Superficial flexors of the forearm
Anatomy and supply
The superficial flexors of the forearm are a group of five muscles found at the anterior forearm. They form the surface of the ulnar side of the forearm where they can also be easily palpated. Their long tendons can be followed very well at the wrist joint, especially during flexion. All five muscles arise from the common flexor tendon located at the medial epicondyle of the humerus; some of them have additional attachment points on the radius and ulna. The following muscles are known to be the superficial flexors:
- Pronator teres muscle: originates at the medial epicondyle of the humerus (humeral head) and coronoid process of the ulna (ulnar head). From there it courses under the brachioradialis and attaches to the lateral side of the radius. That muscle is the most lateral of all superficial flexors. Innervation: median nerve (C6-C7).
- Flexor carpi radialis muscle: runs from the common flexor tendon at the humerus to the bases of the second and third metacarpal bones. Innervation: median nerve (C6-C7)
- Flexor carpi ulnaris muscle: arises from the medial epicondyle of the humerus (humeral head) and the olecranon of the ulna (ulnar head). The muscle has 3 insertion points: first, its tendon inserts into the pisiform bone, where the pisiform bone functions as a sesamoid bone; from there, it relays towards its other 2 insertion points at the hook of the hamate and the base of the 5th metacarpal bone. This muscle is the most medial of all superficial flexors and is mainly responsible for the contour of the ulnar side of the forearm. Innervation: ulnar nerve (C7-Th1).
- Palmaris longus muscle: is a very slender muscle originating from the common flexor tendon and inserting at the flexor retinaculum and palmar aponeurosis. This muscle is variable and can be, in some cases, missing on one or both arms or have an alternative course. Innervation: median nerve (C7-Th1).
- Flexor digitorum superficialis muscle: has a large origin point attached to the medial epicondyle of the humerus (humeral head), the coronoid process of the ulna (ulnar head) and distally from the radial tuberosity (radial head). Its four insertion tendons split into two smaller end tendons each of which then insert on both sides of the middle phalanges of the second to fifth fingers. This is the deepest muscle of all superficial flexors. Innervation: median nerve (C7-Th1).
All superficial flexors of the forearm are supplied by the median nerve except the flexor carpi ulnaris. In the elbow this nerve runs underneath the bicipital aponeurosis and between the two heads of the pronator teres. From there, it courses under the flexor digitorum superficialis between the superficial and deep flexors and through the carpal tunnel at the wrist joint, and finally branches off into sensory and motor branches at the palm of the hand. The carpal tunnel is a passage formed by the carpal bones and a densification of the antebrachial fascia (flexor retinaculum). In addition to the median nerve, the carpal tunnel also contains the tendons of the flexor digitorum superficialis, flexor digitorum profundus and flexor pollicis longus.
The superficial flexors support various movements of the antebrachial and hand joints. As the name reveals, (almost) all of them do flexion of the hand joint (except the pronator teres). Another thing these muscles have in common is that they are weak flexors of the elbow joint. Furthermore, the pronator teres rotates the radius medially (pronation). The contraction of the flexor carpi radialis moves the hand radially (radial abduction) whereas the flexor carpi ulnaris moves the hand ulnarly (ulnar abduction). The flexor digitorum superficialis does flexion of the metacarpophalangeal (MCP) and proximal interphalangeal joints (PIP) of the second to fifth finger. The palmaris longus' major function is to support the palmar aponeurosis.
A chronic false strain of the superficial flexors often leads to inflammation and increased connective tissue in the common flexor tendon at the medial epicondyle of the humerus (medial epicondylitis). Particularly golfers are affected by this condition as they permanently have to flex their hand wrist for the swing. For that reason, this condition is also referred to as the golfer’s elbow. Classic symptoms are pain which increases during hand movements and trouble performing day-to-day tasks (e.g. pressing the door handle, hand shaking). Noticeably, adolescent baseball players often suffer from the medial epicondylitis as well (“little league elbow”). The reason is that children’s bones have ossification centers as they are still growing. When these experience massive false strain, the pressure is carried forward to the apophysis causing inflammation (apophysitis) or even deformation.