Biceps brachii Anatomy
Biceps brachii muscle is also called Musculus Biceps brachii which simply means the two-headed muscle of the arm. It is one of the flexor muscles of the forearm; in fact, it is the primary and most powerful flexor muscle of the forearm. These three (3) flexors are located in the anterior or flexor compartment of the arm are all innervated by the musculocutaneous nerve.
The Biceps brachii has two heads: the long and short heads that arise by tendons from two origins. These tendons then expand into two fleshy bellies that lie side by side and loosely connected by areolar tissue but do not merge until just above the elbow joint, below the main convexity of the muscle bellies. This is where they unite to form a flat tendon whose lower end rotates (anterior surface turning laterally) as it passes through the cubital fossa to its insertion into the posterior border of the tuberosity of the radius.
Biceps brachii Origin and Insertion
The Biceps brachii muscle has two heads with different origins: the long head of this muscle originates as a tendon from the supraglenoid tubercle and adjoining part of the glenoid labrum of the scapula with the tendon passing over the head of the humerus to enter the intertubercular sulcus in the axilla. Here, it is held by the transverse humeral ligament that runs between the greater and lesser tubercle; the tendon then forms a muscle belly in the upper part of the arm.
The short head arises from the apex of the coracoid process of the scapula together with the origin of coracobrachialis and to the lateral side of the coracobrachialis. It then passes through the axilla to the arm where it joins with the long head in the distal part of the arm to form a single tendon where it inserts into the radial tuberosity of the forearm. There is a triangular membranous band known as the bicipital aponeurosis which runs from the biceps tendon across the cubital fossa and merges with the antebrachial fascia (deep fascia) this aponeurosis covers the flexor muscles in the medial side of the forearm.
Biceps brachii Action and Function
It is a powerful flexor of the forearm at the elbow joint and a powerful supinator of the forearm. The muscle is also an accessory flexor of the arm at the glenohumeral joint because the origin of the two heads of biceps brachii arises from the scapula. The long head of biceps brachii prevents the upward movement of the humerus on the glenoid cavity.
Biceps brachii Nerve and Blood Supply
The musculocutaneous nerve innervates the biceps brachii through the C5 and C 6 segments of the spinal cord, with one branch to each belly of the muscle. The blood supply is provided by the brachial artery.
Biceps Brachii Test
The test for Biceps brachii is carried out with the forearm supinated and the elbow flexed against resistance which helps the contracted muscle in the arm and the tendon and aponeurosis at the elbow to be palpated easily.
Clinical importance of Biceps brachii
The tendon of biceps brachii could be inflamed, injured, or ruptured or the musculocutaneous nerve could be injured, all of which leads to a reduction in function or paralysis with associated clinical symptoms and signs of inability to flex the forearm.
Injury to Musculocutaneous Nerve
There could be an injury to the musculocutaneous nerve which could occur by a stab leading to paralysis of biceps brachii, coracobrachialis and brachialis muscles with signs and symptoms such as weakened flexion of the elbow and supination of the forearm with loss of sensation occurring on the lateral surface of the forearm that is supplied by the lateral cutaneous nerve of the forearm.
Inflammation of the Biceps brachii (Biceps Tendinitis)
The continuous movement of the tendon of the long head of this muscle through the synovial sheath in the intertubercular sulcus (groove) of the humerus leads to wear and tear that causes shoulder pain. This could occur during repetitive micro-trauma in sports involving throwing such as baseball.
Rupture of Tendon of Long Head of Biceps
When there is inflammation of the long head without treatment, it may rupture. Usually, the rupture of the tendon occurs at its attachment to the supraglenoid tubercle of the scapula. This rupture is commonly dramatic and is associated with a snap or pop of which the detached muscle belly forms a ball near the center of the distal part of the anterior aspect of the arm known as the Popeye deformity.
The Bicipital Myotatic Reflex
The biceps reflex is one of the several deep-tendon reflexes that are routinely tested during physical examination and it is carried out when the relaxed limb is passively pronated and partially extended at the elbow while the thumb of the examiner is firmly placed on the biceps tendon and the reflex hammer is tapped briskly at the base of the nail bed of the thumb of the examiner.
A positive response (the normal response) is said to be elicited when there is an involuntary contraction of the biceps brachii which is felt as a momentarily tensed tendon, usually with a brief jerk-like flexion of the elbow; this positive response confirms the integrity of the musculocutaneous nerve and the C5 and C6 spinal cord segments. When there is a negative response such as excessive response or diminished response or prolonged (hung) responses, this may points to central or peripheral nervous system disease.