Dominic Stone
The lumbricals are a group of four intrinsic muscles of the hand that originate from the flexor digitorum profundus (FDP) tendon. They are responsible for flexion at the metacarpophalangeal joints and extension at the interphalangeal joints. The name lumbrical comes from the Latin lumbricus, meaning worm, which describes the appearance of the muscle. These muscles are unique in that they attach to tendons rather than bone, and they play a role in complex hand movements and dexterity.
| Characteristics | Values |
|---|---|
| Number | Four |
| Type | Intrinsic muscles of the hand |
| Origin | Tendons of flexor digitorum profundus (FDP) |
| Insertion | Extensor expansion on the dorsum of the hand |
| Function | Flexion at the metacarpophalangeal joint and extension at the interphalangeal (IP) joints |
| Innervation | Lateral two lumbricals (index and middle fingers) by the median nerve; medial two lumbricals (little and ring fingers) by the ulnar nerve |
| Blood Supply | Superficial palmar arch, common palmar digital artery, deep palmar arch, and dorsal digital artery |
| Structure | Lumbricals 1-2 are unipennate; Lumbricals 3-4 are bipennate |
| Muscle Spindles | High density |
| Clinical Relevance | May have implications for trauma injuries, especially replantation for zone III amputations |
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What You'll Learn
The four lumbrical muscles are intrinsic to the hand
The four lumbricals are numbered and associated with a specific digit. Lumbrical 1 is associated with the index finger, Lumbrical 2 with the middle finger, Lumbrical 3 with the ring finger, and Lumbrical 4 with the little finger. Lumbricals 1 and 2 are typically unipennate, meaning their fibres arise from one tendon, while Lumbricals 3 and 4 are bipennate, arising from two adjacent tendons.
The lumbrical muscles play a crucial role in the complex movement and dexterity of the hand. They are involved in fine motor control and proprioception, thanks to their high density of muscle spindles. These muscles are also clinically significant in certain trauma injuries, particularly replantation for zone III amputations, where damage to the lumbrical muscles can lead to poor functional outcomes.
In terms of innervation, the first and second lumbricals are typically innervated by the median nerve, while the third and fourth lumbricals are innervated by the deep branch of the ulnar nerve. However, individual variation exists, with some people exhibiting different patterns of innervation.
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They are associated with the flexor digitorum profundus (FDP) tendon
The lumbrical muscles of the hand are intrinsic muscles of the hand that are associated with the flexor digitorum profundus (FDP) tendon. The FDP is a muscle in the forearm of humans that flexes the fingers, also known as digits. It is considered an extrinsic hand muscle because it acts on the hand while its muscle belly is located in the forearm. The FDP is the major muscle for hand-gripping power. It 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, to the palmar base of the distal phalanx.
The lumbricals, on the other hand, are four small, worm-like muscles on each hand. They are unusual in that they do not attach to bone but rather to tendons. They attach proximally to the tendons of the FDP and distally to the extensor expansions. The first and second lumbricals are unipennate, while the third and fourth are bipennate. The first lumbrical arises from the radial side and palmar surface of the tendon of the index finger, while the second arises from the same areas for the middle finger. The third and fourth lumbricals arise from the ulnar side of the middle and ring finger tendons, respectively, and the radial side of the ring and little finger tendons, respectively.
The lumbricals' innervation always follows the innervation pattern of the associated muscle unit of the FDP. This means that if the muscle units supplying the tendon to the middle finger are innervated by the median nerve, the second lumbrical will also be innervated by the median nerve. The lumbricals, with the help of the interosseous muscles, simultaneously flex the metacarpophalangeal joints while extending both interphalangeal joints of the digit on which it inserts. This combined movement plays a role in the complex movement of the hand and contributes to its general dexterity.
After detachment of the distal tendons, the lumbricals serve as the new insertion surface of the FDP. This means that even though a person consciously activates the flexor muscle, they actually move the lumbricals instead. Since the FDP and the lumbricals are antagonists in the proximal interphalangeal (PIP) and distal interphalangeal (DIP) joints, the intended fist closure paradoxically leads to an extension of the fingers. This oddity is clinically referred to as the lumbrical-plus finger and can occur after injuries or amputations.
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Lumbricals are involved in complex hand movement
The lumbricals are intrinsic muscles of the hand that play a role in complex hand movements. There are four lumbrical muscles in each hand, and they are small, worm-like muscles that do not attach to bones. Instead, they attach proximally to the tendons of the flexor digitorum profundus and distally to the extensor expansions. This unique attachment allows the lumbricals to perform two different actions: flexion at the metacarpophalangeal joints and extension at the proximal and distal interphalangeal joints. These combined movements are essential for complex hand movements, such as holding a pen, and contribute to the overall dexterity of the hand.
The first lumbrical arises from the radial side and palmar surface of the tendon of the index finger, while the second lumbrical originates from the same areas of the tendon of the middle finger. Lumbricals 1 and 2 are typically unipennate, meaning their fibres arise from a single tendon. On the other hand, the third and fourth lumbricals are bipennate, arising from two adjacent tendons. The third lumbrical is associated with the ulnar side of the middle finger tendon and the radial side of the ring finger tendon. Meanwhile, the fourth lumbrical is connected to the ulnar side of the ring finger tendon and the radial side of the little finger tendon.
The lumbricals' ability to facilitate complex hand movements is further enhanced by their role in proprioception. The high number of muscle spindles found in the lumbricals suggests that they are involved in sensory feedback, which is crucial for precise pinch movements and object manipulation. This sensory feedback mechanism contributes to the overall dexterity and precision of hand movements.
Additionally, the lumbricals are associated with the flexor digitorum profundus (FDP) tendon. After an injury or amputation that results in the detachment of distal tendons, the lumbricals become the new insertion surface for the FDP. This anatomical arrangement leads to an interesting phenomenon known as the "lumbrical-plus finger." When an individual consciously activates the flexor muscle, the lumbricals move instead, resulting in an unintended extension of the fingers.
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They are weak extensors of the PIP joint
The lumbrical muscles are indeed intrinsic muscles of the hand. They are a group of four small, worm-like muscles on each hand, with the first and second lumbricals being unipennate and the third and fourth being bipennate. The name "lumbrical" comes from the Latin "lumbricus", meaning "worm", which describes the gross appearance of the muscle.
The lumbricals are associated with the flexor digitorum profundus (FDP) tendon and insert into the extensor expansions. This means that both of their attachment points are quite mobile, allowing the muscles to perform two different actions: flexion at the metacarpophalangeal joints and extension in both the proximal (PIP) and distal interphalangeal joints (DIP).
While the lumbrical muscles do contribute to PIP joint extension, they are considered weak extensors when compared to the interosseous muscles. Schreuder and Stam measured the strength of the lumbrical muscles in 12 patients with complete ulnar nerve paralysis and found that the ulnar nerve patients lost approximately 90% of intrinsic muscle strength when compared to the opposite hand. This led them to conclude that the lumbricals are weak extensors of the PIP joint.
The relatively small size of the lumbrical muscles and their limited motor function further support the idea that they are weak extensors. Biomechanical studies have shown that the lumbrical muscle primarily functions to extend the PIP and DIP joints, but its small size and limited range of motion suggest that it plays a more supportive role in hand movement rather than being the primary driver of extension.
In summary, while the lumbrical muscles are intrinsic hand muscles that contribute to PIP joint extension, they are considered weak extensors when compared to other muscle groups in the hand, such as the interosseous muscles. Their small size and limited motor function suggest that they play a more supportive role in hand movement and dexterity.
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Lumbricals are important in certain trauma injuries
Lumbricals are a group of four small, worm-like muscles on each hand. They are intrinsic muscles, meaning they originate and insert in the hand. Lumbricals are unique because they do not attach to bones; instead, they attach proximally to the tendons of the flexor digitorum profundus and distally to the extensor expansions. This gives the muscles two different actions: flexion at the metacarpophalangeal joints and extension in both the proximal and distal interphalangeal joints. This complex movement plays a role in the general dexterity of the hand, such as holding a pen.
The structure of the lumbricals is also important in understanding how they get injured. The lumbricals of the pinky and ring fingers have a bipennate structure, while the middle and index fingers are unipennate. A bipennate muscle is like a feather, with the "quill" being a tendon and the "barbs" being muscle fibres. The muscle fibres of a bipennate lumbrical attach to the extensor tendon of one finger and the flexor tendons of two fingers. This can lead to problems if those two fingers are moved in opposite directions while under force.
Lumbrical injuries typically occur when pockets are involved or when hitting small holds with just 2-3 fingers. This causes some fingers to remain in the extended position while the adjacent finger or two goes into a flexed position. If there is a significant tear with substantial pain and loss of function, it may be a high-level injury that requires immediate medical attention.
It is important to note that lumbrical injuries should not be treated like pulley injuries. While you can massage your fingers to help with the pain, taping is not supported by research as a method for healing. Therefore, understanding the structure and function of the lumbricals and how they can be injured is crucial for climbers and others who engage in similar activities.
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Frequently asked questions
Yes, the lumbrical muscles are intrinsic to the hand.
The lumbricals are four small, worm-like muscles on each hand. They are unusual in that they attach to tendons, not bone.
The lumbrical muscles flex the metacarpophalangeal joints and extend the interphalangeal joints. They are used during an upstroke in writing and play a role in complex hand movements and dexterity.
The name "lumbrical" comes from the Latin "lumbricus", meaning "worm", which describes the gross appearance of the muscle.
