Biceps brachii muscle
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The biceps brachii muscle (also known simply as biceps) is a two-headed muscle in the anterior compartment of the arm that flexes at the elbow and supinates the forearm.
short head: coracoid process of the scapula
long head: supraglenoid tubercle of the scapula
insertion: radial tuberosity of the proximal radius
innervation: musculocutaneous nerve
arterial supply: muscular branches of the brachial artery
Given the muscle has two proximal tendons (hence its name), there are two origins:
the long head originates at the supraglenoid tubercle of the scapula before passing laterally through the rotator cuff interval and then inferiorly through the intertubercular (bicipital) groove of the proximal humerus.
the short head tendon is much simpler, originating from the apex of the coracoid process of the scapula, lateral to the origin of the coracobrachialis muscle.
The two muscle bellies remain separate until merging in the distal arm above the elbow joint.
The distal insertion of the biceps tendon has two components:
a bifid tendon descends onto the posterior margin of the radial tuberosity of the proximal radius which is associated with a small bursa to facilitate supination and pronation.
a bicipital aponeurosis (lacertus fibrosus) that arises from the medial aspect of the tendon before it dives deep towards the radial tuberosity. The aponeurosis spans across the brachial artery and median nerve before inserting onto the antebrachial fascia overlying the flexor muscles.
Separate branches from the musculocutaneous nerve (C5,6) supply each muscle belly.
As the muscle has several heads and crosses both the shoulder and elbow joints, the biceps muscle has a variety of actions:
with the elbow is extended, biceps solely produces elbow flexion
with any degree of elbow flexion, biceps will produce supination with triceps brachii resisting elbow flexion
if triceps does not counteract elbow flexion, biceps will produce flexion at both the shoulder and elbow
biceps can help initiate shoulder abduction if deltoid is weak or paralyzed
Overall, these complex movements can be summarized as:
long head: arm abduction
short head: arm adduction and flexion
supination of the pronated forearm
flexion of the supinated forearm
duplicated long head of biceps tendon: can mimic longitudinal tear 1
bifid/bifurcated insertion: anatomic variant that arises from persistent division between the short head and long head of the distal biceps brachii tendon 3
Brasseur 5 describes nine individual zones (from proximal to distal) that should be assessed with ultrasound:
glenoid insertion of the long head
extension to the upper pole of the head of the humerus
reflection to the upper bicipital groove
upper myotendinous junction
lower myotendinous junction