Dominic Stone
The elbow joint is a complex structure that enables a wide range of movements, including flexion and extension. Elbow flexion, or the act of bending the arm at the elbow joint, is primarily achieved through the contraction of three muscles: the biceps brachii, the brachialis, and the brachioradialis. These muscles work in tandem to produce the flexion movement, with the biceps brachii taking centre stage as the most prominent flexor. The pronator teres also contributes to elbow flexion, albeit to a lesser degree, earning it the title of a secondary flexor. The unique functions of these muscles and their attachments to the radius or ulna result in distinct actions, such as pronation or supination of the forearm. Understanding the anatomy of these muscles and their specific roles is crucial, especially in clinical settings when assessing muscle strength or designing targeted stretches for physical therapy.
| Characteristics | Values |
|---|---|
| Definition of a flexor | A muscle that contracts to perform flexion, a movement that decreases the angle between the bones converging at a joint. |
| Elbow flexion | Performed by the biceps brachii, brachialis, and brachioradialis muscles. |
| Knee flexion | Performed by the hamstrings, gracilis, sartorius, gastrocnemius, plantaris, and popliteus. |
| Dorsiflexion | Bringing the foot upward toward the shin. |
| Plantar flexion | Depressing the foot. |
| Isometric exercises | Strengthen muscles by keeping them still. |
| Iliopsoas muscles | Allow for flexion in a sitting position. |
| Third-class levers | Increase the distance moved by a load, requiring greater force to be exerted by the muscle. |
| Antagonists | Muscles that produce the opposite effect on the same bones. |
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What You'll Learn
The flexor pollicis brevis flexes the thumb
The muscles of the thumb can be divided into two groups: the extrinsic hand muscles, with their muscle bellies located in the forearm, and the intrinsic hand muscles, with their muscles located in the hand. The flexor pollicis brevis is an intrinsic hand muscle.
The flexor pollicis brevis is a two-headed muscle. The superficial head arises on the flexor retinaculum, while the deep head originates on three carpal bones: the trapezium, trapezoid, and capitate. The muscle is inserted onto the radial sesamoid bone of the metacarpophalangeal joint.
The flexor pollicis brevis is the smallest and most distal of the thenar muscles. The thenar eminence refers to the group of muscles on the palm at the base of the thumb. The three muscles composing the thenar eminence are the abductor pollicis brevis, flexor pollicis brevis, and opponens pollicis. The abductor pollicis brevis pulls the thumb away from the index finger, and the opponens pollicis performs the important function of bringing the thumb away from the fingers so that we can grasp objects.
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The flexor pollicis longus flexes the thumb
The thumb is moved by a combination of extrinsic and intrinsic muscles. The extrinsic muscles are located in the forearm, while the intrinsic muscles are located in the hand itself. The flexor pollicis longus is an extrinsic muscle that arises from the mid-forearm from the radial shaft and allows us to bend the tip of our thumb. It is the ninth tendon to pass through the carpal tunnel on its way to the thumb.
The intrinsic muscles of the thumb can be divided into two groups: the thenar eminence and other muscles. The thenar eminence refers to the group of muscles on the palm at the base of the thumb. The three muscles composing the thenar eminence are the abductor pollicis brevis, flexor pollicis brevis, and opponens pollicis. The abductor pollicis brevis pulls the thumb away from the index finger, and the flexor pollicis brevis bends the thumb toward the small finger. The opponens pollicis performs one of the most important functions of the human hand: the ability to bring the thumb away from the fingers so that we can grasp objects. It helps pull the thumb away from the index finger, rotating it so that the tip of the thumb is opposite, or "opposes," the tips of the other fingers.
The other two muscles that influence the movement of the thumb are the adductor pollicis and the first dorsal interosseous muscle. The adductor pollicis is a large triangular muscle with two heads. One head originates from metacarpal III, and the other originates from the capitate and adjacent areas of metacarpals II and III. Both attach to the base of the proximal phalanx of the thumb. The first dorsal interosseous lies between the thumb and the first metacarpal (or index finger). This muscle primarily acts on the index finger but also assists the adductor pollicis in thumb adduction.
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The opponens pollicis flexes the thumb's metacarpal bone
The opponens pollicis muscle is a small, triangular muscle in the hand. It is one of three thenar muscles, along with the abductor pollicis brevis and the flexor pollicis brevis. The opponens pollicis muscle originates from the flexor retinaculum of the hand and the tubercle of the trapezium bone. It passes downward and laterally, inserting into the entire length of the thumb's metacarpal bone on its radial side.
The function of the opponens pollicis muscle is to oppose the thumb, allowing it to touch the tips of the other digits. This movement, known as opposition, is a combination of flexion, internal rotation, and palmar abduction of the thumb, which enables the hand to cup objects. The opponens pollicis muscle works in conjunction with the other thenar muscles to control the positioning and coordination of the thumb.
The opponens pollicis receives its blood supply from the superficial palmar arch and is innervated by the recurrent branch of the median nerve. In 20% of the population, the opponens pollicis is innervated by the ulnar nerve instead. Damage to the median nerve can result in atrophy of the thenar muscles, impacting the function of the opponens pollicis and affecting hand movements.
The opponens pollicis muscle is responsible for the flexion of the thumb's metacarpal at the first carpometacarpal joint. This specific action cups the palm and is an essential component of true apposition, which refers to the thumb touching the tips of the other digits. While the opponens pollicis muscle plays a crucial role in this movement, it is important to note that the actions of several other muscles are also required at the thumb's metacarpophalangeal joint for true apposition to occur.
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The abductor pollicis brevis pulls the thumb away from the index finger
The abductor pollicis brevis is a muscle in the hand that functions as an abductor of the thumb. It is a flat, thin muscle located just under the skin. The abductor pollicis brevis pulls the thumb away from the index finger. This movement is called abduction, which is defined as the movement of the thumb in a direction perpendicular to the palm. The abductor pollicis brevis achieves this by acting across the carpometacarpal joint and the metacarpophalangeal joint.
The abductor pollicis brevis originates from the flexor retinaculum of the hand, the tubercle of the scaphoid bone, and, in some cases, the tubercle of the trapezium. It runs laterally and downward and is inserted by a thin, flat tendon into the lateral side of the base of the first phalanx of the thumb and the capsule of the metacarpophalangeal joint.
The abductor pollicis brevis is supplied by the recurrent branch of the median nerve (Roots C8-T1). It cooperates with the abductor pollicis longus to stabilise the trapezium to the carpus in movements of the thumb. This cooperation is necessary to maintain tension in the deep APL. The abductor pollicis brevis also provides some assistance in the opposition and extension of the thumb.
The abductor pollicis brevis can be exercised by forcing it to abduct against resistance, such as a small resistance band or elastic band. This exercise helps train the natural range of motion of the muscle. To perform this exercise, hold the elastic band between your two thumbs and keep your hands as still as possible while abducting your thumb out and away from the rest of your hand. It is important to keep the movement smooth and controlled and adjust the resistance as needed.
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The adductor pollicis rotates and opposes the thumb
The adductor pollicis is a muscle in the hand that functions to adduct the thumb. It is the most powerful of the intrinsic muscles of the hand. The thumb is one of the most important parts of the hand, and its versatility in movement compared to the other digits makes the hand the ultimate tool of elite dexterity.
The adductor pollicis has two heads: transverse and oblique. It is a fleshy, flat, triangular, and fan-shaped muscle deep in the thenar compartment beneath the long flexor tendons and the lumbrical muscles at the center of the palm. The oblique head arises from the capitate bone, the bases of the second and third metacarpals, the intercarpal ligaments, and the sheath of the tendon of the flexor carpi radialis. The transverse head, on the other hand, originates from the palmar base of the third metacarpal bone.
The adductor pollicis muscle is larger than and deeper than the thenar muscles. It is also the sole muscle of the adductor compartment of the hand and does not form part of the thenar muscle group. The thenar muscle group is found at the base of the thumb and forms the muscle bulk on the thumb side of the hand. The adductor pollicis evolved from the contrahens I muscle as man's ancestors' thumbs and big toes became opposable.
The main function of the adductor pollicis is to adduct the thumb, which refers to bringing the thumb into a position of opposition at the center of the palm. It can also bring the thumb alongside the palm and index finger. This movement is a combination of actions, namely adduction, medial rotation, flexion, and adduction of the thumb so that it can touch each fingertip of the same hand.
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Frequently asked questions
The biceps brachii, brachialis, and brachioradialis flex the elbow.
The flexor group of the forearm flexes the wrist and fingers.
The gracilis, sartorius, hamstrings, gastrocnemius, plantaris, and popliteus are all muscles that flex the knee.
