Dominic Stone
The human hand is a complex system of muscles, tendons, and joints that work together to allow for a wide range of movements and dexterity. One of the key muscles responsible for finger flexion is the flexor digitorum profundus (FDP), a powerful flexor that pulls the lateral four digits (the index, middle, ring, and small fingers) towards the hand, causing flexion at the metacarpophalangeal and interphalangeal joints. The FDP works in synergy with other muscles such as the flexor digitorum superficialis and lumbricals to enable a full range of finger movements and grip strength. The tension generated by the FDP is influenced by the position of the wrist, with flexion of the wrist reducing the tension that can be generated in the fingers.
| Characteristics | Values |
|---|---|
| Muscle Name | Flexor Digitorum Profundus |
| Alternative Name | Flexor Digitorum Communis Profundus |
| Location | Forearm |
| Function | Flexes the fingers |
| Type of Muscle | Extrinsic hand muscle |
| Muscle Belly Location | Forearm |
| Tendon Count | Four |
| Flexion | Flexes the wrist, metacarpophalangeal and interphalangeal joints |
| Gripping Power | The main gripping muscle |
| Injury | May lead to a weak grasp and difficulty in making a full fist |
| Test | Pinch Grip test |
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What You'll Learn
Flexor digitorum profundus
The flexor digitorum profundus (FDP) is a muscle in the forearm of humans 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. It is the main gripping muscle and acts best when the wrist is extended. It is the sole flexor of the distal interphalangeal (DIP) joint of the second to fifth digits. It also acts as a flexor of the midcarpal (wrist), metacarpophalangeal, and proximal interphalangeal (PIP) joints of the index, middle, ring, and little fingers.
The FDP 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 for each of the second to fifth fingers, which attach to the palmar base of the distal phalanx. The FDP has a common muscle belly for the middle, ring, and small fingers, which typically prevents us from bending the tip of one of these fingers without the others bending as well. However, there is a separate muscle belly for the index finger FDP, allowing for its independence.
The FDP is a composite muscle innervated by the anterior interosseous nerve and ulnar nerves. The medial aspect of the muscle (which flexes the fourth and fifth digits) is supplied by the ulnar nerve, while the lateral aspect (which flexes the second and third digits) is innervated by the median nerve via the anterior interosseous branch. It is one of two flexor muscles that is not exclusively supplied by the median nerve (the other being the flexor carpi ulnaris). The tension generated by the FDP at the more distal joints is determined by wrist position. Flexion of the wrist causes muscle shortening, reducing the tension that can be generated more distally, and preventing full flexion of the fingers when the wrist is fully flexed.
Injuries to the FDP can result in conditions such as Quadrigia and Jersey finger. Quadrigia occurs when the FDP tendons to the fifth, fourth, and third fingers are interconnected with the separate tendon that runs to the index finger, resulting in the patient's inability to make a full fist and associated weak grasp. Jersey finger is a zone 1 flexor tendon injury that involves the ring finger in 75% of cases and results in a loss of active flexion of the distal interphalangeal joint.
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Flexor digitorum superficialis
The flexor digitorum superficialis (FDS) is an extrinsic flexor muscle of the fingers at the proximal interphalangeal joints. It is located in the anterior compartment of the forearm, arising from the medial epicondyle (an elbow bone) between the palmaris longus and the flexor carpi ulnaris muscles. The FDS has four independent muscle bellies, from which four tendons arise. These tendons attach to the anterior margins on the bases of the intermediate phalanges of the four fingers (excluding the thumb). The primary function of the FDS is to bend the middle phalanges of the four fingers (excluding the thumb) at the proximal interphalangeal joints. However, under continued action, it also flexes the metacarpophalangeal and wrist joints.
The FDS is sometimes considered to be the deepest part of the superficial layer of the anterior compartment of the forearm. Alternatively, it is considered a distinct, "intermediate layer" of this compartment. The muscle has two classically described heads – the humeroulnar and radial (or ulnar) – and it is between these heads that the median nerve and ulnar artery pass. The ulnar collateral ligament of the elbow joint gives its origin to part of this muscle.
The FDS is one of two flexor muscles that is not exclusively supplied by the median nerve (the other being the flexor carpi ulnaris). In the forearm, the median nerve travels distally between the FDS and the flexor digitorum profundus. The independence of each finger's FDS contributes to the hand's skill in performing tasks, such as using chopsticks.
The FDS can be tested by flexing one finger at the proximal interphalangeal joint against resistance while holding the remaining three fingers fully extended (to inactivate the flexor digitorum profundus). It is relatively common for the FDS to be missing from the little finger, which can cause problems when diagnosing a little finger injury. A tight fibrous arch of the FDS can compress the median nerve, causing anterior interosseous nerve (AIN) syndrome.
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Lumbrical muscles
The lumbrical muscles are a set of four intrinsic hand muscles that flex the metacarpophalangeal (MCP) joints and extend the interphalangeal joints. Each finger is attached to two lumbricals, except for the thumb, which is associated with only one lumbrical muscle. The lumbricals are unusual muscles in that they do not attach to bone. Instead, they attach proximally to the tendons of the flexor digitorum profundus and distally to the extensor expansions.
The first and second lumbricals (the most radial two) are innervated by the median nerve. The third and fourth lumbricals (the most ulnar two) are innervated by the ulnar nerve. This is the most common innervation pattern of the lumbricals, occurring in 50-60% of individuals. However, 20-30% of individuals have the median nerve innervating only the first lumbrical, while the rest have the median nerve innervating the lateral three lumbricals.
The lumbricals are essential for the fingers' precision grip and fine motor control. By coordinating with other hand muscles, the lumbricals help control the delicate movements of the fingers. The actions of these muscles are important for gripping objects, typing, and performing various dexterous hand movements.
The lumbricals are also believed to play a role in proprioception and sensory feedback, which is crucial for precision pinch movements and the precise manipulation of objects.
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Forearm muscles
The forearm muscles are an essential part of hand and finger function, allowing for a wide range of movements and actions. The flexor digitorum profundus (FDP) is a powerful forearm muscle that plays a crucial role in finger flexion. It is considered an extrinsic hand muscle, with its muscle belly located in the forearm. The FDP arises from the ulna and interosseous membrane, and its four tendons pass through the carpal tunnel to insert into the tips of the index, middle, ring, and small fingers. This allows the FDP to bend all three finger joints, making it an important muscle for gripping and grasping actions.
The flexor digitorum superficialis (FDS) is another key forearm muscle that works in synergy with the FDP. The FDS arises from the medial epicondyle (an elbow bone) and has four independent muscle bellies, each supplying a tendon to the middle, ring, index, and little fingers. The FDS is responsible for bending the middle joint of each finger, except the thumb, and its independence allows for skilled finger movements such as using chopsticks.
The flexor carpi ulnaris is a large forearm muscle that arises from the ulna and crosses the wrist to attach to the pisiform bone at the base of the palm. This muscle is built for power, bending and deviating the wrist away from the thumb, which is useful in activities such as throwing and hammering. The brachioradialis (BR) is another forearm muscle that acts as a pure flexor of the elbow, allowing the forearm to twist between the thumb-up and palm-up positions.
The thenar muscle group is found at the base of the thumb and is responsible for important thumb movements. It includes the abductor pollicis brevis, which pulls the thumb away from the index finger, and the flexor pollicis brevis, which bends the thumb towards the small finger. The opponens pollicis allows the thumb to move away from the fingers, enabling us to grasp objects. These forearm muscles work together to provide the fine motor skills and grip strength necessary for a wide range of human activities.
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Grip strength
The flexor digitorum profundus (FDP) is the primary muscle responsible for grip strength. It is a forearm muscle that flexes the fingers and acts on the hand while being located in the forearm. The FDP has four tendons that attach to the second to fifth fingers, allowing them to bend at the joints. The independence of each finger's FDP enables us to perform precise tasks, such as using chopsticks.
Injuries to the FDP can result in conditions like Quadrigia, where the patient cannot make a full fist due to interconnected tendons, or Jersey finger, which involves the rupture of the FDP tendon at the base of the distal phalanx.
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Frequently asked questions
The flexor digitorum profundus muscle is a powerful flexor of the fingers. It pulls the lateral four digits (the index, middle, ring, and small fingers) towards the hand.
The primary function of the flexor digitorum profundus is to bend the fingers. It also acts as a flexor of the midcarpal (wrist), metacarpophalangeal, and proximal interphalangeal joints of the index, middle, ring, and little fingers. It is the chief gripping muscle.
If the flexor digitorum profundus is injured, it can lead to a condition called Quadrigia, where the patient is unable to make a full fist and has a weak grasp. Another possible injury is Jersey finger, where there is no active flexion of the distal interphalangeal joint of the ring finger in 75% of cases.
