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The muscles that flex the fingers are known as flexor muscles. There are two types of flexor muscles: the flexor digitorum superficialis and the flexor digitorum profundus. The flexor digitorum superficialis is the largest of the extrinsic flexors of the forearm and is responsible for flexing the middle phalanges of the medial four digits at the proximal interphalangeal joints. The flexor digitorum profundus is a powerful flexor muscle that pulls the lateral four digits towards the hand, causing flexion of the second to fifth digits at the metacarpophalangeal and interphalangeal joints.
| Characteristics | Values |
|---|---|
| Muscle group | Thenar muscle group |
| Muscle name | Flexor digitorum profundus |
| Alternative names | Flexor digitorum communis profundus |
| Location | Forearm |
| Function | Flexes the fingers |
| Primary action | Bends the index, middle, ring, and small fingers |
| Joints | Metacarpophalangeal, interphalangeal, midcarpal, and proximal interphalangeal |
| Gripping strength | High |
| Tendon slips | Four |
| Digits | 2-5 |
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What You'll Learn
Flexor digitorum profundus
The flexor digitorum profundus (FDP) is a muscle in the forearm that flexes the fingers. It is considered an extrinsic hand muscle because it acts on the hand while its muscle belly is located in the forearm. The muscle is named from the Latin 'deep bender of the fingers'.
The muscle originates in the upper 3/4 of the anterior and medial surfaces of the ulna, interosseous membrane, and deep fascia of the forearm. The muscle fans out into four tendons (one to each of the second to fifth fingers) to the palmar base of the distal phalanx. The tension generated by flexor digitorum profundus at the more distal joints is determined by wrist position. Flexion of the wrist causes muscle shortening at that point, reducing tension that can be generated more distally. Fingers cannot be fully flexed if the wrist is fully flexed.
The lumbricals, intrinsic muscles of the hand, attach to the tendon of flexor digitorum profundus. Thus, the flexor muscle is used to aid the lumbrical muscles in their role as extensors of the interphalangeal joints. As the lumbrical muscles originate on the palmar side of the hand and attach on the dorsal aponeurosis, power is transferred from the flexor digitorum profundus muscle to fully extend the fingers as well as flex the metacarpophalangeal joints.
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Flexor digitorum superficialis
The flexor digitorum superficialis is an extrinsic flexor muscle of the fingers at the proximal interphalangeal joints. It is the largest muscle of the anterior compartment of the forearm, where it is located at the superficial volar/anterior aspect. The flexor digitorum superficialis is sometimes considered to be the deepest part of the superficial layer of the anterior forearm compartment, and sometimes considered to be a distinct, "intermediate layer".
The muscle has two heads: the humeroulnar and the radial. The humeroulnar head originates from the medial epicondyle of the humerus via the common flexor tendon, as well as the medial margin of the coronoid process of the ulna. The radial head originates on the anterior oblique line of the shaft of the radius. The median nerve and ulnar artery pass between these two heads. The ulnar collateral ligament of the elbow joint gives its origin to part of the muscle.
The flexor digitorum superficialis courses distally across the anterior forearm, separating into two planes of muscular fibres: superficial and deep. The superficial plane gives off tendons directed to digits 3 and 4, while the deep layer gives off tendons to digits 2 and 5. The deep layer also gives off a small slip that joins the tendon of the superficial layer directed to digit 2. The tendons of the flexor digitorum superficialis then pass through the carpal tunnel, after which they diverge into two pairs.
The primary function of the flexor digitorum superficialis is flexion of the middle phalanges of the four fingers (excluding the thumb) at the proximal interphalangeal joints. However, under continued action, it also flexes the metacarpophalangeal joints and wrist joint.
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Lumbrical muscles
The lumbricals are a set of four intrinsic hand muscles that are attached to the flexor digitorum profundus (FDP) tendons proximally and the digits' extensor expansions distally. These muscles are cylindrical and worm-like, and they do not attach to bone. Each finger is attached to two lumbricals, except for the thumb, which is associated with only one lumbrical muscle.
The lumbricals flex the metacarpophalangeal (MCP) joints and extend the proximal interphalangeal (PIP) and distal interphalangeal (DIP) joints. By coordinating with other hand muscles, the lumbricals help control the fine and delicate movements of the fingers. These movements are essential for gripping objects, typing, and performing various dexterous hand movements.
The lumbricals are also important for sensory feedback, which is crucial for precision pinch movements and the precise manipulation of objects. They contain many muscle spindles, which indicates that they play a role in proprioception. This is further supported by the high number of muscle spindles in the lumbricals, suggesting that they are involved in proprioceptive monitoring of the fingers.
The lumbricals are sometimes damaged in crush injuries to the hand, and they have also been shown to be relevant in carpal tunnel syndrome.
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Flexor carpi ulnaris
The flexor carpi ulnaris (FCU) is a muscle in the forearm that flexes and adducts at the wrist joint. It is the most medial of the superficial flexors in the forearm. The FCU has two heads: the smaller humeral head and the larger ulnar head. The humeral head originates from the medial epicondyle of the humerus via the common flexor tendon. The ulnar head originates from the olecranon and the upper two-thirds of the dorsal border of the ulna. The FCU inserts onto the pisiform, hook of the hamate, and the base of the fifth metacarpal.
The FCU is the only muscle in the anterior compartment of the forearm that is fully innervated by the ulnar nerve. It receives its arterial blood supply via three routes: the ulnar collateral arteries, and the anterior and posterior ulnar recurrent arteries. The FCU flexes and adducts the hand at the wrist joint, and it can also move the hand sideways. It works with the flexor carpi radialis to flex the wrist and with the extensor carpi ulnaris to adduct the wrist.
The FCU tendon can be palpated beneath the skin near the wrist joint. It serves as a landmark for locating the ulnar nerve and ulnar artery, which lie lateral to the tendon at the wrist joint. The FCU is susceptible to involvement in Volkmann's contracture, where ischemic damage compromises the muscle, leading to fibrosis, contracture, and impaired function in the affected limb.
The FCU can be strengthened through exercises such as using a wrist roller and performing wrist curls with dumbbells. These exercises help to prevent injury to the ulnar collateral ligament of the elbow joint.
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Flexor carpi radialis
The flexor carpi radialis is a muscle in the forearm that acts to flex and abduct the hand. The name comes from the Latin 'carpus', meaning wrist, and so the flexor carpi is a flexor of the wrist. It is one of four muscles in the superficial layer of the anterior compartment of the forearm.
The flexor carpi radialis originates from the medial epicondyle of the humerus, via the common flexor tendon, and surrounding fascia. The muscle belly courses obliquely, crossing from the ulnar to the radial part of the forearm. In the lower third of the forearm, it gives off a long tendon that passes below the flexor retinaculum into the palmar surface of the hand. The flexor carpi radialis tendon passes through a tunnel and is prone to tendonitis or even rupturing. The tendon is visible on the anterior surface of the forearm, just proximal to the wrist, when the wrist is flexed. It is the tendon seen most lateral, closest to the thumb.
The muscle is supplied by a branch arising high in the forearm from the anterior or posterior recurrent ulnar arteries. The rest of its nutritional needs are fulfilled by 6-8 branches of the radial artery. The radial artery runs between the tendons of the flexor carpi radialis and brachioradialis muscles, marking a common site for its palpation in order to measure someone’s radial pulse.
The flexor carpi radialis is innervated by the median nerve, specifically by axons from cervical nerve roots C6 and C7. It is lateral to palmaris longus, while being medial to pronator teres in its proximal part and to brachioradialis in its distal part.
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Frequently asked questions
The flexor digitorum profundus is a powerful muscle in the forearm that flexes the fingers. It is also known as flexor digitorum communis profundus, derived from the Latin 'deep bender of the fingers'.
This muscle pulls the lateral four digits (the index, middle, ring, and small fingers) towards the hand, causing flexion of the second to fifth digits at the metacarpophalangeal and interphalangeal joints. It is the main gripping muscle.
Injury to the flexor digitorum profundus can lead to conditions such as Quadrigia, where the patient cannot make a full fist due to the interconnected tendons of the 5th, 4th, and 3rd fingers. Another condition is Jersey Finger, where there is a rupture of the tendon from the distal attachment at the base of the distal phalanx, resulting in a loss of active flexion of the distal interphalangeal joint.
