Muscle fibers can broadly be divided into 3 muscles types: skeletal, cardiac, and smooth muscle. This summary discusses the basic anatomy of skeletal muscle, key features of skeletal muscle histology and physiology, and important presentations of muscular disease.
Most of the skeletal muscular system is arranged into groups of agonists and antagonist muscles that work in concert to provide efficient and controlled motion. This is achieved through the complex interaction of the musculoskeletal system with the pyramidal, extrapyramidal, and sensory components of the nervous system. This article mainly focuses on the end organ of this complex interaction, the muscle fiber (myofiber). The different types of myofibers and their functional responsibilities are also discussed.
Skeletal muscles can vary greatly in size depending on location and responsibility. Muscles may measure as small as 2 mm (eg, stapedius muscles) or up to 2 feet (eg, large muscles of the thigh). Likewise, the number of muscle fibers within each of these, as well as the shape of muscles (depending on their function), can also vary greatly. What remains constant, however, is that the muscle fibers are aligned in the same direction so that individual muscle fibers can work in concert.
Muscles can also be classified according to their relationship with a tendon. When the muscles fibers and the associated tendon are arranged along the same axis, the muscle is termed a parallel muscle. Parallel muscles may be flat or cylindrical shaped. Cylindrical muscles typically have greater mass at the center of the muscle, leading to a central body or muscle belly (eg, biceps brachii). Some muscles are spread out over a large area and converge on a relatively small tendon, appropriately termed convergent muscles (eg, pectoralis muscles of the chest).
Some muscles insert on their respective tendons at an oblique angle. This arrangement leads to a feathered appearance, which is termed pennate. Unipennate muscles insert on only one side of the tendon. Bipennate muscles have muscles fibers inserting at an angle on both sides of a central tendon. A well-known example of a bipennate muscle is the rectus femoris. Finally, some muscles are circular in shape and contract around an opening. Examples of skeletal circular muscles include the orbicularis oris and orbicularis oculi.
Gross inspection of a skeletal muscle reveals collections of muscle fascicles surrounded by a layer connective tissue termed the epimysium. Each muscle fascicle represents a group of muscle fibers bound together by a layer of connective tissue termed the perimysium. Another layer of connective tissue, termed the endomysium, surrounds each individual myofiber. It is at the level of the myofiber where the basic contractile element of muscle, the sarcomere, is found.
In gross anatomy, the nerves to skeletal muscles are branches of mixed peripheral nerves. The branches enter the muscles about one third of the way along their length, at motor points. Motor points have been identified for all major muscle groups for the purpose of functional electrical stimulation by physical therapists, in order to increase muscle power. Only 60% of the axons in the nerve to a given muscle are motor to the muscle fibers that make up the bulk of the muscle. The rest are sensory in nature, although the largest sensory receptors, the neuromuscular spindles, have a motor supply of their own.
The nerve supply branches within the muscle belly, forming a plexus from which groups of axons emerge to supply the muscle fibers. The axons supply single motor endplates placed about halfway along the muscle fibers.
A motor unit comprises a motor neuron in the spinal cord or brainstem together with the squad of muscle fibers it innervates. In large muscles (eg, the flexors of the hip or knee), each motor unit contains 1200 or more muscle fibers. In small muscles (eg, the intrinsic muscles of the hand), each unit contains 12 or fewer muscle fibers. Small units contribute to the finely graded contractions used for delicate manipulations.
Muscle fibers are long and cylindrical in shape. However, their borders are not perfectly round, accounting for the polygonal appearance seen when examined in cross-section. Each fiber contains multiple nuclei, which are pushed to the periphery of the cell. When examined in cross-section, a typical muscle cell reveals between 4 and 6 nuclei, which lie just underneath the plasma membrane of the muscle fiber, the sarcolemma. Every millimeter of muscle fiber contains approximately 30 nuclei.
In addition to the nuclei, other key structures that are specific to muscle cells within the sarcoplasm include sarcoplasmic reticulum and the contractile apparatus made of thick and thin filaments. The sarcoplasmic reticulum is not identifiable on routine hematoxylin and eosin stains. Its principle function is to store and regulate release of calcium, the key ion for initiation of contraction. Proper staining of myofibers oriented in a longitudinal view reveals the striated nature of skeletal muscle. The striated appearance results from a repeated pattern of protein deposition that makes up the functional unit of the muscle, the sarcomere.
To fully appreciate the sarcomere, an electron microscope is needed to distinguish the ultrastructure of the cell. At this magnification, repeating series of light and dark bands are appreciated. The darkest of these bands is termed the Z-line, which serves as the anchoring point for the contractile elements and helps to mark the border of each sarcomere. Just inside each Z-line is a band that is lighter and broader, termed the I-band.
Named for its properties under polarizing light (isotropic), the I-band is chiefly made up of actin, the principal protein of the so-called thin filament. Inside the I-band is darker broad band termed the A-band (anisotropic). This A-band represents the actin-containing thin filaments and the myosin-containing thick filaments overlapping. Within each A-band is a lighter region, termed the H-band, which has a dark line through the center, termed the M-band. The M-band serves to crosslink the thick filaments.
A full discussion of the contractile mechanism of skeletal muscle is beyond the scope of this article. Briefly, when a motor neuron is depolarized, acetylcholine is released into the neuromuscular junction. Acetylcholine binds the nicotinic receptors on the surface of the skeletal muscles, leading to depolarization of the muscle cell. This depolarization causes the sarcoplasmic reticulum to release large stores of calcium. This calcium influx results in a conformational change that releases tropomyosin from actin. Actin is then free to cross-link with myosin. In addition to this, the intracellular calcium activates the adenosine triphosphate (ATP)–myosin system, which powers the contraction. At an ultrastructural level, contraction results in a shortening of the H-band.
Muscle fibers can be functionally classified as either type I or type II fibers. Type I fibers are characterized principally by relatively high number of mitochondria, allowing for these muscle fibers to participate in aerobic respiration. Functionally, type I muscle fibers contract relatively slow but are able to provide sustained contraction. Conversely, type II muscle fibers have relative few mitochondria and higher glycogen stores. These muscle fibers are better suited for quick contraction but are not as well suited for prolonged activity. Type II muscle fibers can be identified on biopsy based on the myofibrillar ATPase reaction in an alkaline environment. Under these conditions, type II fibers stain dark, while type I fibers appear more light (Figure 2).
Muscle fibers can also be classified into 3 different types based on their physiological functions, as follows:
Slow-twitch, oxidative fibers are small and rich in mitochondria and blood capillaries (hence, red). They exert small forces and are resistant to fatigue. They are deeply placed and suited to sustained postural activities, including standing.
Fast glycolytic (FG) fibers are large, mitochondria-poor, and capillary-poor (hence, white). They produce brief, powerful contractions. They predominate in superficial muscles.
Intermediate (fast, oxidative-glycolytic [FOG]) fibers have properties intermediate between the other two.
Every muscle contains all 3 kinds of fiber, but a given motor unit contains only one kind. The fibers of each unit interdigitate with those of other units.
Progressive wasting of muscles seen in elderly individuals results mainly from loss of motor neurons from the spinal cord and brainstem, often due in part to low-grade peripheral neuropathy arising from vascular disease and/or nutritional deficiency.
Electromyographic records taken from contracting muscles show giant motor unit potentials during the seventh and eighth decades of life. The extra-large potentials result from takeover of vacated motor endplates of lost motor neurons by collateral sprouts from the axons of adjacent healthy motor units. 
At the myoneural junction, the axon divides into a handful of branchlets that groove the surface of the muscle fiber. The underlying sarcolemma is thrown into junctional folds. The basement membrane of the muscle fiber traverses the synaptic cleft and lines the folds. The underlying sarcoplasm shows an accumulation of nuclei, mitochondria, and ribosomes known as a sole plate.
Each axonal branchlet forms an elongated terminal bouton containing thousands of synaptic vesicles loaded with acetylcholine. Synaptic transmission takes place at active zones facing the crests of the junctional folds.
Vesicular acetylcholine is extruded at great speed by exocytosis into the synaptic cleft. The acetylcholine diffuses through the basement membrane to bind with acetylcholine receptors in the sarcolemma. Activation of the receptors leads to depolarization of the sarcolemma. The depolarization is led into the interior of the muscle fiber by T tubules. The sarcoplasmic reticulum liberates Ca2+ ions that initiate contraction of the sarcomeres. Acetylcholinesterase enzyme is concentrated in the basement membrane, and about 30% of released acetylcholine is hydrolyzed without reaching the postsynaptic membrane. Following hydrolysis, the choline moiety is returned to the axoplasm. The best known is calcitonin gene-related peptide, a potent vasodilator.
Muscle spindles are up to 1 cm in length and vary in number from a dozen to several hundred in different muscles. They are abundant (1) in the antigravity muscles along the vertebral column, femur, and tibia; (2) in the muscles of the neck; and (3) in the intrinsic muscles of the hand.
All of these muscles are rich in slow, oxidative muscle fibers. Spindles are scarce where FG or FOG fibers predominate. Muscle spindles contain up to a dozen intrafusal muscle fibers. The larger intrafusal fibers emerge from the poles (ends) of the spindles and are anchored to connective tissue (perimysium). Smaller ones are anchored to the collagenous spindle capsule. At the spindle equator (middle), the sarcomeres are replaced almost entirely by nuclei, in the form of “bags” (in wide fibers) or “chains” (in slender fibers). Muscle spindles have both a motor and a sensory nerve supply.
The motor fibers, called fusimotor, are in the Aγ size range, in contrast to the Aα fibers supplying extrafusal muscle. The fusimotor axons divide to supply the striated segments at both ends of the intrafusal muscles. A single primary sensory fiber of type Ia caliber applies annulospiral wrappings around the bag or chain segments of the intrafusal muscle fibers. Secondary ”flower spray” sensory endings on one or both sides of the primary are supplied by type II fibers.
Muscle spindles are stretch receptors. Ion channels in the surface membrane of the sensory terminals are opened by stretch, creating positive electronic waves that summate close to the final heminode of the parent sensory fiber. Summation produces a receptor potential that fires off nerve impulses when it reaches threshold. Muscle spindles may be stretched either passively or actively. Passive stretch of muscle spindles occurs when an entire muscle belly is passively lengthened. For example, in eliciting a tendon reflex such as the knee jerk, the spindles in the belly of the quadriceps muscle are passively stretched when the tendon is struck.
The type Ia and type II fibers discharge to the spinal cord, where they synapse on the dendrites of α motor neurons. (α motor neurons are so called because they give rise to axons of Aα diameter.)
The response to the positive feedback from spindles is a twitch of contraction in the extrafusal muscle fibers of quadriceps. The spindles, because they lie parallel to the extrafusal muscle, are passively shortened; they are described as being unloaded. Tendon reflexes are monosynaptic reflexes. They have a latency (stimulus-response interval) of about 15-25 ms. In addition to exciting homonymous motor neurons (ie, motor neurons supplying the same muscles), the spindle afferents inhibit the α motor neurons supplying the antagonist muscles, through the medium of inhibitory internuncial (interposed) neurons. This effect is called reciprocal inhibition. The inhibitory neurons involved are called Ia internuncials.
Spindle primary afferents are most active during the stretching process. The more rapid the stretch, the more impulses they fire off. They therefore encode the rate of muscle stretch. Spindle secondary afferents are more active than the primaries when a given position is held. The greater the degree of maintained stretch, the more impulses they fire off. They therefore encode the degree of muscle stretch.
Active stretch is produced by the fusimotor neurons, which elicit contraction of the striated segments of the intrafusal muscle fibers. Because the connective tissue attachments are relatively fixed, the intrafusal fibers stretch the spindle equators by pulling them in the direction of the spindle poles. During all voluntary movements, Aα and Aγ motor neurons are coactivated by the corticospinal (pyramidal) tract. As a result, the spindles are not unloaded by extrafusal muscle contraction. Through ascending connections, the spindle afferents on both sides of the relevant joints are able to keep the brain informed about contractions and relaxations during any given movement.
Patellar reflex is elicited as follows:
- A tap to the patellar ligament stretches the spindles in quadriceps femoris.
- Spindles discharge excitatory impulses to the spinal cord.
- Alpha motor neurons respond by eliciting a twitch in quadriceps, with extension of the knee.
- Ia inhibitory internuncials respond by suppressing any activity in the hamstrings.
Golgi tendon organs are found at muscle-tendon junctions. A single Ib caliber nerve fiber forms elaborate sprays that intertwine with tendon fiber bundles enclosed within a connective tissue capsule. A dozen or more muscle fibers insert into the intracapsular tendon fibers, which are in series with the muscle fibers. The bulbous nerve endings are activated by the tension that develops during muscle contraction.
Because the rate of impulse discharge along the parent fiber is related to the applied tension, tendon endings signal the force of muscle contraction. The Ib afferents exert negative feedback to the homonymous motor neurons, in contrast to the positive feedback exerted by muscle spindle afferents. The effect is called autogenetic inhibition, and the reflex arc is disynaptic because of the interpolation of an inhibitory neuron. If need be, there follows reciprocal excitation of motor neurons supplying antagonist muscles. An important function of tendon organ afferents is to dampen the inherent tendency of moving limb segments to oscillate. Dampening introduces an element known to physiologists as joint stiffness.
Free nerve endings
Muscles are rich in freely ending nerve fibers, distributed to the intramuscular connective tissue and investing fascial envelopes. They are responsible for pain sensation caused by direct injury or by accumulation of metabolites, including lactic acid.
Table 1. Muscle Localization (Open Table in a new window)
|Muscle||Peripheral Nerve||Nerve Roots||Muscle Localization||Muscle Action|
|Abductor Digiti Quinti (hand)||Ulnar||C8, T1||Directly at the medial border of the hand, at the midpoint between the distal wrist crease and the metacarpophalangeal crease. It is the first muscle encountered.||Abduction of digit 5.|
|Abductor Pollicis Brevis||Median||C8, T1||Parallel to first metacarpal shaft, in line with the mid-shaft of the extended first phalanx of the thumb, where it is the first muscle met by the electrode.||Abduction of thumb, ie, movement of thumb out of the plane of the palm.|
|Abductor Pollicis Longus and Extensor Pollicis Brevis||Posterior interosseous branch of radial nerve||C7, C8||In the distal 25% of the dorsal forearm, overlying the radius||Abduction and extension of the proximal phalanx of the thumb|
|Adductor Pollicis||Ulnar, deep palmar branch||C8, T1||Immediately proximal to the first metacarpophalangeal joint, the electrode is inserted in the groove between the metacarpal bone and first dorsal interosseous muscle and toward the depth of the web space. At this fairly distal location, the bulk of the first dorsal is avoided||Thumb adduction within the plane of the palm.|
|Anconeus||Radial||C7, C8||The electrode is inserted midway between the olecranon process and the lateral epicondyle. No other muscle is found at this location.||Elbow extension.|
|Brachioradialis||Radial||C5, C6||Place index finger in the cubital fossa, pointing proximal. Brachioradialis is the first muscle lateral to your finger.||Elbow flexion, with the forearm in mid pronation-supination.|
|Extensors Carpi Radialis Brevis and Longus||Radial||C6, C7||Visualize the line connecting the lateral epicondyle and the radial styloid process. In the proximal half of the forearm, this line separates the extensor digitorum (communis) from the wrist extensors, with a groove between them. The extensors are therefore approached just lateral to this line (ie, to the thumb side) and are superficial.||If the electrode is too lateral, it will be in the brachioradialis. If it is too medial, it will be in the extensor digitorum (communis).|
|Extensor Carpi Ulnaris||Posterior interosseous branch of radial nerve||C7, C8||In the proximal half of the forearm, just dorsal to the ulnar shaft, and superficial||Wrist extension combined with ulnar deviation.|
|Extensor Digitorum (Communis)||Posterior interosseous branch of radial nerve||C7, C8||Brachioradialis and the radial wrist extensors compromise a "movable mass" of muscles. Just medial to this group is a groove separating it from the extensor digitorum (communis), which itself is relatively immovable. The division occurs in the proximal half of the forearm, along the line connecting the lateral epicondyle and radial styloid. The electrode is therefore inserted just medial to and parallel to that groove, in the proximal forearm, where the extensor digitorum (communis) is superficial.||Extension of digits 2 through 5.|
|Extensor Indicis||Posterior interosseous branch of radial nerve||C7, C8||In the distal 20% of the forearm, midway between the radius and ulna. At this distal location, extensor indicis is the only dorsal muscle that is not primarily tendinous.||Extension of digits 2 through 5.|
|Extensor Indicis||Posterior interosseous branch of radial nerve||C7, C8||In the distal 20% of the forearm, midway between the radius and ulna. At this distal location, extensor indicis is the only dorsal muscle that is not primarily tendinous.||Extension of the index finger.|
|Extensor Pollicis Longus||Posterior interosseous branch of radial nerve||C7, C8||Insert the electrode at the junction of the middle and lower thirds of the dorsal forearm, midway between the ulna and radius. At this point, extensor pollicis longus lies immediately beneath the distal muscle bellies of extensor digitorum (communis).||Extension of distal phalanx of thumb.|
|First Dorsal Interosseous (hand)||Ulnar, deep palmar branch||C8, T1||The electrode is inserted parallel to the second metacarpal shaft, superficially, directly into the middle of the dorsal web space.||Abduction of digit 2 within the plane of the palm.|
|Flexor Carpi Radialis||Median||C6, C7||Place index finger in the cubital fossa, pointing proximal. Flexor carpi radialis is the first muscle medial to your finger at the level of the apex of the cubital fossa (where brachioradialis and the muscle converge) and is superficial at that point.||Wrist flexion.|
|Flexor Carpi Ulnaris||Ulnar||C8, T1||Middle third of the forearm, superficial and directly medial.||Wrist flexion with ulnar deviation.|
|Flexor Digitorum Profundus, Ulnar (medial) Heads||Ulnar||C8, T1||In the middle one-third of the forearm, immediately ventral to the ulnar shaft. Here, the muscle lies just below the thin aponeurosis of flexor carpi ulnaris.||Flexion of the distal phalanges of digits 4 and 5.|
|Flexor Digitorum Superficialis||Median||C7, C8, T1||At mid-forearm, halfway from the ventral midline to the medial border of the forearm. At this location, it is the first muscle reached.||Finger or wrist flexion.|
|Flexor Pollicis Longus||Anterior interosseous branch of median nerve||C7, C8||In the middle of the ventral forearm, the electrode is inserted just distal to the convergence of the muscle bellies of flexor carpi radialis and brachioradialis, virtually at the midline-ie, needle placement us just distal to the apex of the cubital fossa. Direct the needle perpendicular to the skin and deep until bone is reached (the flat anterior surface of the radius). The last muscle traversed is flexor pollicis longus, so pull the needle out a few millimeters after reaching bone.||Flexion of distal phalanx of thumb.|
|Opponens Pollicis||Median||C8, T1||At the midpoint of the first metacarpal shaft, in the groove between the metacarpal bone and abductor pollicis brevis. The muscle is studied where it attaches to the medial side of the bone. If abductor pollicis brevis is moved aside, no other muscle overlies the opponens at this point.||Opposition of thumb across the palm.|
|Pronator Quadratus||Anterior interosseous branch of median nerve||C7, C8, T1||The muscle width is the same as its length, covering the distal 20% or so of the forearm, anterior to the interosseous membrane. Insert the electrode just anterior to the distal ulnar shaft, perpendicular to it, and direct the electrode horizontally to meet the thick medial border of the muscle.||Forearm Pronation|
|Pronator Teres||Median||C6, C7||With the index finger in the cubital fossa pointing proximally, pronator teres is the first muscle medial to your finger, immediately distal to the median cubital vein.||Elbow flexion or, if necessary, forearm pronation.|
|Supinator||Radial||C5, C6||In the proximal 20% of the dorsal forearm, insert the electrode in the groove between the radial wrist extensors (movable) and extensor digitorum (communis) (immovable). The electrode is directed deep, where supinator is found lying against the radius.||Forearm supination.|
|Biceps Brachii||Musculocutaneous||C5, C6||Middle one-third of the arm, directly into and paralleling the muscle belly, approaching biceps from its lateral side||Elbow flexion, with the forearm in supination.|
|Brachialis||Musculocutaneous||C5, C6||In the distal one-third of the arm, push the biceps medially and insert the electrode in the groove between biceps and triceps. Direct it down and medially, toward the anterior aspect of the humeral shaft.||Elbow flexion; the degree of forearm pronation-supination is irrelevant.|
|Deltoid, Anterior||Axillary||C5, C6||Midpoint of the line connecting the lateral one-third of the clavicle and the deltoid insertion||Arm abduction or shoulder flexion.|
|Deltoid, Middle||Axillary||C5, C6||One-third of the distance down the line between the acromion process and the deltoid insertion. Deltoid is the only muscle encountered in this location.||Arm Abduction.|
|Deltoid, Posterior||Axillary||C5, C6||Midpoint of the line connecting the distal scapular spine and the deltoid insertion||Arm abduction or shoulder extension.|
|Infraspinatus||Suprascapular||C5, C6||Halfway between the scapular spine and the inferior tip of the scapula, midway between the lateral and medial borders of the scapula-ie, directly in the center of the infraspinous fossa. The electrode should first gently touch the posterior surface of the scapula, then be pulled back slight to examine the infraspinatus.||External rotation of the arm. Activation is usually possible simply by the patient lifting the arm off the table.|
|Latissimus Dorsi||Thoracodorsal (middle subscapular)||C6, C7, C8||Posterior axillary fold, directly lateral to the inferior tip of the scapula||Extension/adduction of the humerus.|
|Levator Scapula||Cervical plexus||C3, C4, C5||Midpoint of the line connecting the superior medial scapular border and the nuchal line. Levator scapula is found deep to the overlying upper trapezius.||Scapular elevation. Have the patient shrug the shoulder.|
|Pectoralis Major||Medial and lateral pectoral nerves||C7, C8, T1||Anterior axillary fold, in direct vertical line with the coracoid process||Adduction of the arm.|
|Rhomboid Major||Dorsal scapular||C5||At the level of the midpoint of the medial scapular border, midway between the border and the high thoracic (T1-T4) spinous processes. The muscle lies deep to middle trapezius.||Scapular adduction. Have the patient lift the elbow off the table against resistance.|
|Rhomboid Minor||Dorsal scapular||C5||Midpoint of the line connecting the superior medial scapular border and the cervical prominence. Middle trapezius fibers overlie rhomboid minor.||Scapular adduction. Have the patient move the scapulae closer together.|
|Serratus Anterior||Long thoracic||C5, C6, C7||In the mid or anterior axillary line, isolate one rib by placing two fingers in the adjacent interspaces, anterior to the bulk of the latissimus dorsi but posterior to the breast tissue in a woman. Needle electrode insertion is directly between your fingers, as serratus anterior is the only muscle between the skin and the rib.||Elevation and reaching forward with the arm, ie, scapular protraction. Providing resistance is sometimes necessary.|
|Supraspinatus||Suprascapular||C5, C6||At the medial one-third of the scapular spine, insert the electrode immediately superior to the scapular spine. Aim the electrode perpendicular to the skin (not parallel to it) into the depth of the supraspinous fossa, where only supraspinatus is encountered. The aponeurosis of the lateral trapezius fibers is pierced first.||Arm abduction.|
|Teres Major||Lower subscapular||C5, C6||Immediately lateral to the lower one-third of the lateral scapular border.||Internal rotation of the arm.|
|Teres Minor||Axillary||C5, C6||Immediately lateral to the middle third of the lateral scapular border.||External rotation of the arm.|
|Trapezius, Middle||Spinal accessory, cervical (subtrapezial) plexus||Cranial nerve XI, C3, C4||Directly medial to the medial edge of the scapular spine. Keep the electrode superficial, just under the subcutaneous tissue.||Scapular adduction.|
|Trapezius, Upper||Spinal accessory, cervical (subtrapezial) plexus||Cranial nerve XI, C3, C4||Superior border of the shoulder, immediately medial to the acromioclavicular joint. The free border of the upper trapezius can be grasped between two fingers at this point, and the electrode parallels the slope of the shoulder.||Shoulder elevation. Have the patient shrug the shoulder.|
|Triceps, Lateral Head||Radial||C7, C8||Distal one-third of arm, directly in line with lateral epicondyle, and superficial||Elbow extension.|
|Triceps, Long Head||Radial||C7, C8||At the level of the midshaft of the humerus, the electrode is inserted just medial to the posterior midline of the arm.||Elbow extension.|
|Abductor Digiti Quinti (foot)||Lateral plantar branch of tibial nerve||S1, S2||At the lateral border of the foot, locate the base of the fifth metatarsal bone, the prominence of which is easily felt. The electrode is inserted immediately proximal to and to the plantar side of the prominence, parallel to the long axis of the foot.||Small toe abduction. Ask the patient to fan the toes. Voluntary activation of this muscle can be difficult.|
|Abductor Hallucis||Medial plantar branch of tibial nerve||S1, S2||Halfway between the prominence of the navicular bone and the plane of the sole, where it is the most superficial muscle. Insert the electrode parallel to the long axis of the foot.||Can be difficult. Ask the patient to fan or curl the toes.|
|Anterior Tibialis||Deep branch of fibular nerve||L4, L5||At the junction of the middle and upper thirds of the leg, one quarter of the distance from the tibial shaft to the lateral border of the leg. In this location, it is the only muscle encountered.||Ankle dorsiflexion. The patient will sometimes reflexively extend the toes in the same motion, and extensor digitorum longus can substitute for anterior tibialis in producing ankle dorsiflexion. If necessary, hold the toes in plantarflexion while the patient dorsiflexes the ankle.|
|Extensor Digitorum Longus||Deep branch of fibular nerve||L5, S1||At the junction of the middle and upper thirds of the leg, halfway between the tibial shaft and lateral border of the leg. At this point, extensor digitorum longus is the first muscle encountered.||Extension of digits 2 through 5.|
|Extensor Hallucis Longus||Deep branch of fibular nerve||L5, S1||At the junction of the middle and lower thirds of the leg, one third of the distance from the tibial shaft to the lateral border of the leg. The electrode is directed deep and medially.||Great toe extension; be certain the needle is pulled back into the subcutaneous tissue before the patient contracts this muscle.|
|First Dorsal Interosseous (foot)||Lateral plantar branch of tibial nerve||S1, S2||Place your index finger in the dorsal web space between the first and second toes, pointing distally. Pull your finger proximal until it wedges between the first two metatarsal heads. Insert the electrode immediately distal to your finger and angle it slightly toward the second toe. The muscle is at the depth of the metatarsals; no other muscle is encountered.||Have the patient curl or fan the toes. Many cannot voluntarily activate the first dorsal interosseous.|
|Gastrocnemius, Lateral head||Tibial||S1, S2||Midway between the fibular head and the posterior midline of the leg, and superficial||Ankle plantarflexion.|
|Gastrocnemius, Medial Head||Tibial||L5, S1, S2||Medial border of the leg, junction of the upper and middle thirds, and superficial.||Ankle plantarflexion.|
|Fibularis Longus||Superficial branch of fibular nerve||L5, S1||Straddle the fibular head with your index and middle fingers, pointing proximal. Pull straight down to the junction of the upper and middle thirds of the leg; your fingers will be surrounding fibularis longus, which is the first muscle encountered.||Eversion/plantarflexion of the ankle.|
|Posterior Tibialis||Tibial||L5, S1||There are two acceptable approaches: 1. At the junction of the middle and lower thirds of the leg, insert the electrode under the medial tibial shaft and direct it along the bone and deep, where the muscle lies against the interosseous membrane. The full width of flexor digitorum longus is traversed before posterior tibialis is entered. The illustration depicts this approach. 2. Through anterior tibialis, directly against the lateral border of the tibial shaft, at the junction of the middle and lower thirds of the leg. The electrode crosses the full width of anterior tibialis against the periosteum of the tibia until the interosseous membrane is reached and pierced. Beyond the membrane is posterior tibialis. At the junction of the middle and lower thirds of the leg, the needle electrode is inserted immediately adjacent (either medial or lateral) to the posterior midline.||Plantarflexion/inversion of the ankle.|
|Soleus||Tibial||S1, S2||At the junction of the middle and lower thirds of the leg, the needle electrode is inserted immediately adjacent (either medial or lateral) to the posterior midline.||Ankle plantarflexion. If the examiner holds the patient's knee in flexion during activation, gastrocnemius contribution to ankle plantarflexion is minimized.|
|Adductor Longus||Obturator||L2, L3, L4||In the proximal 20% of the thigh, one-quarter the distance from the medial border to the anterior border of the thigh.||Thigh adduction.|
|Adductor Magnus||Obturator and sciatic||L2, L3, L4||Upper one-third of thigh, immediately posterior to the medial border of the thigh.||Thigh adduction.|
|Gluteus Maximus||Inferior gluteal||L5, S1, S2||Midpoint of the line connecting the posterior inferior iliac spine and greater trochanter. Gluteus maximus is the first muscle underlying the subcutaneous tissue.||Hip extension. Flex the knee to 90° to minimize hip extensor action of the hamstrings, and then have the patient lift the knee off the table. As an alternative, hip abduction.|
|Gluteus Medius||Superior gluteal||L4, L5, S1||The anterior border of gluteus medius is defined by the line joining the anterior superior iliac spine (ASIS) and greater trochanter. The electrode is inserted parallel to this line, at its midpoint and just posterior to it. The muscle is the first reached.||Internal rotation of the thigh. Needle insertion as described above places it in the anterior fibers of gluteus medius, allowing internal rotation to be used for activation. This motion can be carried out smoothly, as opposed to thigh abduction, which a cruder motion and which less easily allows for smooth recruitment of motor units.|
|Gracilis||Obturator||L2, L3, L4||At the junction of the upper and middle thirds of the thigh, directly medial. At this point, gracilis can usually be surrounded by two fingers, facilitating localizing.||Thigh adduction.|
|Hamstring External, Biceps Femoris Long Head||Tibial portion of sciatic nerve||L5, S1, S2||At midthigh, there is a palpable groove from the iliotibial band between vastus lateralis and the external hamstrings. The needle electrode is inserted just posterior to (ie, above in the prone position) the groove and parallel to the femur. At this location, the long head is the first muscle reached.||Knee flexion; be certain the electrode is first pulled back into subcutaneous tissue. A strongly contracting muscle can easily bend a imbedded EMG electrode.|
|Hamstring External, Biceps Femoris Short Head||Fibular portion of sciatic nerve||L5, S1||At the level of the superior crease of the popliteal fossa, immediately medial or lateral to the tendon of biceps femoris long head. The electrode is directed down and under the tendon. At this distal level, long head is tendinous and short head is muscular. The tendon of the long head is shown in dashed outline.||Knee flexion.|
|Hamstring Internal, Semimembranosus and Semitendinosus||Tibial portion of sciatic nerve||L4, L5, S1, S2||At mid-thigh, at or just medial to the midline and immediately subcutaneous.||Knee flexion.|
|Iliopsoas||Femoral||L2, L3, L4||Immediately distal to the inguinal ligament, halfway between the femoral artery pulse and the anterior superior iliac spine. The electrode is directed laterally, away from the neurovascular bundle.||Hip flexion.|
|Quadriceps, Rectus Femoris||Femoral||L2, L3, L4||At the midpoint of the line connecting the anterior superior iliac spine (ASIS) and the superior pole of the patella. This places the electrode insertion slightly lateral to the geographic center of the anterior thigh.||Knee extension.|
|Quadriceps, Vastus Lateralis||Femoral||L2, L3, L4||Mid-thigh, directly lateral. In most patients there is a visible and palpable groove between the external hamstring group and vastus lateralis, caused by the iliotibial band. The needle is therefore inserted just anterior to (ie, above the supine position) the groove.||Knee extension. Have the patient push the back of the knee down into the table or into your hand. Alternatively, have the patient lift the entire leg off the table with the knee straight.|
|Quadriceps, Vastus Medialis||Femoral||L2, L3, L4||L4 The distal 20% of the medial thigh. At this level, the oblique fibers of vastus medialis are angled at nearly 45° toward the patella, and the electrode should parallel them.||Knee extension. Have the patient push the back of the knee into the table or your hand. If necessary, have the patient lift the leg off the table with the knee straight and the thigh in external rotation.|
|External Anal Sphincter||Inferior rectal branch of pudendal nerve||S2, S3, S4||With a gloved finger in the rectum, insert the electrode at the mucocutaneous junction, and angle it toward your finger.||Ask the patient to tighten the sphincter around your finger. Relaxation is best obtained by having the patient strain, simulating pushing to have a bowel movement.|
|Diaphragm||Phrenic||C3, C4, C5||Anterior axillary line, eighth or ninth rib interspace. The intercostal muscles encountered first, then the diaphragm, identified by its cyclic contractions with breathing. If there are no voluntary contractions originating from the diaphragm, correct localization relies on recognizing that the first insertional activity heard is from the intercostal muscles, followed by an electrically silent gap, then the insertional activity from the targeted muscle.||Respiration.|
|Orbicularis Oculi||Temporal and zygomatic branches of facial nerve.||Cranial nerve VII.||Two-thirds the distance from the anterior border of the ear to the lateral edge of the orbit. From that point, direct the electrode toward the lateral canthus of the eye and remain superficial.||Closing or squeezing of the eyelids.|
|Orbicularis Oris||Buccal branches of facial nerve||Cranial nerve VII.||Two-thirds the distance from the angle of the jaw to the corner of the mouth. From that point, direct the electrode toward the corner of the mouth and remain superficial.||Whistling motion of the lips.|
|Paraspinals, Cervical (Erector Spinae)||Posterior primary rami||C1 through T1||Adjacent to the cervical spine, in vertical line with the midpoint of the nuchal ridge. The electrode is inserted perpendicular to the skin and must travel through trapezius before reaching the paraspinals. This transition is a fascial plane separating the two. The insertion point shown is for the midcervical paraspinal muscles.||Gentle isometric neck extension, with the electrode in subcutaneous tissue first.|
|Paraspinals, Lumbosacral (Erector Spinae)||Posterior primary rami||L1 though S1, (S2)||The point halfway between the posterior superior iliac spine and the midline corresponds to the low lumbar paraspinal muscles. Needle electrode insertion for more proximal or distal levels is through the same point and along the line parallel to the spine. The electrode is directed perpendicular to the skin and somewhat medially, toward the deeper paraspinal layers.||Hip extension. This will secondarily cause the paraspinal muscles to contract.|
|Sternocleidomastoid||Spinal accessory, cervical plexus||Cranial nerve XI, C2, C3||Midway between the mastoid and clavicular attachments of the muscle. Enter it from its lateral side and parallel to its course.||Have the patient turn the head to the opposite side, against your hand.|
|Tongue (Genioglossus)||Hypoglossal||Cranial nerve XII||Midpoint between tip of chin and the angle of the jaw, medial to the mandible. The tongue is found deep here, after the electrode passes through mylohyoid and geniohyoid muscles.||Protraction of the tongue. Ask the patient to stick out the tongue.|