps brachii; the triceps brachii and anconeus are antagonistic.