The arm muscles of the upper limb act on the elbow and shoulder joints to produce the various movements of the forearm. Five arm muscles play a role in these movements: biceps brachii, brachialis, coracobrachialis, triceps brachii and anconeus. The first three are primarily involved in forearm flexion, hence they are called flexors. They occupy the anterior, or flexor, compartment of the arm. The triceps brachii is mainly involved in forearm extension, making it an extensor muscle. It is located in the posterior, or extensor, compartment of the arm. The anconeus is not physically located in the arm, but it is very closely related to the triceps in terms of function. In addition to their primary involvement in forearm movements, some of these five muscles also help with arm adduction, flexion, extension and abduction.
However, what is the ‘arm’? It is the entire upper limb from the shoulder to the fingertips, right? Wrong! In terms of gross anatomy, the ‘arm’ refers to the portion between the shoulder and elbow joints. It is the scaffold of the arm and its main functions include forearm movement, motion transmission and passage of neurovascular structures along the upper limb.
Flexors: Biceps brachii, brachialis, coracobrachialis
Extensors: Triceps brachii, anconeus
Flexors: Musculocutaneous nerve, except the brachialis muscle which is also innervated by the radial nerve
Extensors: Radial nerve
Primary functions: Forearm flexion and extension
Secondary functions: Arm adduction, flexion, extension and adduction
In this article, we’ll explore the muscles in the arm, including their attachments, innervation and actions.
Before diving into the nitty-gritty details about every muscle, here’s an overview diagram with the arm muscles labeled in order to get your bearings:
The biceps brachii is the most superficial muscle of the anterior compartment of the arm. Therefore, it is easily palpable and clearly visible as a round bulge when you flex your forearm against resistance.
The muscle consists of two heads, long and short. The long head originates from the supraglenoid tubercle of the scapula. The short head starts from the coracoid process of the scapula. The two heads blend together distally, ending in a single biceps tendon which inserts into the radial tuberosity. An aponeurotic sheeth called the bicipital aponeurosis emerges from the distal end of the muscle and inserts at the deep fascia of forearm, separating the superficial from deep structures in the cubital fossa.
As you can see from the attachments, the biceps brachii traverses the shoulder and elbow joints, thus acting on both. However, it primarily moves the forearm around the elbow joint by pulling on the radius. During contraction, it causes forearm flexion and supination, exhibiting the greatest force when the forearm is close to 90 degrees. This muscle is also a weak arm flexor by acting on the shoulder joint. The biceps brachii is innervated by the musculocutaneous nerve (C5-C6).
Long head: Supraglenoid tubercle of scapula
Short head: Coracoid process of scapula
Radial tuberosity of radius
Deep fascia of forearm (insertion of the bicipital aponeurosis)
|Innervation||Musculocutaneous nerve (C5- C6)|
Elbow joint: Forearm flexion and supination
Shoulder joint: Weak arm flexion
Thanks to your biceps brachii muscle, you can insert a corkscrew into a wine bottle to open it. More details about the muscle are provided in the following resources:
Next on the list of upper arm muscles is the brachialis. It lies deep in the anterior compartment of the arm, underneath the biceps brachii.
Quite evidently, the brachialis acts only on the elbow joint. During contraction, it flexes the forearm by pulling on the ulna. The brachialis is the most powerful flexor of the forearm, irrespective of the forearm position. It is innervated by the musculocutaneous and radial nerves (C5-C7).
|Origins||Inferior half of anterior surface of humerus|
|Insertions||Ulnar tuberosity, Coronoid process of ulna|
Radial nerve (C5-C7)
|Function||Elbow joint: Forearm flexion (in all positions)|
The following video and article explore the anatomy of the brachialis muscle in greater detail.
The coracobrachialis muscle also lies deep within the anterior compartment of the arm, in the same plane as the brachialis underneath the biceps brachii.
It originates from the coracoid process of the scapula, passes through the axilla and inserts into the anteromedial surface of the humeral shaft.
Therefore, the coracobrachialis acts on the shoulder joint. During contraction, it flexes and adducts the arm by pulling on the humerus. It also stabilizes the shoulder joint, preventing its dislocation while carrying heavy loads, such as your shopping bags. The coracobrachialis is innervated by the musculocutaneous nerve (C5-C7).
|Origins||Coracoid process of scapula|
|Insertions||Anteromedial surface of humeral shaft|
|Function||Shoulder joint: Arm flexion, arm adduction|
Do you want to learn more details about the coracobrachialis structures, such as the endangered neurovascular structures that the muscle can easily compress? Take a look below!
The anatomy of the arm muscles would be incomplete without learning about the triceps brachii, the largest muscle of this region. It is the sole occupant of the posterior compartment of the arm, being clearly visible as a horseshoe on the back of the arm, especially in well-trained athletes.
Studying arm muscles anatomy seems daunting to you? Our muscle anatomy reference charts can help you memorize the attachments, innervation and functions in no time!
As its name suggests, the triceps consists of three heads: long, lateral and medial. The long head originates from the infraglenoid tubercle of the scapula. The lateral and medial heads begin from the posterior surface of the humerus, superior and inferior to the radial groove, respectively. The three heads blend together into a single tendon that attaches to the olecranon of the ulna and the forearm fascia.
The attachments of the triceps brachii imply its involvement to two joints, the shoulder and elbow. The primary function of this muscle is forearm extension by pulling on the ulna, making it the main flexor antagonist. The medial head produces the greatest amount of force, followed by the lateral one. The long head is the least active extensor, but it performs other functions like arm extension and adduction around the shoulder joint. The triceps brachii is innervated by the radial nerve (C6 - C8).
Long head: Infraglenoid tubercle of scapula
Lateral head: Posterior surface of humerus (superior to radial groove)
Medial head: Posterior surface of humerus (inferior to radial groove)
|Insertions||Olecranon of ulna and fascia of forearm|
|Innervation||Radial nerve (C6 - C8)|
Elbow joint: Forearm extension
Shoulder joint: Arm extension and adduction (long head)
More details about the triceps brachii are covered below:
Last but not least, the anconeus, which is the fifth arm muscle. Strictly speaking, it is not located within the posterior compartment of the arm, but rather posterolateral to the elbow.
The anconeus originates from the lateral epicondyle of the humerus and attaches to the lateral surface of the ulnar olecranon.
Why is the anconeus included with the arm muscles? As you can see from the attachments, it acts on the elbow joint. In fact, the function of this muscle is closely related to the triceps brachii. The anconeus helps the triceps by participating in forearm extension and prevents the joint capsule of the elbow from getting pinched during this movement. The anconeus is innervated by the radial nerve (C7, C8).
|Origins||Lateral epicondyle of humerus|
|Insertions||Lateral surface of olecranon|
|Innervation||Radial nerve (C7, C8)|
|Function||Elbow joint: Assists in forearm extension; Stabilization of elbow joint|
The anconeus is explained in detail down below. Overview videos and quizzes that combine all the information within this article have also been included, so you can easily start building your knowledge of arm anatomy.