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Anatomy & Physiology Of The Elbow Flexors
Biceps Brachii:
This muscle is formed by two heads (this is the anatomical reason it was given the name: "Biceps") that originate proximally & distinctly with 2 separate tendons. They run vertically on the humerus where they fuse to each-other and end up on the forearm. They share the same tendon on the distal insertion.
Function: flexion and supination of the elbow.
It shows its best efficiency during the flexion of the elbow when the forearm is completely supinated. If the forearm is pronated the distal tendon of the biceps brachii will be wound around the neck of the radius.
While in this position the musle tension is slackened which leads to less efficiency on the flexion. Therefore, as the elbow flexion distances itself from the full supination position, efficiency on the elbow flexion decreases.
To get the best efficiency on the elbow flexion, supination is needed to tense up the muscle. This can be done by having the distal tendon unwound around the neck of the radius.
The two heads of the biceps-brachii are:
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Long Head: with its proximal origin on the supraglenoid tubercle of the scapula and its distal insertion on the tuberosity of the radius. It crosses 2 joints: scapulo-humeral and elbow joints. The long head represents the lateral portion of the biceps brachii.
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Short Head: with its proximal origin on the coracoid process of the scapula and its distal insertion on the tuberosity of the radius, crosses the elbow joint only. The short head represents the medial (inner portion) of the biceps brachii.
Brachialis:
The brachialis is placed anteriorly on the arm on its lower part. This muscle is not easily visible since it's almost totally covered by the biceps brachii. Its origin starts from the anterior aspect of the lower half of the humerus and inserts into the tuberosity of the ulna.
The brachialis only crosses one joint, so it is called a mono-articular muscle. It represents one of the few muscles of the body with only a single function; flexion of the elbow.
Function: it's active in the elbow flexion only, that's to say it brings the forearm toward the arm.
The brachialis is called into action in every movement during flexion of the elbow, independently by the forearm position (supinated or pronated), that's why it's named the "pure flexor" of the elbow.
Important Note:
The forearm rotation is determined only by the rotation of the radius around the ulna and not vice versa.
The distal insertion of the brachialis into the ulna bone means no possibility to rotate effectively. Therefore it does not get effected by the prono-supination movements of the forearm.
Important Note: The musclular development of this muscle optimizes the shape of the biceps brachii. In fact, by virtue of its anatomic disposition underlying the biceps brachii, its development will consequently results in a kind of "anatomical lift," pushing the biceps brachii upward helping to create that particular arm shape better known as the "peak of the bicep."
Brachio-Radialis:
This muscle is evident in the anterior-lateral region of the forearm with its long inferior tendon placed on the radius. It has its origin on the lateral supracondylar ridge of the humerus and its insertion on the styloid process of the radius.
Function: is a flexor of the elbow, supinates the forearm when in extreme pronation, and pronates the forearm when in extreme supination.
Its action for this reason (in normal conditions), guarantees an equilibrium between the supinator and pronator forces resulting in the neutral position of the forearm.
Having its distal insertion far from the joint fulcrum, it has the advantage to exert its traction from the very beginning through its dynamic component which is favorable to start flexion of the elbow.
Therefore its contribution in flexion of the elbow is maximal in the first degrees of movement and due to a muscular lever. This flexion is more favorable at the beginning than that of the biceps brachii. This muscle also is effected by the rotation of the forearm because of its distal insertion on the radius (on its distal end).
The strengthening of the brachio-radialis occurs while exercising the biceps brachii (during the flexion of the elbow). The pronation of the forearm, while the elbow is extended, allows the muscle to be fully lengthened (due to its distal insertion on the radius), resulting in maximal stimulation and efficiency in the following concentric contraction (upward phase); during elbow flexion, in conjunction with its supination.
Pre-lengthening and pre-shortening comprehension will enable you to understand how the arm exercises differ from each other in stimulating the arm muscles and their growth.