Dominic Stone
Pronation and supination are two unique movements that occur at the radioulnar joints, allowing the human body to flip the palm either face up or face down. The movement of pronation involves placing the palms into the prone (face-down) position, like someone would do when looking at the backs of their hands. This movement is produced by forearm muscles, specifically the pronator teres, pronator quadratus, and flexor carpi radialis. The pronator quadratus is the main pronator of the forearm, and it works together with the pronator teres to achieve pronation by pulling on the radius bone of the forearm.
| Characteristics | Values |
|---|---|
| Definition | Pronation is the movement of the hand that turns the palm downwards. |
| Upper limb | In the upper limb, pronation is the rotatory movement during which the palm and forearm face downward. |
| Foot | Pronation is the natural movement of the foot when it rolls inward. |
| Bones involved | The forearm has two bones: radius and ulna. |
| Joints involved | The proximal and distal radioulnar joints formed between the upper and lower ends of the radius and ulna. |
| Muscles involved | Pronator teres, pronator quadratus, and brachioradialis muscles. |
| Nerves involved | The median nerve commonly passes between the two heads of the pronator teres. |
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What You'll Learn
Pronator Teres
The pronator teres is a muscle that is mainly located in the forearm. It is a long, round muscle that rotates the forearm so that the palm faces downwards. This movement is called pronation. The pronator teres has two heads, the humeral head and the ulnar head, and two points of origin: the medial humeral supracondylar ridge and the medial side of the coronoid process of the ulna. The humeral head is larger and more superficial, while the ulnar head is deeper and joins the other head at an acute angle. The ulnar head is sometimes absent.
The median nerve, which innervates the pronator teres, enters the forearm between the two heads of the muscle. The median nerve is at risk of compression at this site, which can lead to pronator teres syndrome, causing pain and numbness, and possibly motor dysfunction. The nerve is also at risk of nerve entrapment, which can cause pain and paresthesia.
The pronator teres works together with the pronator quadratus to pronate the forearm. It also assists in flexion of the elbow joint. If the elbow is flexed at a right angle, the pronator teres will turn the hand so that the palm faces inferiorly. If the elbow is fully flexed, the muscle fibres will be shortened and less able to produce force.
The pronator teres is an important landmark for the cubital fossa, a triangular area located on the anterior upper limb between the arm and forearm. It forms the medial border of the cubital fossa.
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Pronator Quadratus
The pronator quadratus is a thin, short, flat, quadrilateral muscle that is located in the anterior (flexor) compartment of the forearm. It is part of the deep group of forearm flexors, which also includes the flexor digitorum profundus and flexor pollicis longus. The pronator quadratus is the deepest muscle in this group, lying underneath the other two muscles.
The pronator quadratus originates from the anterior surface of the distal shaft of the ulna and an aponeurosis that partially covers the muscle. Its fibres project laterally and distally towards the anterior surface of the distal shaft of the radius, where they also insert. The deeper fibres of the muscle insert superiorly to the ulnar notch of the radius. The muscle extends across the distal parts of the radius and ulna, with its fibres running perpendicular to the direction of the arm.
The main function of the pronator quadratus is forearm pronation, which is the movement that turns the forearm and palm so that they face downwards. This muscle works together with the pronator teres to achieve pronation by pulling on the radius bone of the forearm. When the pronator quadratus contracts, it pulls the lateral side of the radius towards the ulna, resulting in the pronation of the radioulnar joint.
The pronator quadratus is innervated by the anterior interosseous nerve of the forearm, with contributions mainly from C7 and C8 spinal nerves. The anterior interosseous nerve is a branch of the median nerve, which arises from the brachial plexus. The muscle receives arterial blood from the anterior interosseous artery, a branch of the common interosseous artery. The lateral corticospinal tract is responsible for the motor pathway of the pronator quadratus, transmitting signals from the upper motor nerve to the lower motor neurons of the skeletal muscles.
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Brachioradialis
The brachioradialis is a muscle in the forearm that flexes the forearm at the elbow. It is one of the muscles that enable pronation of the upper limb, along with the pronator teres and pronator quadratus muscles. The brachioradialis is also capable of supination, depending on the position of the forearm. When the forearm is pronated, the brachioradialis tends to supinate as it flexes, and vice versa. This muscle is also synergistic with the brachialis and biceps brachii, and it assists the biceps brachii when the forearm is in a midposition between supination and pronation.
The brachioradialis is a fusiform muscle, meaning it is wider at the muscle belly and narrows distally to insert via a thin tendon at its attachment site. It is considered a posterior or extensor-compartment muscle but functions as a flexor. It is one of only two forearm extensor-compartment muscles that do not cross the wrist, the other being the supinator. The brachioradialis is located in the mobile wad compartment, along with the extensor carpi radialis longus and extensor carpi radialis brevis muscles.
The brachioradialis is attached to the distal styloid process of the radius by the brachioradialis tendon and to the lateral supracondylar ridge of the humerus. It is a superficial muscle on the radial side of the forearm and is often fused proximally with the brachialis. It forms the lateral side of the cubital fossa, or elbow pit, and contributes to the muscle mass overlying the anterolateral forearm. The brachial artery, median nerve, and biceps tendon all pass through this region on the elbow's flexor surface.
The brachioradialis is innervated by the radial nerve, receiving input directly from this nerve. It is one of the first muscles to recover innervation following an injury to the radial nerve, making it important to check for radial nerve recovery. The radial nerve passes down the spiral groove of the humerus and emerges anteriorly between the brachialis and brachioradialis. The nerve then enters the supinator muscle and divides into superficial and deep branches.
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Flexor digitorum profundus
The flexor digitorum profundus is a muscle in the forearm that flexes the fingers. The name is derived from Latin and means "deep bender of the fingers". It is considered an extrinsic hand muscle as it acts on the hand while being located in the forearm.
The muscle originates from four sites: the superior three-quarters of the anterior surface of the ulna, the adjacent part of the interosseous membrane, the coronoid process of the ulna, and the aponeurosis of the flexor carpi ulnaris muscle. The muscle then takes an inferior course towards the hand, giving off a broad tendon at the level of the distal third of the forearm. This tendon crosses the superficial surface of the pronator quadratus and enters the hand by passing beneath the flexor retinaculum. Upon entering the hand, the tendon splits into four slips that attach to the palmar surfaces of the bases of the distal phalanges of digits 2-5. The muscle fibres are arranged so that the medial part of the muscle inserts into digits 4 and 5, while the lateral part inserts into digits 2 and 3.
The flexor digitorum profundus is innervated by the anterior interosseous nerve and ulnar nerves. The medial aspect of the muscle, which flexes the 4th and 5th digits, is supplied by the ulnar nerve (C8, T1). The lateral aspect, which flexes the 2nd and 3rd digits, is innervated by the median nerve, specifically the anterior interosseous branch (C8, T1). It is one of two flexor muscles that is not exclusively supplied by the median nerve (the other being the flexor carpi ulnaris). The median nerve travels distally between the flexor digitorum superficialis and the flexor digitorum profundus.
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Flexor pollicis longus
Pronation is the rotatory movement that occurs at the radioulnar joints, during which the palm and forearm face downward. It is important to note that pronation and supination movements in the foot differ from those in the upper limb. In the foot, pronation refers to the natural movement of the foot rolling inward during walking or running.
Now, let's focus on the Flexor Pollicis Longus (FPL):
The FPL is a long muscle located in the forearm and hand, belonging to the deep flexors of the forearm. It is unique to humans, being either rudimentary or absent in other primates. The FPL is responsible for flexing the thumb at the interphalangeal joint, which is essential for gripping. This muscle arises from the anterior surface of the radius and adjacent structures, crossing three joints before inserting into the distal phalanx of the thumb. It is classified as an extrinsic muscle of the hand due to its origin in the forearm and function in thumb movement.
The FPL receives dual blood supply, with its medial part supplied by the anterior interosseous artery (a branch of the ulnar artery) and its lateral part receiving blood from the radial artery. Additionally, the median artery may contribute to its blood supply if the muscle is well-developed. The FPL is innervated by the anterior interosseous branch of the median nerve, derived from spinal roots C7 and C8.
An accessory head of the FPL, known as Gantzer's muscle, is occasionally present. This accessory head may cause compression of the anterior interosseous nerve. It arises from the adjacent part of the interosseous membrane of the forearm and, in some cases, from the medial border of the coronoid process of the ulna.
The FPL is a vital muscle for hand function, particularly in activities that require gripping. Its unique presence in humans highlights its importance in our evolutionary history.
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Frequently asked questions
Pronation is the movement of the forearm and hand that turns the palm to face downwards. It is the opposite of supination, which turns the palm to face upwards.
The muscles involved in pronation are the pronator teres, pronator quadratus, and brachioradialis. The pronator teres is a long, round muscle that rotates the radius and turns the palm down. The pronator quadratus and brachioradialis assist in pronation and flexion of the elbow joint.
Pronation and supination are important movements that help us perform various daily activities. Examples of functional movements that use pronation include turning a screwdriver and turning a key.
