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Chapter 5 - Atlas of Limb Prosthetics: Surgical, Prosthetic, and Rehabilitation Principles

Kinesiology and Functional Characteristics of the Upper Limb

Shahan K. Sarrafian, M.D. 

The functional capacity of the upper limb is determined by the shoulder complex, elbow, wrist, and hand developing multiple integrated spheres of action. Given the normal proportion of limb segments, this capacity is limited in relation to the surrounding space. In the standing position the upper-limb field of motion reaches the midthigh region. Any more distal point on the lower extremity or on the ground is reached through mobility provided by the hip, knee, ankle, and trunk (Fig 5-1). Furthermore, a distant point in space comes within the reach of the upper-limb action through a functional integration with gait.

A maximum arcuate field or envelope of action termed "Ex" (Fig 5-2) is traced by the most distal point of the upper extremity through the motion of the shoulder complex, all other joints being held in extension. Within this envelope, the elbow, wrist, and hand develop their own fields of motion, E₂, E₃, and E₄. These contained capabilities enrich the functional performances of the upper extremity.

SHOULDER COMPLEX

Motion in the Frontal or Coronal Plane

When the arm and forearm are held in the anatomic position, the antecubital surface facing anteriorly, the upper limb sweeps a circular surface in the frontal plane. The very distal point of the extremity traces an envelope of action E₁ (Fig 5-3).

In position 1 the shoulder is in neutral rotation, and the extremity can be elevated in the outer half of the circle to positions 2 and 3. The elbow does not contribute to functional exploration in this segment of the arc of motion. If the wrist is initially held in neutral rotation, the hand sweeps the space E₃, and the digits explore the interior of this space through E₄. Beyond position 3 the shoulder externally rotates, and complete elevation is achieved at position 4. In this second arc of motion the elbow explores the segment of the space through its action envelope E₂. The sweeping of the inner half of the coronal circle is now possible from position 4 to 5 through internal rotation of the shoulder. The elbow action dissipates. From position 5 to 6 the shoulder externally rotates, and elbow functional capability in this plane reappears, whereas with further external rotation from position 6 to 1, the elbow action dissipates again.

When the upper limb is maintained in neutral rotation at the shoulder, the motion is quite restricted (Fig 5-4), and no elbow action is possible in this plane. Maintaining the extremity in complete external rotation permits exploration of the outer half of the frontal circle with ease, whereas any functional development in the inner half is very restricted. The elbow envelope of action is clearly visible now in all positions (Fig 5-2).

Placement of the limb in complete internal rotation significantly restricts the field of motion (Fig 5-5). Elbow action is possible from position 1 to 2. The coronal plane is also explored posteriorly in the inner half space (Fig 5-6). With a position of internal rotation at the shoulder, the limb traces a small arc of displacement just enough for the elbow, wrist, and hand to sweep the surface corresponding to the gluteal area and up to the opposite scapular region. From position 3 the elbow envelope of action scans the posterior aspect of the head, neck, and shoulder.

During elevation of the upper extremity in the frontal plane, motion is determined by the scapulohumeral joint and scapulothoracic upward rotation. The acromioclavicular and sternoclavicular joints also participate in a synchronized manner (Fig 5-7). External rotation accompanies the elevation for the performance of a smooth motion. Beyond 90 degrees of elevation this external rotation is necessary to free the greater tuberosity from the acromial process, and more humeral articular surface is offered to the opposing glenoid (Fig 5-8).

From 0 to 30 degrees of elevation (Fig 5-7) the motion occurs at the scapulohumeral joint, and the scapular motion is variable. This is the "setting phase" of the scapular motion. In the remaining arc of motion of 150 degrees, the scapulohumeral (SH) joint motion and the scapulothoracic (ST) motion of upward rotation participate at a ratio of SH/ST=2/1 as measured in the frontal plane. The total contribution of the scapulohumeral joint is 130 degrees. The clavicle does not remain still. In the initial 90 degrees of motion the clavicle is elevated at the sternoclavicular joint for about 40 degrees, and in the second half of the arc of motion the clavicle rotates on its long axis for another 40 to 50 degrees. A combined acromioclavicular motion of 20 degrees occurs during the initial and terminal phases of elevation.

The motor units responsible for scapulohumeral elevation are the middle segment of the deltoid muscle and the components of the rotator cuff: the supraspina-tus, infraspinatus, teres minor, and subscapularis muscles (Fig 5-9).

The deltoid acts as the upper vector component of a force couple, whereas the rotator cuff stabilizes the humeral head and acts as the lower vector force of the couple. Electromyographic study of these muscles (Fig 5-10) indicates that the deltoid action potential increases steadily with elevation, reaches a maximum at 110 degrees, and maintains a plateau level of activity with a final peak at full elevation. The supraspinatus also reaches a peak at 110 degrees, and beyond this point its activity diminishes and traces a sine wave. The subscapularis reaches peak activity at 100 degrees, maintains a plateau level up to 130 degrees, and diminishes rapidly in action. The teres minor reaches the maximum at 120 degrees and from there maintains the high level of activity, whereas the infraspinatus increases steadily in activity from the initial position to that of full elevation. The action of these two last muscles is necessary to continue the external rotation of the humerus during the last stage of the elevation. The posterior segment of the deltoid also participates as an external rotator (Fig 5-11).

The motor units acting during upward rotation of the scapula are the upper and lower segments of the trapezius and the lower digitations of the serratus anterior. They act on the scapula as a force couple (Fig 5-12).

When the upper limb moves in the lower and inner quadrant of the envelope of action E₁, it is adducted and internally rotated. The internal rotation is brought about by the subscapularis, pectoralis major, and anterior segments of the deltoid (Fig 5-11). Adduction is determined by the latter two muscles, supplemented by the action of the coracobrachialis (Fig 5-13). During the anterior adduction-internal rotation, the scapula is abducted. This motion is controlled by the serratus anterior and the pectoralis minor (Fig 5-14). When the upper limb moves in a similar lower and inner quadrant but posterior to the body, the limb is once more adducted and internally rotated. The posterior adduction is brought about by the latissimus dorsi, teres major, long head of the triceps, and posterior segment of the deltoid (Fig 5-13). The latissimus dorsi and teres major also determine the associated internal rotation (Fig 5-11). During this same motion, the scapula is adducted by the middle segment of the trapezius and the combined action of the rhomboidei and latissimus dorsi(Fig 5-15). When the upper limb is in a maximum position of elevation and is brought down in the frontal plane in the outer half circle, the scapula makes a downward rotation. This is determined by the combined action of the latissimus dorsi, lower segment of the pectoralis major (the pectoralis minor acting as the lower component for a force couple), and the levator scapulae, with the rhomboidei acting as the upper component of the rotational couple (Fig 5-16). Downward stabilization of the limb in the frontal plane is also of important functional significance, such as in crutch walking or parallel bar exercising. This function is determined by the depressors of the shoulder complex: la-tissimus dorsi, lower segment of the trapezius, lower segment of the pectoralis major, pectoralis minor, and subclavius (Fig 5-17).

The upward stabilization in the frontal plane is also necessary for functional purposes, as in carrying heavy loads on the shoulders. This is controlled by the elevators of the scapula: levator scapulae, upper segment of the trapezius, and rhomboidei (Fig 5-18).

Motion in the Sagittal Plane

From a neutral rotational position the upper limb moves in the sagittal plane and sweeps the surface from position 1 to 3 (Fig 5-19). The elbow, wrist, and hand are capable of functioning in this plane through their envelopes of action E₂, E₃, and E₄.

In position 3 the elbow action extends farther posteriorly, the hand reaching the posterior aspect of the shoulder. Further movement in the posterior half of the field is possible through the internal rotation of the shoulder followed by gradual external rotation to bring the limb to its neutral initial position (Fig 5-20). Elevation of the upper limb, or flexion from position 1 to 3, is determined by the anterior segment of the deltoid, biceps, coracobrachialis and clavicular head of the pecto-ralis major (Fig 5-21). The rotator cuff is also active in stabilizing the humeral head. The scapulothoracic mechanism participates in the motion through upward scapular rotation at a ratio of SH/ST=2/1. From the elevated position 3 the upper limb is brought down by the posterior segment of the deltoid, long head of the triceps, latissimus dorsi, and pectoralis major (Fig 5-22). Beyond neutral the motion continues as extension, and all motors continue their action except the pectoralis major. The range of extension is 60 degrees (Fig 5-23).Contributors to this motion are gravity and downward rotators of the scapula.

Motion in the Horizontal Plane

When the upper extremity is elevated to 90 degrees in the frontal plane, the distal point of the limb scans the horizontal plane and traces an arc of 165 degrees (Fig 5-24). The flexors and extensors of the scapulo-humeral joint control the motion.

Rotary Capability of the Shoulder Complex

When the upper extremity is held in the neutral rotational position at the shoulder and the elbow is flexed at 90 degrees, the distal point traces an arc of internal rotation of 80 degrees and an arc of external rotation of 60 degrees. With the shoulder elevated 90 degrees in the frontal plane, this rotary capability changes to 90 degrees of external rotation and 70 degrees of internal rotation (Fig 5-25).

ELBOW

The elbow joint determines an arc of motion, E₂, of 150 degrees. The orientation of the plane of action is closely influenced by the rotational position of the shoulder joint. When the arm is elevated in the frontal plane, for example, the envelope of action E₂ of the elbow is located in this plane if the shoulder is in external or internal rotation.

The main flexors of the elbow are the brachialis and the biceps. The brachioradialis and pronator teres are the accessory flexors (Fig 5-26). There is an intricate interplay and a wide range of participation in the elbow flexors. The brachialis is the baseline flexor and is active at any rotational position of the forearm and any speed, with or without load applied to the flexing forearm (Fig 5-27). It is also active in flexed elbow posture or during extension of the forearm; it then acts as an an-tigravity muscle, The biceps is a flexor of the supine forearm, and its activity is evident as soon as slight resistance is applied. Deactivation occurs when the forearm is pronated unless significant resistance is applied to the pronated flexing forearm.

The biceps is minimally active as an antigravity muscle or in maintaining a static flexed position. The brachioradialis is active when the forearm is flexing rapidly at any rotational position. It is also a reserve flexor during flexion against resistance, especially in neutral rotation of the forearm. The pronator teres does not participate as a flexor unless resistance is encountered during flexion.

The extensor of the elbow is the triceps assisted by the anconeus (Fig 5-28). The baseline worker during extension is the medial head of the triceps. Without load being applied, the long head is not active, whereas the lateral head is minimally active. These last two reserve extensors come into play when resistance is applied to the motion of extension (Fig 5-29).

FOREARM ROTATION

The average range of pronation-supination of the forearm with the elbow flexed at 90 degrees is 173 degrees measured at the level of the hand. The corresponding rotation measured at the wrist is 156 degrees.The difference of 17 degrees indicates participation of the radiocarpal and midcarpal joints. When the distal end of the radius and the head of the ulna are aligned in the vertical plane delineating the neutral position at the level of the wrist, the hand is in a position of minimal supination of 11 degrees. The average range of pronation is 62 degrees and ranges from 49 degrees to 84 degrees. The average range of supination is 104 degrees and ranges from 86 degrees to 122 degrees.

The axis of pronation-supination is variable in location. It extends from the center of the radial head to the distal end of the radius and ulna and passes "anywhere between the radial and ulnar styloid processes."In the average habitual motion, the axis passes through the distal end of the radius in line with the third metacarpal or the long finger. During this rotary motion the distal third of the radius and the head of the ulna trace arcs of motion quite comparable in size (Fig 5-30). Starting from the position of supination, the head of the ulna is extended and laterally displaced in the neutral position. In pronation the ulnar head is flexed and further displaced laterally. When the hand rests on its ulnar border on a surface and rotation is initiated, the motion occurs around the axis passing through the head of the ulna and the little finger. The head of the ulna remains still. The hand then makes a circumferential transposition. The styloid process of the radius traces a large arc of motion (Fig 5-30). When rotational motion occurs along an axis passing through the middle finger and near the radial styloid process, the head of the ulna traces a much larger arc of motion than the radius. One can easily appreciate the shift of the rotational axis by supinating and pronating the forearm, the elbow being held at 90 degrees flexion, with the tip of an extended finger applied against the wall or the border of a table. In other words, the peripheral point of fixation through the finger or through a tool held in the hand determines the location of the axis of pronation-supination. When the rotation occurs along the oblique axis passing through the head of the ulna, the radial styloid traces an arc corresponding to the base of a cone. In full pronation the styloid process then appears to be less distal relative to the head of the ulna.

The interosseous membrane uniting the radius and ulna relaxes or tenses during pronation-supination. The interosseous distance measured in the distal, middle, and proximal thirds of the forearm is the largest in neutral position and the smallest in full pronation (Fig 5-31). The tension in the membrane is thus minimal in full pronation. During a fall on the outstretched pronated hand, the interosseous membrane is not the main element of pressure transmission to the elbow through the ulna. When load is applied to the forearm from a distoproximal direction, the radius transmits 57% of the load directly to the humerus and 43% to the ulna.

The forearm is pronated by the pronator quadratus and pronator teres (Fig 5-32). The main pronator is the pronator quadratus, the action of the muscle being independent of the position of the elbow. The pronator teres is a reserve pronator reinforcing the power when speed is required or resistance is applied to the motion (Fig 5-33). The participation of the accessory pronators, flex or carpi radialis and palmaris longus, is controversial. The forearm is supinated by the supinator (Fig 5-34). The biceps is the reserve supinator and reinforces the action when fast supination is required or resistance is encountered (Fig 5-35). The extensor carpi radialis longus and brevis are accessory supinators.

WRIST

The wrist acts as a universal joint. It develops a spheroid type of motion envelope E₃ (Fig 5-36) that permits the hand to move without digital motion. The wrist flexes, extends, deviates laterally, and participates minimally in pronation-supination. The wrist traces an arc of 121 degrees of flexion-extension with a minimum of 84 degrees and a maximum of 169 degrees. The average arc of extension is 55 degrees and ranges from 31 degrees to 79 degrees; the average arc of flexion is 66 degrees and ranges from 38 degrees to 102 degrees, as measured on 55 normal adult wrists. The radiocarpal and midcarpal joints participate in this motion, and both flexion and extension are initiated in the midcarpal joint (Fig 5-37). Starting from the neutral position, when the wrist flexes, the average range of flexion is 40 degrees at the midcarpal joint and 26 degrees at the radiocarpal joint. The midcarpal joint contributes 60% of the arc of flexion, and the radiocarpal joint contributes 40%. During extension the average range of extension is 19 degrees at the midcarpal joint and 37 degrees at the radiocarpal joint. The midcarpal joint contributes 33.5% of the arc of extension, and the radiocarpal joint contributes 66.5% (Fig 5-38). The scaphoid belongs anatomically to both rows, and yet functionally it is part of the distal row in extension and part of the proximal row in flexion. This behavior of the scaphoid correlates well with the concept of the carpus becoming a rigid "close-pack" mass in extension and "loose-pack" mass in flexion. The rigidity of the carpal mass in extension favors fracture of the scaphoid or the distal end of the radius on impact. The combination of wrist extension and pronation-supination permits the hand to explore the outer half of a circle (Fig 5-39). The flexed wrist, when rotated, permits the hand to explore the inner half of a circle (Fig 5-40). This latter motion is concerned more with functional activities related to the body. Functionally the hand is used more frequently with the wrist extended and radially deviated or with flexion combined with ulnar deviation.

The wrist is flexed by the flexor carpi radialis, flexor carpi ulnaris, and palmaris longus. The long digital flexors are the accessory flexors of the wrist. The wrist extenders are the extensor carpi radialis longus and brevis and the extensor carpi ulnaris. The digital extensors are the accessory extensors of the wrist. The motion of lateral deviation of the wrist averages 40 degrees, with 30 degrees in the ulnar direction and 15 degrees on the radial side. The proximal and distal rows of the carpus participate and move in the opposite direction. During ulnar deviation the distal row rotates with the metacarpals ulnaward, and the proximal row, including the scaphoid, turns radialward. The reverse motion occurs during radial deviation. The range of ulnar deviation is greater when the hand is supinated. During radial deviation the scaphoid rotates posteroanteriorly, the proximal pole turning dorsally and the distal pole with its tuberosity anteriorly. The lunate follows the scaphoid and flexes. In ulnar deviation, the scaphoid derotates and exposes its full profile (Fig 5-41).

Pronation occurs when the hand extends in a radial direction starting from a neutral rotation position. Supination accompanies the motion of flexion with ulnar deviation. This combination of motion becomes quite evident during manipulative functions of the hand and wrist when involved in power-type performance (hammering, casting a fishing line, swinging a club, etc.).

The center of rotation during radioulnar deviation is located in the head of the capitate. The radial deviators of the wrist are the abductor pollicis, extensor pollicis brevis, extensor carpi radialis longus and brevis, long extensors of the index, and the flexor carpi radialis. The ulnar de viators are the extensor carpi ulnaris, flexor carpi ulnaris, and long extensors of the middle, ring, and little fingers.

The wrist is a key joint with regard to the functional activities of the hand. Grip power is maximal when the wrist is extended to 35 degrees and minimal with the wrist maximally flexed. The degree of participation of the digital motors determines, on the other hand, recruitment of the wrist motors. When the wrist is in extension and the fingers make a soft fist, the following wrist motors are active in a descending order: extensor carpi radialis brevis, extensor carpi ulnaris, and extensor carpi radialis longus. With a tight fist, all three extensors are maximally active (Fig 5-42).

When the fingers are gently extended and the wrist is held in extension, the extensor carpi ulnaris and flexor carpi ulnaris are active. The forceful opening of the fingers brings into action, in a descending order, the following additional wrist motors: extensor carpi radialis brevis, palmaris longus, extensor carpi radialis longus, and flexor carpi radialis (Fig 5-43).

HAND

Fingers

Located at the end of a multisegmented system, the hand functions within the action envelope E₃ of the wrist. The flexing finger traces an action envelope, E₄, that is an equiangular spiral (Fig 5-36).

When the wrist is extended, the field of motion of the fingers is within the wrist envelope E₃. With wrist flexion the action envelope E₄ of the fingers extends beyond the field of motion of the wrist (Fig 5-36).

If the fingers are to be used for the purpose of prehension, the interphalangeal and metacarpophalangeal joints must flex in a coordinated fashion to permit wrapping of the digital palmar surface over the surface of the object. Separately the distal joint is flexed by the flexor profundus, the middle joint by the flexor superficialis and the metacarpophalangeal joint by the intrinsic muscles. The coordination of flexion at the interphalangeal joints and the metacarpophalangeal joint is brought about by the instantaneous participation of the extrinsic-intrinsic motors commanded by the motor cortex.

Furthermore, a fine mechanism of coordination is present locally in the fingers at the level of the interphalangeal joints as presented by Landsmeer. As soon as flexion is initiated at the level of the distal joint (Fig 5-44) by the flexor profundus, the terminal extensor tendon is displaced distally, and the extensor trifurca-tion is carried distally through the lateral tendons, thus relaxing the middle slip. Simultaneously, the oblique retinacular ligament attached to the terminal tendon also increases in tension and, passing volar to the axis of motion at the proximal interphalangeal joint, automatically flexes the middle phalanx. This is a passive mechanism of interphalangeal joint coordination. When the finger reaches a position of flexion close to 70 degrees at the proximal interphalangeal joint, the previously relaxed middle slip goes under tension, and the extensor trifurcation is displaced further distally. This displacement relaxes the lateral slips, lateral tendons, and terminal tendon. This unloading of the extensor tendon at the distal joint allows completion of the flexion at this joint without encountering undue resistance. Any break in this system of activation and coordination interferes immediately with the function of prehension.

The absence of intrinsic muscle action not only breaks the contour of the longitudinal arch of the finger but also creates an abnormal pattern of function. The three joints flex successively from a distoproximal direction rather than simultaneously, and this pattern of flexion prevents the palmar skin from making the necessary surface contact with the object.

The opening of the fingers is an essential prerequisite for the act of prehension. Extension of the metacarpophalangeal joint is controlled by the long extensor. The mechanism is dual. An indirect action of extension is exerted by the long extensor on the proximal phalanx through the volar attachment of the transverse or quadrilateral lamina. A direct action is present through a tendinous attachment of the long extensor to the dorsum of the proximal phalanx. This band is present in only 38.5% of dissected hands.

The proximal interphalangeal joint is extended by the long extensor middle slip and spiral fibers arising from the intrinsic tendons. The distal joint is extended by the terminal tendon, which is essentially formed by the long extensor lateral slip but also receives a contribution from the corresponding intrinsic tendons. The oblique retinacular ligaments participate in the constitution of this tendon (Fig 5-45).

When the middle joint extends actively, the oblique retinacular goes under tension and automatically extends the distal joint. This is another mechanism of coordination on the extensor side of the finger. The flexing finger increases gradually in skeletal length due to the noncircular contour of the metacarpal head. This creates undue tension in the extensor system, but immediate adjustment occurs by the distal shift of the entire extensor mechanism and the volar displacement of the lateral slips at the level of the middle joint. In maximum flexion, the lateral slips are at the level of the axis of motion of the joint. The side motion and rotation of the fingers are determined by the intrinsic muscles. The dorsal interossei abduct or spread the fingers, whereas the volar interossei adduct the fingers relative to a functional axis passing through the third metacarpal. There is more abduction to the finger in extension and less in flexion.

A final passive mechanism of flexion-extension of the finger is present through a tenodesis effect: wrist extension flexes the fingers, and wrist flexion extends them.

Thumb

The thumb sweeps a conoid surface through circumduction. This curved surface is flattened on the palmar aspect (Fig 5-46). All functional activities of the thumb occur within this envelope. Through flexion-adduction the thumb traces the segment of the base of the cone along the palmar surface. The curve traced during this motion is an equiangular spiral (Fig 5-47). Through extension-abduction the ray returns to its initial position.

A fundamental function of the thumb is opposition with the fingers. This occurs as the pad of the thumb is set against the pulp of a corresponding finger. To bring about this opposition, the thumb is abducted in a plane perpendicular to the palm and flexed and rotated pronated) on its long axis (Fig 5-48). The thumb and the pulp of the finger make contact along the equiangular spiral curve of the finger.

There are two phases to the opposition. In stage I the thumb is positioned against the pulp of a corresponding finger. This is determined by the abductor pollicis bre-vis, opponens, and superficial head of the short flexor. Stage I is a function of the median nerve. Stage II of the opposition is the clamping of the thumb pad against the opposed finger. This phase provides the power for the opposition. It is controlled by the adductor and deep head of the short flexor and is a function of the ulnar nerve (Fig 5-48).

Functional Activities

The functional activities of the hand are extensive but can be grouped into nonprehensile and prehensile activities. The former includes touching, feeling, pressing down with the fingers, tapping, vibrating the cord of a musical instrument, lifting or pushing with the hand, stirring, etc. Prehensile activities are grouped into precision and power grips. Precision grip involves participation of the radial side of the hand with the thumb, index, and middle finger to form a three-jaw chuck. When the pulp of these digits comes into contact, the grip is of the palmar type, whereas for very precise work contact with the tip of the same digits, creates a tip type of grip. A lateral, or key, grip involves contact of the pulp of the thumb with the lateral aspect of the corresponding finger in its distal segment.

Power grip predominantly involves the ulnar aspect of the hand with involvement of the little and ring fingers. The radial three digits also participate actively either in a pure power pattern form or by adding an element of precision to the power grip. A typical power grip is the cylindrical grip. All fingers are flexed maximally, for example, around the handle of a tool, and the counterpressure to the flexing fingers is provided by the thenar eminence. More power is provided to this grip when the thumb wraps around the flexed fingers. If an element of precision is necessary, the thumb will adopt a longitudinal position of adduction that allows for small adjustments of posture. In general, the pattern of the grip during prehension is determined by the intention and not necessarily by the shape of the object. A scalpel is held in a precision grip for exact work or in a power grip for bold cuts.

The hook power grip involves flexion of both inter-phalangeal joints and minimal participation of the metacarpophalangeal joint. This pattern is used in carrying a suitcase.

The spherical grip is an interesting grip. If the object held by the digits is large, the grip is of the power type with minimal flexion of the fingers, which are abducted and rotated, and the thumb participates at the opposite pole by stabilizing the object and providing the necessary counterpressure. With a smaller spherical object the fingers are adducted, and the thumb is in opposition; this pattern of prehension is of the precision type.

Despite the multitude of functional activities of the hand, any prehensile act when arrested instantaneously might fit in one of these patterns in a pure or combined form. In the cylindrical grip the motors responsible are the flexor profundi and the intrinsic muscles except for the second dorsal interosseous and the three radial lumbricales. The flexor superficialis is a reserve flexor and participates when more power is necessary. The index finger is an exception; here the flexor superficialis pattern predominates. The thumb brings its contribution with the thenar muscles and the long motors, except for the abductor pollicis longus.

In the hook type of prehension, the radial intrinsics are silent. The long flexors, fourth dorsal interosseous, lumbricales, and the abductor digiti quinti, are active.

During soft opposition of the thumb with the index finger-palmar prehension-the opponens, abductor pollicis brevis, and short flexor are active in a decreasing order (Fig 5-49). When pressure is exerted, the short flexor becomes the more active, followed by the opponens and abductor pollicis brevis. In the lateral grip the flexor pollicis brevis and the opponens are very active. The activity of the abductor pollicis brevis is negligible.

References:

1. American Academy of Orthopaedic Surgeons: Joint Motion- Method of Measuring and Recording. Chicago, 1969.
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4. Darcus HD, Salter N: The amplitude of pronation and supination with the elbow flexed to a right angle. J Anat 1953;87:169-184.
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Chapter 5 - Atlas of Limb Prosthetics: Surgical, Prosthetic, and Rehabilitation Principles

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