Human Anatomy (with the Fundamentals of Dynamic and Sports Morphology) - Ivanitsky, M. F. 2008

Myology
Muscles of the Upper Extremity

The Muscles of the Upper Limb are divided into those that produce Movements of the Pectoral Girdle (primarily at the sternoclavicular joint), movements at the shoulder, elbow, wrist joints, and the JOINTS OF THE hand (Fig. 54).

Muscles producing movements of the pectoral girdle

Schematically, movements of the pectoral girdle (scapula and clavicle) are subdivided into:

1. Forward and backward movement with abduction of the scapula from the spinal Column and adduction toward it.

2. Elevation and depression of the scapula and clavicle.

3. Rotation of the scapula around the sagittal axis with its inferior angle moving medially and laterally.

4. Circular motion of the lateral end of the clavicle accompanied by the scapula.

Six functional Muscle groups participate in these movements.

Forward movement

Forward movement of the pectoral girdle is produced by muscles that intersect the vertical axis of the sternoclavicular joint and are located anterior to it. These include:

1) the pectoralis major, which acts on the pectoral girdle via the humerus;

2) the pectoralis minor;

3) the serratus anterior.

The pectoralis major muscle (see Fig. 37) is considerable in both thickness and width. It covers the upper Ribs anteriorly and participates in forming the anterior wall of the axillary fossa.

This muscle originates from the medial half of the clavicle (clavicular part), the anterior surface of the Sternum and the cartilaginous PARTS OF THE upper five or six ribs (sternocostal part), the anterior wall of the rectus sheath (abdominal part), and inserts into the crest of the greater tubercle of the humerus.

It belongs to the muscles extending from the trunk to the free upper limb. Only when the humerus is fixed in the shoulder joint by the contraction of other muscles does the pectoralis major pull the scapula forward and abduct it from the spinal column. This function is secondary. Primarily, it participates in movements of the humerus. When the trunk is fixed, this muscle adducts, pronates, and flexes the humerus. When the upper limb is fixed, such as during a pull-up on a horizontal bar (proximal support), this muscle contributes—mainly through its lower portion—to pulling the trunk upward. The lower portion of the muscle can also assist in elevating the ribs, participating in the respiratory mechanism. During hanging, the muscle stabilizes the shoulder joint, counteracting gravity. If the arms are positioned close to the midline at this time, the greater part of the work is expended on overcoming body weight, and less on joint stabilization; if the arms are abducted, the opposite is true. With the arms lowered and secured (support on parallel bars), the pectoralis major can elevate the ribs because the direction of its fibers coincides with that of the intercostal muscles.

The external shape of the upper anterior trunk depends to a large extent on the form of the pectoralis major muscle. With good development of this muscle and thin Skin, not only its upper and lower boundaries during contraction, but also the direction of its individual bundles become clearly visible. Between the pectoralis major and the laterally situated deltoid muscle lies the clearly visible and palpable deltopectoral groove, which expands directly beneath the clavicle into the infraclavicular fossa (see Fig. 37). One of the Structural Features of the pectoralis major is that its lower bundles run not only inferiorly but also posteriorly relative to the middle and upper bundles. On the humerus, the lower bundles attach higher than the upper ones originating from the clavicle. Due to this structural feature, the upper and lower bundles are evenly stretched and somewhat untwisted during arm abduction, which is especially pronounced when the arm is raised upward (Fig. 55).

The pectoralis minor muscle lies deep to the pectoralis major. It originates from the 2nd to 5th ribs and, passing superiorly and laterally, reaches the coracoid process of the scapula, to which it attaches.

The pectoralis minor moves the pectoral girdle forward and downward and participates in rotating the scapula with its inferior angle medially (toward the spinal column). If the scapula is fixed, this muscle elevates the ribs and contributes to expanding the thoracic cavity during inspiration. When supporting the body on parallel bars, it stabilizes the trunk relative to the pectoral girdle, working with a proximal support (on the BONES OF THE shoulder girdle).

The serratus anterior muscle (see Figs. 37, 54) is located on the lateral surface of The thoracic cage and is covered by the pectoralis Major and minor muscles. It originates by digitations from the upper nine or eight ribs and attaches to the medial border and inferior angle of the scapula.

This is the primary muscle that moves the scapula anteriorly and laterally. Its significance for the forward movement of the pectoral girdle is especially great during a straight punch in boxing, a lunge in fencing, etc. The lower digitations of this muscle depress the scapula and pull its inferior angle not only downward but also forward.

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Fig. 55. Diagram of The Structure of the pectoralis major muscle can produce opposing actions, rotating the scapula with either its superior or inferior angle anteriorly. The serratus anterior muscle is particularly well visible when the arm is raised. Covering the lateral surface of the thoracic cage, this muscle forms the medial wall of the axillary fossa.

Backward movement

Backward movement of the pectoral girdle is performed by muscles that intersect the vertical axis of the sternoclavicular joint and lie posterior to it. This muscle group includes:

1) trapezius muscle;

2) rhomboid major and minor muscles;

3) latissimus dorsi muscle.

The trapezius muscle is triangular in shape. It is located on the posterior aspect of the trunk and neck (see Fig. 38).

The origin sites of the muscle are: the Occipital bone (superior nuchal line), the nuchal ligament extending from the occipital bone to the spinous processes of the cervical vertebrae, as well as the spinous processes of all thoracic vertebrae. The insertion site is the pectoral girdle. The upper fibers of the muscle, running laterally and downward, attach to the acromial end of the clavicle; the middle fibers, running horizontally, attach to the acromion of the scapula; and the lower fibers attach to its spine. The direction of the fibers in all three parts of the trapezius muscle varies, which accounts for their distinct Functions.

The function of the trapezius muscle depends on which part of the Skeleton is currently fixed. If THE Vertebral Column and HEAD are fixed, the upper part of the muscle helps elevate the scapula, the middle part retracts it toward the vertebral column, and the lower part depresses the pectoral girdle. Furthermore, simultaneous contraction of the upper and lower parts creates a force couple that rotates the scapula with its inferior angle laterally around the sagittal axis (see Fig. 48). If the pectoral girdle is fixed, bilateral contraction of the trapezius muscle extends the head and vertebral column, whereas unilateral contraction tilts the head and cervical spine toward the contracting muscle. During exercises such as the front support, the tension in the lower part of this muscle is clearly visible. Its underdevelopment can lead to round shoulders, whereas asymmetrical development between the right and left sides may result in Scoliosis with a shift of the spinous processes toward the less developed muscle.

The contours of the trapezius muscle are clearly visible in a living person when the arms are raised upward.

The rhomboid major and minor muscles (see Fig. 38) are somewhat rhomboid in shape. They originate from the spinous processes of the lower two cervical and upper four thoracic vertebrae and insert into the medial border of the scapula, forming a single common muscle. It is usually divided into two parts: the upper part, running from the cervical vertebrae (rhomboid minor muscle), and the lower part, running from the thoracic vertebrae (rhomboid major muscle).

The function of these muscles is to retract and slightly elevate the scapula. During retraction, the rhomboids act as synergists to the middle part of the trapezius and direct antagonists to the serratus anterior muscle. Isolated contraction of the lower part of the rhomboid major rotates the scapula by pulling its inferior angle inward, bringing it closer to the vertebral column. When the scapulae are fixed, the rhomboid muscles extend the vertebral column, and during unilateral tension, they contribute to its lateral flexion.

Like the trapezius, the latissimus dorsi muscle (see Fig. 38) is located directly beneath the skin in the lower region of the posterior trunk, being covered by the trapezius only in its upper portion. The latissimus dorsi originates from the spinous processes of the lower five to six thoracic vertebrae, all lumbar vertebrae, the upper sacral vertebrae, and the posterior part of the iliac crest. In addition, it originates by four digitations from the lower four ribs. Its fibers run laterally and upward, covering the inferior angle of the scapula, and insert into the crest of the lesser tubercle of the humerus. The upper border of the latissimus dorsi is clearly visible when the arm is raised upward and abducted. The anterior border is visible during shoulder adduction, especially when overcoming some resistance simultaneously. In the lumbar region, the latissimus dorsi features a broad aponeurosis that forms part of the very strong thoracolumbar fascia. The posterior layer of this fascia serves as the origin site for the latissimus dorsi.

The muscle's function is diverse. By adducting and medially rotating the humerus, it causes depression of the pectoral girdle and retraction of the scapula toward the vertebral column; by covering the inferior angle of the scapula, it helps fix the scapula against the thoracic cage; the portion of the muscle originating from the ribs can elevate them and exert some influence on increasing thoracic volume during inspiration. Furthermore, this muscle causes trunk movement, which is particularly evident during pull-ups on a horizontal bar or gymnastics rings, where its origin and insertion points shift relative to each other. The muscle experiences maximal tension in the L-sit position.

As a rule, the latissimus dorsi is very well developed in cross-country skiers and rowers. If the pectoral girdle is fixed, its unilateral contraction (hanging by one arm) can facilitate lateral flexion of the vertebral column. When the arms are raised and the vertebral column is strongly extended (for example, in the bridge position), this muscle is heavily stretched and under tension in its lower part. Here, its function is reduced to stabilizing THE POSITION OF the upper limb.

Upward movement

The muscles that elevate the pectoral girdle intersect the sagittal axis of the sternoclavicular joint and lie lateral to it. They run toward the scapula and clavicle from top to bottom.

Elevation of the pectoral girdle is produced by the following muscles:

1) the upper fibers of the trapezius muscle, which pull the lateral end of the clavicle and the acromion of the scapula upward (see p. 148);

2) the levator scapulae muscle;

3) the rhomboid muscles, the resolution of whose resultant force yields a certain upward component (see p. 149);

4) the sternocleidomastoid muscle, which, by attaching with one of its heads to the clavicle, pulls it and consequently the scapula upward.

The levator scapulae muscle originates from the transverse processes of the upper four cervical vertebrae and runs toward the superior angle of the scapula, into which it inserts. With a fixed vertebral column, this muscle elevates the superior angle of the scapula; with a fixed pectoral girdle, it rotates the cervical vertebrae, tilting and turning the head toward its side (in unilateral contraction), or extends the head and cervical spine (in bilateral contraction). Acting as a head-turning muscle, it works simultaneously with the sternocleidomastoid and scalene muscles of the opposite side. When tilting the head laterally, this muscle works in synergy with the aforementioned muscles of the same side.

The sternocleidomastoid muscle (see Fig. 37) is the strongest muscle in the anterolateral region of the neck. According to its origin sites, this muscle has two heads: the sternal head, originating from the manubrium of the sternum, and the clavicular head, originating from the sternal end of the clavicle. Between them lies the lesser supraclavicular fossa, where the coracoid process of the scapula can be palpated. The insertion site of the muscle is the mastoid process of the Temporal bone.

The function of the muscle is rather complex. If the Head and Neck are fixed by the tension of other muscles, the sternocleidomastoid can participate in elevating the pectoral girdle and, partly, the rib cage. Only the clavicular head of this muscle takes part in elevating the pectoral girdle. Because the resultant force vector of the muscle passes very close to the anteroposterior axis of the sternocleidomastoid joint, the torque of this muscle as an elevator of the pectoral girdle is small.

The leverage of this muscle is significantly greater with respect to the axes of rotation of the intervertebral joints, as well as the joints between the ribs and vertebrae and between the occipital bone and the atlas. When both the right and left muscles work simultaneously, flexion of the cervical spine and extension of the head (tilting it backward) at the atlanto-occipital joint occur. This action is explained by the fact that its resultant force vector passes posterior to the transverse axis of the atlanto-occipital joint and anterior to the transverse axes of the joints between the cervical vertebrae. If the muscle contracts on one side, it produces a rotation of the head to the opposite side and a lateral tilt toward its own side.

Downward movement

Merely relaxing the muscles that elevate the pectoral girdle is sufficient for it to descend under METABOLISM/18.html">The Influence of gravity—both its own and that of the free upper limb. Depression is facilitated by muscles running from bottom to top, extending from the thoracic cage or vertebral column to the bones of the pectoral girdle:

1) pectoralis minor (see p. 147);

2) subclavius;

3) lower fibers of the trapezius (see p. 148);

4) lower slips of the serratus anterior (see p. 147).

In addition, depression is assisted by muscles extending from the trunk to the shoulder, namely the pectoralis major (see p. 146) and latissimus dorsi (see p. 150), primarily through their lower portions.

Subclavius. This muscle is located between the first rib and the clavicle. It originates from the cartilaginous part of the first rib and inserts into the Inferior surface of the acromial end of the clavicle. The function of the muscle is to stabilize the sternoclavicular joint and to pull the clavicle downward and forward.

Rotation of the Scapula

(movement of the inferior angle inward and outward)

Rotation of the scapula inward, with the inferior angle moving toward the vertebral column, is produced by a force couple formed by:

1) the pectoralis minor (see p. 147) and

2) the lower part of the rhomboideus major (see p. 149).

Rotation of the scapula outward, moving the inferior angle away from the vertebral

column in a lateral direction, results from the action of a force couple formed by the upper and lower parts of the trapezius (see Fig. 48).

This movement is assisted by:

1) the lower and middle slips of the serratus anterior (see p. 147);

2) the teres major (see p. 155) when the free upper limb is fixed.

Circumduction

Circumduction of the pectoral girdle occurs As a result of the sequential contraction of all its muscles.

Muscles Producing Movements at the Shoulder Joint

Movements around three mutually perpendicular axes are possible at the shoulder joint:

1) abduction and adduction around the anteroposterior axis;

2) flexion and extension around the transverse axis;

3) pronation and supination around the vertical axis;

4) circumduction.

These movements are provided by six functional muscle groups.

Abduction of the arm

The muscles that abduct the arm cross the sagittal axis of rotation at the shoulder joint and lie lateral to it. The humerus is abducted by the following muscles:

1) the deltoid, and

2) the supraspinatus.

The deltoid muscle (see Fig. 54) originates from the clavicle (anterior part of the muscle), the acromion (middle part), and the spine of the scapula (posterior part), and inserts into the deltoid tuberosity of the humerus. In shape, this muscle somewhat resembles an inverted Greek letter "delta", which is THE ORIGIN OF its name.

The function of the deltoid muscle is as follows. When its anterior and posterior parts contract alternately, the upper limb moves forward and backward, i.e., flexion and extension occur. However, when the entire muscle contracts, its anterior and posterior parts form a resultant force vector whose direction coincides with that of the fibers of the middle part, contributing to the abduction of the arm up to the horizontal level.

The deltoid muscle contains numerous Connective Tissue septa, relative to which individual muscle fascicles run at a certain angle, making the muscle multipennate and increasing its lifting power. Since the muscle inserts into the deltoid tuberosity, which is located laterally and anteriorly on the humerus, it can also participate in the rotation of this bone around the vertical axis. Specifically, the anterior part of the muscle not only elevates the arm anteriorly (flexion) but also pronates it, whereas the posterior part not only extends but also supinates it. If the anterior part contracts together with the middle part, According to the parallelogram of forces rule, the muscle flexes and slightly abducts the arm to the side. If the middle part contracts together with the posterior part, simultaneous extension and abduction of the arm take place. During hanging by straight arms, the deltoid muscle fixes the pectoral girdle to the humerus. Because it attaches to the humerus closer to the point of application of the gravity force, the muscle is able to exert greater force with a smaller range of motion due to the longer lever arm. It has been established that the middle part of the deltoid muscle, consisting of short muscle bundles, is capable of exerting greater force through small arcs and is primarily adapted for static work; the anterior and posterior parts, consisting of longer fibers, can produce movements with less force but through larger arcs, i.e., perform dynamic work.

The deltoid muscle strengthens the shoulder joint and defines the contours of this body region.

The supraspinatus muscle is located in the supraspinous fossa of the scapula and has a triangular shape. It originates from this fossa and the overlying fascia, and inserts into the greater tubercle of the humerus and partially into the capsule of the shoulder joint.

The function of the muscle is to abduct the arm and tense the Joint Capsule of the shoulder. By attaching to the humerus close to the axis of rotation—that is, the fulcrum—and far from the point of application of the gravity force, the supraspinatus acts on a short lever arm, producing diverse movements through large arcs, but with limited force.

In a living person, this muscle is not visible because it is completely covered by other muscles (trapezius, deltoid).

Adduction of the arm

There are no dedicated muscles that cross the sagittal axis of the shoulder joint and lie medial to it; therefore, adduction of the arm, following the parallelogram of forces rule, is performed by the simultaneous contraction of muscles located anterior (pectoralis major) and posterior to the shoulder joint (latissimus dorsi and teres major). These muscles are assisted by:

1) the infraspinatus;

2) the teres minor;

3) the subscapularis;

4) the long head of the triceps brachii (see p. 160);

5) the coracobrachialis (see p. 156).

The infraspinatus muscle (see Fig. 38) is located in the infraspinous fossa of the scapula, from which it originates. In addition, the origin of this muscle includes the infraspinous fascia. The muscle inserts into the greater tubercle of the humerus, being partially covered by the trapezius and partially by the deltoid muscle.

The function of the infraspinatus muscle is the adduction, supination, and extension of the arm at the shoulder joint. Since this muscle attaches to the capsule of the shoulder joint, during supination of the arm it simultaneously pulls on the capsule, protecting it from impingement.

The teres minor muscle (see Fig. 38) lies inferior to the infraspinatus muscle. It originates from the scapula and inserts into the greater tubercle of the humerus, assisting in adduction, supination, and extension of this bone.

The teres major muscle (see Fig. 38) originates from the inferior angle of the scapula and inserts into the crest of the lesser tubercle of the humerus, often via a common tendon with the latissimus dorsi. When contracting, the teres major forms a rounded prominence during the adduction of a pronated arm. The function of the muscle is the adduction, pronation, and extension of the humerus.

The subscapularis muscle is located on the anterior surface of the scapula, filling the subscapular fossa from which it originates. The muscle inserts into the lesser tubercle of the humerus. Contracting in synergy with the preceding muscles, it performs adduction of the arm; when acting in isolation, it acts as a pronator. Because this muscle is multipennate, it possesses considerable lifting power.

Flexion of the arm

The flexor muscles of the arm cross the frontal (transverse) axis of the shoulder joint and lie anterior to it.

Shoulder flexion (forward movement of the arm) is performed by the following muscles:

1) the anterior part of the deltoid (see p. 153);

2) the pectoralis major (see p. 146);

3) the coracobrachialis;

4) the biceps brachii (see p. 157).

The coracobrachialis muscle originates from the coracoid process of the scapula, fusing with the short head of the biceps brachii and the pectoralis minor, and inserts into the humerus at the level of the deltoid tuberosity. The function of the coracobrachialis is not only to flex the shoulder anteriorly, but also to adduct and pronate it.

Shoulder Extension

The Muscles responsible for shoulder extension (moving the arm backward), much like the shoulder flexors, cross the frontal axis of the shoulder joint but are located posterior to it. Shoulder extension is performed by the following muscles:

1) the posterior part of the deltoid (see p. 153);

2) the latissimus dorsi (see p. 150);

3) the infraspinatus (see p. 155);

4) the teres minor (see p. 155);

5) the teres major (see p. 155);

6) the long head of the triceps brachii.

Shoulder Pronation

Shoulder pronation, i.e., inward rotation, is performed by muscles that cross the vertical axis of the shoulder joint, attaching anterior to it. These include:

1) the subscapularis (see p. 155);

2) the pectoralis major (see p. 146);

3) the anterior part of the deltoid (see p. 153);

4) the latissimus dorsi (see p. 150);

5) the teres major (see p. 155);

6) the coracobrachialis (see p. 156).

Shoulder Supination

Supination, i.e., outward rotation of the shoulder, is performed by muscles that, like the pronators, cross the vertical axis of the shoulder joint but are located posterior to it:

Elbov Flexion

The forearm is flexed by muscles that cross the transverse axis of the elbow joint and lie anterior to it. These muscles include:

1) the biceps brachii;

2) the brachialis;

3) the brachioradialis;

4) the pronator teres, as well as the muscles originating from the medial epicondyle of the humerus and extending to the forearm and hand.

The biceps brachii (see Fig. 54) has two heads, long and short. The long head originates from the supraglenoid tubercle of the scapula, and the short head from the coracoid process. The muscle inserts into the radial tuberosity and the forearm fascia.

This muscle is biarticular. It flexes the arm and stabilizes the humeral head in the shoulder joint; with respect to the elbow joint, it acts as a flexor and supinator of the forearm.

Since the heads of the biceps originate on the scapula at some distance from each other, their functions regarding arm movement differ: the long head flexes and abducts the arm, while the short head flexes and adducts it. Regarding the forearm, the biceps brachii is a powerful flexor due to its considerable leverage. Its tendon inserts into the radius not so much anteriorly as on its medial side; consequently, flexing the forearm produces its supination. An aponeurotic band extends from the tendon of the biceps brachii medially, which makes the line of action of the muscle's resultant force more central. This helps to smooth out the flexion movement and reduce the supination function.

The biceps brachii is located on the anterior surface of the arm, directly beneath the skin and deep fascia, and is easily palpable. The tendon of this muscle is especially prominent when the forearm is flexed. The medial and lateral sulci of the biceps are clearly visible along its borders, transmitting Blood Vessels and nerves.

The brachialis (see Fig. 54) originates from the lower half of the anterior surface of the humerus and the intermuscular septa of the arm, and inserts into the ulnar tuberosity and its coronoid process. It is covered anteriorly by the biceps brachii and is easily palpable on either side of the biceps tendon. The brachialis is a monoarticular muscle and participates in forearm flexion.

With a proximal pivot point (on the humerus), the brachialis applies force close to the fulcrum of the lever, producing movements through wide arcs and with great force. With a distal pivot point (hanging), the force is applied farther from the fulcrum and is greater in magnitude, although the arcs of movement are smaller due to shorter muscle fibers compared to the biceps.

1) the infraspinatus (see p. 155);

2) the teres minor (see p. 155);

3) the posterior part of the deltoid (see p. 153).

The pronator group is considerably stronger than the supinator group. Consequently, pronation movements can be performed at the shoulder joint with much greater force than supination movements. This is confirmed by analyzing the physiological cross-sectional area of the muscles that supinate and pronate the arm, which characterizes their lifting capacity. For the supinators, it is: supraspinatus — 3.5 cm², infraspinatus — 6.5 cm², teres minor — 1.5 cm² (total — 11.5 cm²); for the pronators: pectoralis major — 6.5 cm², subscapularis — 10.5 cm², teres major — 15 cm², latissimus dorsi — 15.5 cm², coracobrachialis — 1.5 cm² (total — 49 cm²). Assuming that the lifting force of a muscle with a physiological cross-section of 1 cm² is 10 kg, the strength of the pronators reaches 490 kg, while the strength of the supinator group is 115 kg. In The process of physical training, it is necessary to somewhat level out these differences.

The Muscles surrounding the shoulder joint, when the humerus is fixed, stabilize the pectoral girdle (such as hanging with straight arms, etc.).

Circumduction of the Arm

When all the muscles surrounding the shoulder joint act sequentially, circumduction occurs in the joint. Examining these muscles, one can easily notice that they are distributed unevenly; specifically, there are no muscles medial and inferior to the joint—instead, There is a depression known as the axillary fossa.

The axillary fossa is somewhat pyramidal in shape, with its base directed downward and laterally, and its apex upward and medially. It has three walls: the anterior wall is formed by the pectoralis major and minor, the posterior wall by the subscapularis, teres major, and latissimus dorsi, and the medial wall by the serratus anterior. Lying in the groove between the anterior and posterior walls are the coracobrachialis and the short head of the biceps brachii. At its apex, the axillary fossa has an opening located between the first rib and the clavicle (subclavius muscle). When the arm is abducted, the boundaries of this fossa are clearly delineated, especially when the muscles are contracted.

Muscles Producing Movements at the Elbow Joint

With a fixed humerus, the following movements are possible at the elbow joint:

1) flexion and extension of the forearm;

2) pronation and supination of the forearm.

These movements are provided by four functional muscle groups.

The brachioradialis (see Fig. 54) originates from the humerus, proximal to its lateral epicondyle, and from the lateral intermuscular septum, and inserts into the radius proximal to the styloid process. During forearm flexion, particularly against resistance, the muscle stands out clearly and is easily palpable beneath the skin.

This muscle acts not only as a forearm flexor, but also as a supinator when the forearm is pronated, and as a pronator when it is supinated. Because it attaches far from the fulcrum of the lever, it is capable of exerting considerable force when flexing the arm at the elbow joint and producing movement through wide arcs.

The pronator teres (see Fig. 54) runs obliquely from top to bottom, extending from the medial side of the forearm to the lateral. It originates from the medial epicondyle of the humerus and partly from the coronoid process of the ulna, inserting into the middle third of the lateral and anterior surface of the radius.

This muscle is involved in two movements of the forearm: flexion and pronation.

The physiological cross-sectional area of the forearm flexor muscles is as follows: biceps brachii — 3.5 cm², brachialis — 4.5 cm², pronator teres — 16.5 cm², brachioradialis — 1 cm² (total — 25.5 cm²).

Extension of the Forearm

Extension of the forearm is performed by muscles that cross the transverse axis of the elbow joint and lie posterior to it. There are two such muscles:

1) the triceps brachii, and

2) the anconeus.

The triceps brachii (see Fig. 54) has three heads: the long, medial, and lateral heads.

The long head originates from the infraglenoid tubercle of the scapula, while the medial and lateral heads arise from the posterior surface of the humerus and the intermuscular septa.

All three heads converge into a single tendon that inserts into the olecranon process of the ulna. When the muscle contracts, it causes extension and adduction at the shoulder joint (via the long head) and extension at the elbow joint.

The long head of the triceps brachii can function independently. Its extension force relative to the shoulder joint is 1.5 times greater than its force relative to the elbow joint. The combined work of all three heads of the triceps is estimated at 8.5 kgm, which is 2.5 times greater than the action of its long head alone. The most powerful of the three heads is the lateral head.

The triceps brachii lies superficially beneath the skin, serving as the only muscle on the posterior aspect of the arm. Between its medial and lateral heads on one side, and the humerus on the other, lies the musculospiral (radial) groove, through which pass the radial nerve and the profunda brachii artery.

The anconeus muscle originates from the lateral epicondyle of the humerus, the radial collateral ligament, and the antebrachial fascia. It inserts into the upper part of the posterior surface of the ulna and partially into its olecranon process. In individuals with well-developed musculature, this muscle stands out beneath the skin as a small triangular elevation; its primary function is to extend the forearm. If only the heads of the triceps brachii participated in this movement, their resultant force vector would deviate somewhat medially. Furthermore, attaching close to the lever's fulcrum, the triceps would be unable to generate significant force. The anconeus fulfills this function by increasing the area of force application, bringing it closer to the resistance, and shifting the resultant force vector into a more median position, thereby promoting "pure" extension. The physiological cross-sectional area of the forearm extensor muscles is: triceps brachii — 16 cm2, anconeus — 1 cm² (total — 17 cm²).

Studies have shown that the strength of the flexor muscles at the elbow joint exceeds that of the extensors by approximately 1.5 times. Among them, the biceps brachii performs the most work (4.58 kgm), followed by the brachialis (3.84 kgm), and to a significantly lesser extent, the brachioradialis (2.21 kgm).

Pronation of the Forearm

Pronation of the forearm is performed by the following muscles:

1) pronator teres (see p. 159);

2) pronator quadratus;

3) brachioradialis (see p. 159).

The pronator quadratus is located directly on the bones in the distal third of the forearm, originating from the ulna and inserting into the anterior surface of the radius.

The function of this muscle is exclusively dedicated to the pronation of the forearm.

The physiological cross-sectional area of the pronator muscles (pronator teres and pronator quadratus) is 2.5 cm² (1.5 cm² for the pronator teres and 1 cm² for the pronator quadratus).

Supination of the Forearm

The supinators of the forearm are:

1) biceps brachii (see p. 157);

2) supinator muscle;

3) brachioradialis muscle (see p. 159).

The supinator muscle lies directly on the bones of the forearm and is completely covered by other muscles on all sides. Therefore, its contours are not visible in a living person. It originates from the lateral epicondyle of the humerus, the annular ligament of the radius, and the ulna, wraps around the radius in its upper third, and inserts between its tuberosity and the insertion site of the pronator teres.

This muscle produces lateral rotation of the radius in the proximal and distal radioulnar joints, which form a single combined joint, and functions as a forearm supinator.

The physiological cross-sectional area of the forearm supinators is 9.5 cm² (biceps brachii — 3.5 cm², brachioradialis — 1 cm², and supinator — 5 cm²).

Comparing the flexors and extensors of the forearm with its pronators and supinators, one can notice that the first group is significantly stronger than the second, despite the mass of the segment they set in motion being the same. The strength of the supinator muscles exceeds that of the pronator muscles, whereas in the shoulder joint, conversely, the muscles that pronate the arm are stronger than those that supinate it. It should also be noted that during pronation and supination of the forearm, the fulcrum can be located either inferiorly or superiorly. With a superior fulcrum, the ulna is fixed while the radius moves; with an inferior fulcrum (on the hand), the radius along with the hand is fixed, while the ulna and humerus move.

Muscles producing movements in the wrist joint and joints of the hand

Typically, movements in the wrist joint occur simultaneously with movements in the midcarpal, carpometacarpal, and often metacarpophalangeal and interphalangeal joints.

Therefore, it is appropriate to examine the muscles involved in the simultaneous movement of all these joints. In the wrist and midcarpal joints, movements can occur around two mutually perpendicular axes of rotation: transverse and anteroposterior. Accordingly, four functional muscle groups can be distinguished here: wrist flexors, wrist extensors, wrist abductors, and wrist adductors.

Wrist flexion

Wrist flexion involves muscles that cross the transverse axis and are located anterior to it on the anterior surface of the forearm and hand. These include:

1) palmaris longus;

2) flexor carpi radialis;

3) flexor carpi ulnaris;

4) flexor digitorum superficialis;

5) flexor digitorum profundus;

6) flexor pollicis longus.

The palmaris longus muscle (see Fig. 54) is inconstant. Originating from the medial epicondyle of the humerus and the antebrachial fascia, this muscle lies so superficially on the anterior side of the forearm that it is easily visible beneath the Cytology/cytology/66.html">Skin and its tendon can be palpated during contraction. This tendon is very long; upon passing onto the palmar surface of the hand, it continues into the palmar aponeurosis.

Because the muscle occupies a median position on the anterior surface of the forearm, it acts solely as a wrist flexor. By attaching to the palmar aponeurosis and tensioning it, strong contraction of this muscle may also play a minor indirect role in finger flexion.

The flexor carpi radialis (see Fig. 54) originates from the humerus and partially from the antebrachial fascia. This muscle lies superficially beneath the skin; its tendon is easily palpable in the lower third of the forearm. Originating from the medial epicondyle of the humerus and the intermuscular septum, the muscle passes into the hand beneath the flexor retinaculum and inserts into the Base of the 2nd metacarpal bone. As a multijoint muscle, the flexor carpi radialis participates not only in wrist movements, but also in elbow flexion. Because this muscle runs obliquely across the forearm from top to bottom and medially to laterally, it acts partly as a forearm pronator. Its physiological cross-sectional area is 1 cm2.

The flexor carpi ulnaris (see Fig. 54) originates from the medial epicondyle of the humerus, the ulna, and the antebrachial fascia. Its distal end reaches the pisiform bone, to which it inserts. Ligaments extending from the pisiform bone to the hamate and 5th Metacarpal bones serve as a continuation of this muscle's pull.

The pisiform bone, to which the flexor carpi ulnaris attaches, helps increase the muscle's moment arm and, consequently, its torque as a wrist flexor. The physiological cross-sectional area of the flexor carpi ulnaris is larger than that of the radial flexor, measuring 2 cm2.

The flexor digitorum superficialis (see Fig. 54) originates from the medial epicondyle of the humerus, as well as from the ulna and radius. Located in the space between the ulnar and radial wrist flexors, it is partially covered by them, as well as by the palmaris longus, brachioradialis, and pronator teres. The flexor digitorum superficialis has four tendons that pass into the hand through the carpal tunnel beneath the flexor retinaculum and reach, each splitting into two slips, the lateral surfaces of the middle Phalanges of digits 2–5, where they insert.

The function of this muscle is to flex the middle phalanges. Being a multijoint muscle, it also induces flexion in all wrist joints except the distal interphalangeal joints. Because the tendons of this muscle diverge toward the fingers after passing through the carpal tunnel, their flexion is accompanied by adduction toward the middle finger.

When the forearm is extended, the tone of the flexor digitorum superficialis is higher, whereas in a flexed forearm it is lower. Extension of the hand is accompanied by the simultaneous stretching of the muscle, which increases its tone. Consequently, when the hand is extended, fully straightening the fingers is significantly more difficult than when the hand is flexed.

The flexor digitorum profundus lies directly on the anterior surface of the ulna and the pronator quadratus; it originates from the upper two-thirds of the palmar surface of the ulna and partially from the interosseous membrane. Like the superficial flexor, the deep flexor divides into four tendons that pass through the carpal tunnel to the distal phalanges of digits 2–5 of the hand, splitting through the tendons of the flexor digitorum superficialis (Fig. 56).

As a multi-joint muscle, the flexor digitorum profundus produces flexion across all joints of the hand, including the distal interphalangeal joints.

The tendons of the flexor digitorum profundus diverge fan-like across the hand toward the fingers; as a result, this muscle not only flexes the fingers but also adducts them, which is particularly noticeable when flexing spread fingers.

Fig. 56. Muscles of the palmar surface of the hand:

1 — abductor digiti minimi m.; 2 — palmaris brevis m.; 3 — opponens digiti minimi m.; 4 — lumbricales mm.; 5 — flexor digitorum superficialis m.; 6 — tendon of flexor digitorum profundus m.; 7 — first dorsal interosseous m.; 8 — flexor pollicis brevis m.; 9 — adductor pollicis m.; 10 — abductor pollicis brevis m.; 11 — opponens pollicis m.; 12 — flexor carpi radialis m.; 13 — flexor digitorum superficialis m.; 14 — flexor carpi ulnaris m.; 15 — flexor retinaculum

The tendons of the flexor digitorum profundus serve as the origin sites for the lumbrical muscles running toward the fingers. Because these tendons are mobile, contraction of the deep flexor pulls the origins of the lumbrical muscles upward, leading to an increase in their tone.

When the hand is extended, the tone of all finger flexors rises, making extension—and even more so, hyperextension—of the fingers in this position difficult and sometimes impossible.

The flexor pollicis longus is a unipennate, spindle-shaped muscle. It originates from the palmar surface of the radius, passes through the carpal tunnel within a separate synovial sheath, and reaches the distal phalanx of the thumb, where it inserts. The muscle produces flexion in all joints it crosses (specifically flexing the distal phalanx of the thumb).

Extension of the hand

Extension of the hand is performed by muscles that cross the transverse axis of the radiocarpal joint and are located posterior to it on the back of the forearm. These are multi-joint muscles that simultaneously extend all the joints they cross. These muscles include:

1) extensor carpi radialis longus;

2) extensor carpi radialis brevis;

3) extensor carpi ulnaris;

4) extensor digitorum;

5) extensor digiti minimi;

6) extensor indicis;

7) extensor pollicis longus (see p. 168).

The extensor carpi radialis longus (see Fig. 54) is located

superficially beneath the skin and is often clearly visible during forceful extension of the forearm at the elbow joint. This muscle originates from the lateral border of the humerus, the intermuscular septum, and the lateral epicondyle, passes beneath the extensor retinaculum and the tendon of the extensor pollicis longus, and inserts into the base of the 2nd metacarpal bone. Because the line of action of this muscle runs very close to the transverse axis of the elbow joint, its contribution to forearm flexion is minimal. Acting as a powerful wrist extensor, it also produces slight abduction when contracting in isolation.

The extensor carpi radialis brevis (see Fig. 54) lies slightly posterior to the extensor carpi radialis longus, originating from the lateral epicondyle of the humerus and the forearm fascia, and inserts into the base of the 3rd metacarpal bone. While acting as a wrist extensor, the muscle simultaneously abducts the hand. However, compared to the extensor carpi radialis longus, its leverage moment for hand abduction is much smaller because its line of action lies considerably closer to the sagittal axis of the radiocarpal joint.

The physiological cross-sectional area of the radial wrist extensors is 3 cm2.

The extensor carpi ulnaris (see Fig. 54) originates from the lateral epicondyle of the humerus, the radial collateral ligament, and the forearm fascia. Descending toward the hand, the muscle runs between the head and the ulnar styloid process, inserting into the base of the 5th metacarpal bone. Throughout its course, this muscle lies adjacent to the ulna and, in individuals with thin skin and well-developed musculature, is clearly visible and easily palpable. Regarding the elbow joint, much like the preceding muscle, its leverage moment is negligible. As a wrist extensor, the extensor carpi ulnaris also adducts the hand. Its physiological cross-sectional area is 1 cm².

The extensor digitorum (see Fig. 54) is located superficially on the posterior aspect of the forearm. It originates from the lateral epicondyle of the humerus, the radial collateral ligament, the annular ligament of the radius, and the forearm fascia. At the middle of the forearm, this muscle transitions into tendons that pass beneath the extensor retinaculum to the dorsal surface of the proximal phalanges of digits 2–5. Each tendon, in turn, splits into three slips: the middle slip inserts into the middle phalanx, while the two lateral slips reach the distal phalanx. Individual tendons of the extensor digitorum are interconnected by three oblique intertendinous connections. Digits 2 and 5 feature such connections on only one side, whereas digits 3 and 4 have them on both sides (which partly explains the lesser mobility of the latter). Flexing the wrist causes the fingers to extend simultaneously, driven by the increasing tone of the extensor digitorum. A hand clenched into a fist is easier to straighten by flexing it at the radiocarpal joint. In a relaxed hand posture with the arms hanging down, the fingers are usually slightly flexed due to the lower tone of the extensor digitorum compared to its antagonists, the flexors.

The extensor digiti minimi originates from the lateral epicondyle of the humerus, the radial collateral ligament, the annular ligament of the radius, and the antebrachial fascia, running downward to insert into the dorsal aponeurosis of the 5th digit. Along with extending this finger, the muscle also extends and slightly adducts the entire hand.

The extensor indicis originates from the posterior surface of the ulna and the interosseous membrane. Its tendon merges with the tendon of the extensor digitorum leading to the 2nd digit, reaches the dorsal aponeurosis of the index finger, and inserts into its distal and middle phalanges. It extends the index finger and also assists in extending the entire hand.

Comparing the flexors and extensors of the wrist, one can see that among the flexors of the hand, the flexor carpi ulnaris is better developed, whereas among the extensors, the extensor carpi radialis longus and brevis are prominent.

Adduction of the Hand

There are no muscles located on the Medial surface of the ulna that run to the hand strictly along the medial surface of the wrist joint. Adduction of the hand occurs according to the parallelogram of forces rule during the simultaneous contraction of:

1) the flexor carpi ulnaris (see p. 163) and 2) the extensor carpi ulnaris (see p. 166).

The finger flexors and extensors whose tendons run to the 4th and 5th digits may also play a minor role in hand adduction.

Abduction of the Hand

The following muscles are involved in the abduction of the hand:

1) the flexor carpi radialis (see p. 163);

2) the extensor carpi radialis longus (see p. 165);

3) the extensor carpi radialis brevis (see p. 166);

4) the abductor pollicis longus;

5) the extensor pollicis longus;

6) the extensor pollicis brevis. In addition, muscles running from the forearm to the index finger take a minor part in hand abduction.

The abductor pollicis longus muscle (see Fig. 54) is fusiform in shape. It originates from the dorsal surfaces of the radius and ulna as well as the interosseous membrane, and inserts into the base of the 1st metacarpal bone.

This muscle abducts the thumb provided it is not fixed by antagonist muscles. If the thumb is fixed, the muscle abducts the hand instead. When the thumb is fully abducted, further action of the muscle results in the abduction of the hand. It plays a vital role in this movement because its leverage relative to the anteroposterior axis of the wrist joint is significantly greater than that of the radial extensors and the flexor carpi radialis.

The extensor pollicis longus muscle (see Fig. 54) originates from the posterior surface of the ulna and radius and the interosseous membrane of the forearm, inserting into the distal phalanx of the thumb. The tendon of this muscle passes beneath the extensor retinaculum in a separate compartment, crossing the tendons of the wrist radial extensors. By extending the distal phalanx, the muscle simultaneously pulls the thumb slightly backward. If the thumb is fixed, the muscle participates in abducting the entire hand.

The extensor pollicis brevis muscle (see Fig. 54) shares a common origin with the preceding muscle and inserts into the proximal phalanx of the thumb, extending it while simultaneously abducting the entire digit. If the thumb is fixed, this muscle assists in the abduction of the entire hand. Frequently, the tendon of this muscle reaches the base of its distal phalanx, in which case its function is similar to that of the preceding muscle.

Circumduction of the hand successively involves all muscle groups located around the wrist joint. Due to the absence of sufficiently strong muscles running obliquely relative to the longitudinal axis of the wrist joint, active rotation in this joint is impossible. A slight amount of passive rotation is possible thanks to some degree of articular Cartilage deformation.

The function of all muscles traversing the region of the wrist and hand joints is not only to execute movements but also to reinforce these joints. This is especially true for the wrist joint: its capsule is thin and incapable of withstanding tensile loads such as those occurring, for instance, when hanging by straight arms.

On the anterior surface of the upper limb, in the region of the elbow joint, lies the cubital fossa. Its boundaries are formed by the brachioradialis and pronator teres muscles. The fossa is triangular in shape and narrows inferiorly.

On the anterior surface of the forearm lies the radial sulcus, bounded laterally by the brachioradialis muscle and medially by the flexor carpi radialis.

Muscles Producing Finger Movements

The fingers of the hand can perform the following movements: flexion, extension, abduction (movement away from the middle finger), adduction (movement toward the middle finger), and circumduction. In addition, the thumb can perform opposition and reposition, since the head of the first metacarpal bone is not bound by ligaments to the head of the 2nd metacarpal bone.

The flexor muscles of the fingers include the flexor digitorum superficialis, flexor digitorum profundus, and flexor pollicis longus (see pp. 163, 164, 165).

The extensor muscles of the fingers include the extensor digitorum, extensor pollicis longus and brevis, as well as the extensor indicis and extensor digiti minimi. Among these, the extensors of the thumb, along with the abductor pollicis longus, also assist in thumb abduction.

Finger movements are also performed by the intrinsic Muscles of the hand. They form three groups within the hand: one located in the middle section of the palmar surface, another on the thumb side, and a third on the little finger side. The latter two muscle groups form the thenar and hypothenar eminences.

Middle Group of hand muscles

The lumbricals (see Fig. 56) are long, slender muscles that originate from the tendons of the flexor digitorum profundus. These muscles extend to all fingers except the thumb. They lie directly beneath the palmar aponeurosis, on the lateral side of each flexor digitorum profundus tendon. They insert into the dorsal digital expansions of the proximal phalanges.

The function of these muscles is to flex the proximal phalanges of the 2nd–5th digits. At the same time, this action typically causes a slight extension of the middle and distal phalanges, which is related both to the insertion of the lumbricals into the dorsal aponeurosis of these phalanges and to an increase in the tone of the extensor digitorum.

The palmar interossei (three in number) are located in the spaces between the metacarpal bones of the 2nd–5th digits and originate from these bones. They insert into the joint capsules of the metacarpophalangeal joints and the dorsal aponeurosis of the 2nd, 4th, and 5th digits. By flexing their proximal phalanges, these muscles simultaneously adduct the respective digits toward the middle finger.

The dorsal interossei, four in number, are located—like the previous group—in the spaces between the metacarpal bones. They originate from the facing side surfaces of adjacent metacarpal bones. Reaching the dorsal surface of the proximal phalanges, their slender tendons blend into the aponeurotic expansion of the finger extensors. The middle finger has two dorsal interossei inserting on both its ulnar and radial sides, the 2nd digit has only one muscle on the radial side, and the 4th digit also has one muscle, located on the ulnar side.

The function of these muscles is to flex the proximal phalanges of the 2nd–5th digits while simultaneously assisting in the extension of the middle and distal phalanges of these digits. In addition, they abduct the 2nd and 4th digits away from the 3rd, and tilt the 3rd digit toward either the radius or the ulna.

Thumb muscle group (Thenar muscles)

The abductor pollicis brevis (see Fig. 56) has a broad origin from the flexor retinaculum and the scaphoid bone. Inserting into the proximal phalanx of the thumb, it assists in thumb abduction.

The flexor pollicis brevis (see Fig. 56) originates from the flexor retinaculum and the trapezium. This muscle inserts into the sesamoid bone and, by flexing the proximal phalanx of the thumb, facilitates (through the tension of its antagonists) the extension of its 2nd (distal) phalanx. The muscle also participates in the opposition of the thumb.

The opponens pollicis (see Fig. 56) originates from the flexor retinaculum and the trapezium, and inserts into the 1st metacarpal bone. Its function is to oppose the thumb to all the other digits.

The adductor pollicis (see Fig. 56) has two heads: transverse and oblique. The transverse head originates from the palmar surface of the body of the 3rd metacarpal bone, while the oblique head originates from the bases of the 2nd and 3rd metacarpals and the capitate bone. The muscle inserts into the sesamoid bone located anterior to the metacarpophalangeal joint of the thumb, as well as into the joint capsule and the proximal phalanx of the thumb. Its function is to adduct the thumb toward the midline of the palm, thereby assisting in its opposition to the other four fingers.

Little finger muscle group (Hypothenar muscles)

The palmaris brevis (see Fig. 56) is one of the few cutaneous muscles. It is positioned transversely at the ulnar border of the palm's palmar surface, originating from the palmar aponeurosis and inserting into the skin.

When clenching the fist or striking with the palmar surface of the hand, this muscle helps protect the vessels and nerves running along the ulnar side from the anterior forearm onto the hand.

The abductor digiti minimi (see Fig. 56) originates from the pisiform bone and inserts into the base of the proximal phalanx of the 5th digit. The function of the muscle is to abduct this finger, flex its proximal phalanx, and extend its middle and distal phalanges.

The flexor digiti minimi brevis originates from the flexor retinaculum and the hamate bone, and inserts onto the ulnar side of the base of the proximal phalanx of the 5th digit. The function of the muscle is to flex and adduct this digit.

The opponens digiti minimi originates together with the previous muscle and inserts into the body and head of the 5th metacarpal bone, which it slightly flexes and draws closer to the center of the palm.

Connective tissue structures of the upper limb muscles

Fasciae serve to strengthen the muscles of the upper limb and also provide a soft skeletal framework for the attachment of individual muscle bundles. In most cases, the fascia of one region of the upper limb is a direct continuation of the fascia of an adjacent region.

The entire upper limb is enveloped beneath the skin by a very thin superficial fascia. In addition, each upper limb muscle has its own proper, or deep, fascia. The superficial fascia strictly conforms to the shape of the upper limb, whereas the deep fascia divides into a series of distinct sheets (corresponding to specific muscles).

The deltoid fascia covers the outer surface of the deltoid muscle with its superficial layer, while its deep layer separates the muscle from the shoulder joint capsule and neighboring muscles. The supraspinatus fascia spans the edges of the supraspinous fossa and consists only of a superficial layer covering the supraspinatus muscle. The infraspinatus fascia attaches to the borders of the infraspinous fossa and covers the infraspinatus and teres minor muscles. These three fasciae serve as the sites of origin for the aforementioned muscles.

The axillary fascia covers the axillary fossa, continuing distally into the brachial fascia, which forms two compartments for the muscles of the anterior and posterior surfaces of the arm. Between these two muscle groups, the brachial fascia forms two dense intermuscular septa of the arm—medial and lateral. They separate the aforementioned muscle groups from one another and serve as their sites of origin. A deep lamina of the brachial fascia is located between the biceps brachii and brachialis muscles.

The brachial fascia continues directly in the distal direction into the antebrachial fascia, from which the SUPERFICIAL MUSCLES OF the forearm partially originate. The transverse fibers of the antebrachial fascia thicken in the wrist region to form the transverse carpal ligament, the posterior part of which—the extensor retinaculum—is significantly more pronounced than the anterior part. Beneath the extensor retinaculum, six or seven synovial-lined fibro-osseous canals are formed for the extensor tendons.

The antebrachial fascia extends distally into the Fasciae of the hand and fingers. On the PALMAR AND DORSAL aspects of the metacarpus, there is a thin fascia that covers the interosseous muscles, attaching to the metacarpal bones. Between the superficial and deep fasciae of the dorsal aspect of the hand lies a space traversed by muscles, vessels, and nerves. In the middle of the PALMAR ASPECT OF the hand, there is a palmar aponeurosis underlined by adipose tissue, which is particularly well-developed in the central part of the palm. Toward the thenar and hypothenar eminences, it thins out and transitions into a delicate fascia covering these muscle groups.

The palmar portion of the transverse carpal ligament continues distally into a robust band spanning the carpal sulcus—the flexor retinaculum, beneath which lies the carpal tunnel. Near the pisiform bone, the flexor retinaculum splits to form a canal for the passage of the ulnar vessels and nerve.

The flexor and extensor retinacula play a crucial role in securing the position of the muscle tendons passing beneath them, particularly during flexion and extension of the hand.

The gliding of the tendons passing from the forearm to the hand and the reduction of friction are facilitated by synovial tendon sheaths. The greatest number (6—7) of synovial sheaths is located beneath the extensor retinaculum. The flexor tendons within the carpal tunnel are likewise enveloped by synovial sheaths that extend distally into the palm. In this region, the tendons of digits 2–4 briefly lack synovial sheaths but acquire them again within the digits; the synovial sheaths for the tendons of the thumb and little finger run continuously.