Dominic Stone
Flexor muscles are muscles that contract to perform flexion, a movement that bends joints such as the elbow or knee by decreasing the angle between the bones connected by those joints. These muscles are located on the inner side of the limb they move and are essential for movements like gripping or walking. The flexor muscles in the human body include the flexor digitorum superficialis, flexor digitorum profundus, flexor pollicis longus, and pronator quadratus. The attachment points of these muscles vary, with some originating from the medial epicondyle of the humerus and others from the ulna or radius bones. The flexor tendons enter the hand through the carpal tunnel and attach to the base of the phalanges of the fingers or thumb.
| Characteristics | Values |
|---|---|
| Definition | A muscle that contracts to perform flexion, a movement that decreases the angle between the bones converging at a joint. |
| Types | Forearm flexors, Hip flexors, Knee flexors |
| Forearm Flexors | Flexor digitorum superficialis, Flexor digitorum profundus, Flexor pollicis longus, Pronator quadratus |
| Hip Flexors | Iliopsoas, Rectus femoris, Sartorius |
| Knee Flexors | Hamstrings |
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What You'll Learn
Flexor digitorum superficialis
The flexor digitorum superficialis (FDS) is a muscle in the forearm that is responsible for flexing the middle phalanges of the medial four digits (digits 2-5) at the proximal interphalangeal joints. It also assists in flexing the metacarpophalangeal joints and the wrist joint.
The FDS has two origin points, or heads, which are the humeroulnar head and the radial head. The humeroulnar head originates from the medial epicondyle of the humerus and the coronoid process of the ulna. The radial head originates on the upper half of the volar aspect of the radial shaft. From these origin points, the muscle fibres separate into two planes—superficial and deep. The superficial plane provides tendons for the middle and ring fingers, while the deep plane provides tendons for the index and little fingers.
The FDS is the primary flexor of the proximal interphalangeal (PIP) joints of the middle phalanges, but it also assists in flexing the metacarpophalangeal (MCP) joints. The FDS is unique in that it can flex each finger it serves independently. This is in contrast to the flexor digitorum profundus, which flexes the digits slowly. The FDS reinforces this action when speed and flexion are required against resistance.
The median nerve and ulnar artery pass between the two heads of the FDS, and the primary blood supply to the muscle is from the ulnar artery. The FDS is innervated by muscular branches of the median nerve, derived from roots C8 and T1, which arise from the medial and lateral cords of the brachial plexus. The skin that overlies the muscle is supplied by roots C6-8 and T1.
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Flexor digitorum profundus
The flexor digitorum profundus 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, intrinsic muscles of the hand, attach to the tendon of flexor digitorum profundus. The medial half (associated with the ring and little fingers) is innervated by the ulnar nerve, while the lateral half (associated with the middle and index fingers) is innervated by the anterior interosseous nerve (C8, T1)—a motor branch of the median nerve.
The flexor digitorum profundus is the only muscle that can flex the distal interphalangeal joints of the fingers. It also flexes the metacarpophalangeal joints and the wrist. The deeper tendons of the flexor digitorum profundus continue distally to attach to the base of the distal phalanx. The flexor digitorum profundus is a powerful muscle, forming the muscular elevation seen and felt on the posterior surface of the forearm medial to the subcutaneous posterior border of the ulna.
The flexor digitorum profundus is supplied by the anterior interosseous artery, a branch of the common interosseous artery. The lymphatic drainage of the flexor digitorum profundus is part of the upper limb lymph system, consisting of superficial and deep lymphatic vessels. The superficial vessels around the basilic vein go to the cubital lymph nodes, which are proximal to the medial epicondyle of the humerus. The deep lymphatic vessels also drain lymph, following the major deep veins terminating in the humeral axillary lymph nodes.
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Flexor pollicis longus
The flexor pollicis longus (FPL) is a long muscle located in the anterior compartment of the forearm, in the same layer as the flexor digitorum profundus and pronator quadratus. It is one of the three deep flexors of the volar compartment of the forearm. The FPL is unique to humans, being either rudimentary or absent in other primates.
The FPL originates from the mid-half of the anterior surface of the radius and adjacent half of the interosseous membrane. It has also been found to take its origin from the medial epicondyle of the humerus, coronoid process of the ulna, and from flexor digitorum superficialis. An occasionally present accessory long head of the FPL is called Gantzer's muscle.
The FPL forms a flattened large tendon that courses through the carpal tunnel, crossing three joints of the hand to attach at the base of the distal phalanx of the thumb. It is the primary flexor of the thumb, providing flexion at its metacarpophalangeal (MCP) joint and interphalangeal (IP) joint. Additionally, the FPL has minor contributions to radial wrist deviation and wrist flexion.
The FPL is supplied by the anterior interosseous (C8-T1) branch of the median nerve (C5-T1). It receives its blood supply from the anterior interosseous artery, a branch of the ulnar artery. Slips may connect with the flexor digitorum superficialis muscle, flexor digitorum profundus muscle, or the pronator teres muscle.
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Hip flexors
Other hip flexor muscles include the rectus femoris, sartorius, pectineus, and tensor fasciae latae. The rectus femoris is one of the quadriceps and has two functions: to flex at the hip and extend the knee. It has a proximal attachment at the acetabulum and inserts into the tibial tuberosity. The sartorius is the longest muscle in the human body, running from the pelvis to the knee and helping to flex the knee and leg. The tensor fasciae latae can also be assessed for tenderness, taut fibres, and trigger points.
Hip flexor strains are a common injury, often caused by overuse or repetitive over-activation of the muscles and/or tendons, leading to microtears within the tissue. Partial tears can lead to hip flexor strain and pain, and people can also experience "tight" hip flexors, which are not very flexible. Tight hip flexors can increase the risk of injury and can pull on the lumbar spine, potentially contributing to low back pain.
To prevent and treat hip flexor injuries, it is important to stretch and strengthen the hip flexor muscles. Rest is crucial for healing, and specific exercises such as lunges, mountain climbers, and pigeon pose can help strengthen the hip flexors. Warming up before exercise and avoiding activities that overuse the hip flexors are also important considerations.
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Forearm flexors
The forearm is made up of twenty muscles, divided between the anterior (flexor) and posterior (extensor) compartments. The anterior compartment is further divided into three layers: the superficial layer, the middle layer, and the deep layer.
Superficial Layer
The superficial layer contains the pronator teres, palmaris longus, flexor carpi radialis, and flexor carpi ulnaris. These muscles share a common origin on the common flexor tendon, which arises from the medial epicondyle of the humerus. The flexor carpi radialis helps move your arm away from your body, while the flexor carpi ulnaris lets you move your wrist back and forth. The palmaris longus helps flex the wrist, and the pronator teres muscle helps rotate your forearm.
Middle Layer
The middle layer is composed exclusively of the flexor digitorum superficialis. This muscle is sometimes classed as a superficial muscle, but in most individuals, it lies between the deep and superficial muscle layers. It has two heads, one originating from the medial epicondyle of the humerus and the other from the radius. It splits into four tendons at the wrist, which attach to the base of the middle phalanx of the four digits.
Deep Layer
The deep layer is comprised of the flexor digitorum profundus, flexor pollicis longus, and pronator quadratus. The flexor digitorum profundus is a deep muscle that originates at the ulna and acts to bend the fingers near their tips. The flexor pollicis longus allows you to bend your thumb, and the pronator quadratus is a square-shaped muscle that lets you turn your forearm downward.
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Frequently asked questions
The flexor muscles in the forearm attach to the base of the distal phalanx of the thumb. The flexor digitorum superficialis attaches to the base of the middle phalanx, while the deeper tendons of the flexor digitorum profundus attach to the base of the distal phalanx.
The iliopsoas is the body's most important hip flexor. The rectus femoris is a hip flexor muscle that can flex at the hip and extend the knee.
The hamstring muscles are the primary flexors in the thigh.
