Musculoskeletal System
Brachialis: the main elbow flexor in any forearm position
Brachialis is the main elbow flexor (the muscle that bends the arm at the elbow), and because it attaches to the ulna it works in every forearm position.
Remember
Brachialis attaches to the ulna, the forearm bone that does not rotate, so it bends the elbow in every forearm position, while the biceps flexes best with the palm facing up.
Brachialis lies deep to (underneath) the biceps brachii, in the front of the arm. It arises from the humeral shaft (the shaft of the upper arm bone) and attaches to the ulnar tuberosity, a roughened bump on the ulna just below the elbow joint. When it contracts, it pulls the ulna towards the humerus and the elbow bends.
Whether a muscle's action depends on forearm rotation comes down to which forearm bone it attaches to. In pronation (turning the palm down) and supination (turning the palm up), the radius, the other forearm bone, rotates around the ulna. The ulna does not rotate. A muscle attached to the ulna is therefore not twisted by these movements, so brachialis pulls the same way and flexes the elbow in every forearm position.
The biceps brachii is different. It attaches to the radial tuberosity (a bump on the radius) and is also a strong supinator (a muscle that turns the palm up), so it flexes the elbow best with the palm facing up and contributes less when the forearm is pronated. Brachioradialis, a flexor attached to the lower radius, is taught as a helper to brachialis. Coracobrachialis, the third muscle in the front of the arm, acts at the shoulder rather than at the elbow.
If a fall has injured a muscle, weak elbow flexion in every forearm position, with a normal shoulder, points to brachialis rather than to biceps brachii, brachioradialis or coracobrachialis.
Mnemonic
BBC
- B biceps
- B brachialis
- C coracobrachialis
Use it to list the three muscles in the front of the arm, then recall that coracobrachialis acts at the shoulder.
Key facts
- Action
- the main elbow flexor, working in all forearm positions.
- Attachments
- from the humeral shaft to the ulnar tuberosity.
- Position
- beneath the biceps brachii, forming part of the floor of the cubital fossa (the triangular hollow in front of the elbow).
- Why forearm position does not matter
- the ulna is not the bone that rotates; the radius is.
- Contrast with biceps brachii
- it attaches to the radius and is also a supinator, so its flexion varies with forearm rotation.
- Opposing muscle
- triceps brachii extends (straightens) the elbow.
Exam traps
- Biceps brachii flexes the elbow too, yet it is most effective with the forearm supinated and it also supinates; it is not the muscle that works regardless of forearm position.
- Brachioradialis assists with elbow flexion, working best when the forearm is in the mid position (half pronated), but it is not the primary flexor and has a supporting role.
- Brachialis is not a supinator: it is attached to the ulna, which does not turn in the same way as the radius.
- Brachialis is supplied by the musculocutaneous nerve (the nerve that also supplies the biceps), with a contribution from the radial nerve, but this does not explain the constant pull: nerve supply decides whether the muscle can be activated, and the attachment to the ulna is what keeps the pull the same in every forearm position.
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