Functional Anatomy of the Muscle

Fig. 2.1

The hamstring muscle complex. (1) Superior compartment of semitendinosus – (2) Inferior compartment of semitendinosus – (3) Tendinous inscription of semitendinosus – (4) Superior compartment of semimembranosus – (5) Middle compartment of semimembranosus – (6) Inferior compartment of semimembranosus – (7) Medial compartment of the long head of the biceps femoris muscle – (8) Lateral compartment of the long head of the biceps femoris muscle – (9) Short head of the biceps femoris muscle – (10) Common tendon of the semitendinosus and long head of the biceps femoris muscles

The study of neuromuscular partitioning of the extensor carpi radialis muscles was based on extra- and intramuscular innervations. The extensor carpi radialis longus muscle comprises a superficial compartment and a deep compartment, each innervated by a specific branch of the radial nerve, dividing into the anterior and posterior branches. Partitioning of the extensor carpi radialis brevis muscle is more complex comprising two to four compartments. The increased number of neuromuscular partitions in extensor carpi radialis brevis when compared to extensor carpi radialis longus could be due to the need for more differential recruitment in this muscle depending on force requirements [37].

The serratus anterior muscle can be separated into three parts based on its origin and insertion. The superior part originates from the first and second ribs, and is inserted onto the superior angle of the scapula. The middle part, originating from the second and third ribs, inserts on the medial border of the scapula and the inferior part originates from the inferior and third ribs and inserts on the inferior angle of the scapula. The long thoracic nerve supplies the three parts. The superior part receives additional innervation by a nerve branch also innervating the levator scapulae muscle. The inferior part is additionally innervated by branches of the intercostal nerves. Understanding the characteristics of the innervation in each part is useful in identifying the cause of dysfunction of the muscle [33].

2.2.2 Architecture of the Triceps Surae Muscle

The triceps surae is a musculotendinous complex involved in balance and walking. It is a heavily used muscle in athletes. The complexity of its architecture is linked to its fundamental role during the various phases of walking and running.

The triceps surae muscle-tendon complex, innervated by the tibial nerve, is classically composed of two gastrocnemius muscle heads and the soleus muscle. Both gastrocnemius heads are in fact the same muscle. The gastrocnemius muscle is composed of two independent heads, the lateral head and medial head (Fig. 2.2).

Fig. 2.2

Posterior view of the right leg shows the lateral gastrocnemius muscle (1), the medial gastrocnemius muscle (2), the soleus muscle (3), the calcaneal tendon (4), the fibularis brevis muscle (5) and the flexor hallucis longus muscle (6)

The study of compartments enables a different conceptualization of the architecture of the triceps surae complex. Gastrocnemius is a digastric muscle with two juxtaposed bellies and each head has proximal insertions and a common terminal aponeurosis. The lateral head is divided into three muscle compartments [46]. The medial head is organized in a single compartment [27]. The soleus muscle can be divided into two compartments [13, 25]. The dorsal portion is situated between a dorsal aponeurosis and a ventral aponeurosis, which can be incomplete. Short muscular fibers inserted on these aponeuroses are oriented obliquely from the rear towards the front, from top to bottom, converging towards the center of the muscle. The ventral portion is inserted into the ventral concavity of the ventral aponeurosis of the dorsal portion. It is a bi-pinnate muscle with an intramuscular septum at its center [35] (Fig. 2.3). The terminal aponeurosis of the gastrocnemius muscle binds to the dorsal aponeurosis of the soleus muscle to which the intramuscular septum is attached. This distal fibrous unit becomes a powerful tendon, the calcaneal tendon, common to the heads of the gastrocnemius and the soleus muscle. The long tendon of the plantaris muscle slides between the gastrocnemius and soleus muscles, attaching on the medial aspect of the calcaneal tendon.

Fig. 2.3

Schematic drawing of the soleus muscle shows the different compartments. (1) Dorsal portion of the muscle – (2) Dorsal aponeurosis – (3) Ventral aponeurosis – (4) Ventral portion of the muscle – (5) Septum of the ventral portion –(6) Calcaneal tendon

Innervation of the lateral head of the gastrocnemius muscle is provided by two or three nerve branches [35, 45, 46]. Innervation of the medial head is extremely variable and systematization is not feasible [55]. The soleus muscle is innervated by two nerve pedicles. The dorsal nerve for the dorsal compartment divides rapidly in two to three branches that provide branches to the medial and lateral parts of the muscle. The ventral nerve divides usually into two branches which in turn provide branches for each half of the bi-pinnate part of the muscle [28, 35].

The architectural complexity of the muscle is demonstrated by its functional complexity but the physiological action of each part remains unclear [28]. However, this organization of muscle, aponeurosis and tendon demonstrates extensive areas of muscle insertions. Each is susceptible to sustaining injury [3].

2.3 Functional Organization of Muscles

The muscles of the human body are functionally organized. Organization differs significantly between the static muscles of the trunk and the dynamic segmental muscles of the limbs.

Limb muscles are organized in compartments which contain muscles that activate or coordinate movement. It is important to consider the functional terminology of the muscles involved in movements such as flexing the forearm.

Some muscles are both static and dynamic, and their organization is more complex. Periscapular muscles are part of this muscle group and are arranged in a particular muscle system presented in this chapter.

Morphological classification of muscles is antiquated. Muscles were often described at a time when functional aspects were poorly taken into account. Morphological classifications are therefore sometimes ill-adapted to biomechanical reality, as is the case of the deltoid and pectoralis major muscles. This example of functional proximity will also be discussed.

2.3.1 Functional Organization of Limb Muscles

Morphologically, the dynamic muscles of the limbs are mainly long. In each limb segment they are organized into muscular compartments. Each muscular compartment is separated from the adjacent compartments by a septum, a fibrous structure, which inserts on the bone and fascia of the circular envelope of the corresponding member segment. This fascia is of varying thickness depending on the muscle power imposed. For example, in the lower limb, these fasciae are thick so as to contain muscle expansion during contraction and maintain maximum power. In pathological conditions, this barrier to muscle expansion becomes a source of increased pressure in the compartment, creating compartment syndrome, occurring frequently in athletes due to the repeated activity imposed by the sport concerned.

In the same muscle compartment, some muscles only mobilize a single joint, and are called monoarticular muscles. Others, polyarticular muscles, cross several joints, and can be organized in several muscle compartments. Each limb has a primary function, such as flexion-extension of a joint, integrated into a more specific function, such as prehension in the upper limbs or walking and running in the lower limbs.

Furthermore muscles have specific roles in a given movement. The main activated muscle which produces analytical movement is called the agonist muscle. In the same muscular compartment and adjacent compartments, some muscles participate in the same movement without being the main mobiliser. These muscles are called “congener” muscles. To harmonize movement, other muscles or muscle bundles may act in opposition to the agonist and “congener” muscles. These are the antagonist muscles; they control the speed and accuracy of movement, moderating the action of the agonist muscle. Synergist muscle is another type of functional group of muscles. A synergist muscle is involved concomitantly in the considered movement. It may be an agonist or “congener” muscle, which produce and control movement, or antagonists that neutralize or moderate the execution of muscle movement.

To illustrate the functional organization of the segmental muscles, we will consider flexion of the forearm. It involves three bones, the humerus, radius and ulna. Their epiphyses are grouped into a single joint complex, the elbow joint. The three main flexor muscles of the elbow are the biceps brachii, brachialis and brachioradialis. The brachialis muscle, located in the ventral compartment of the arm is strictly mono-articular, crossing only the elbow joint. The biceps brachii muscle, also located in the same compartment, is polyarticular, passes over the proximal joints of the shoulder and the elbow joint complex. The brachioradialis muscle is also polyarticular, passing over the elbow joints and the distal radio-ulnar joint. It belongs to the ventral compartment of the arm and the lateral compartment of the forearm.

The brachialis muscle is the agonist in flexion of the forearm. It is involved only in this movement. The biceps brachii and brachioradialis muscles are “congeners”. They have other functions in addition to that of elbow flexion. Thus, the biceps brachii muscle is involved in the abduction of the arm and supination of the forearm. The brachioradialis is involved in pronation and supination of the forearm. The main antagonist of elbow flexion is the triceps brachii muscle, located in the dorsal compartment of the arm. The radialis extensor carpi muscles are also antagonists, involved in moderating the end of flexion movement. All these muscles are synergist in flexion of the elbow (Fig. 2.4).

Fig. 2.4

Horizontal section of the arm showing the ventral and dorsal compartments separated by the intermuscular septum (IM) and surrounded by the arm fascia (AF). Concerning the forearm flexion, the agonist muscle is the brachialis muscle (1), the biceps brachialis (2) and brachioradialis (3) muscles are “congener” muscles and the triceps brachialis (4) is an antagonist muscle

2.3.2 The Scapulothoracic Functional Complex

Another type of elaborate muscle organization is that of the periscapular muscular complex. This complex has two functions: stabilizing the scapula and optimizing movements of the upper limb. Functionally, the muscles inserted on the scapula are usually classified into two major groups: the scapulohumeral muscles which help to position the arm in space and the scapulothoracic, or axiothoracic, muscles, which coordinate scapulothoracic movements.

The scapulohumeral muscles include the supraspinatus, infraspinatus, teres major, teres minor, deltoid, biceps brachii, long head of triceps brachii and coracobrachialis muscles. They regulate activities of the glenohumeral articulation and provide power to the humerus.

The scapula is essential in coordinating upper extremity activity. Other than its attachment to the acromioclavicular and sternoclavicular joints, the scapula does not have any other attachments to the thorax. Its stability is provided by the surrounding musculature, the scapulothoracic muscles. They include the trapezius, serratus anterior, rhomboid and levator scapulae muscles. The main convergence of the muscles of this group is the medial or spinal border of the scapula. Indeed, the tendinous attachment of the serratus anterior muscle combines with the ventral tendinous insertions of the levator scapulae and rhomboid muscles, enveloping the entire medial border of the scapula. Fibers of the latissimus dorsi muscle cover the inferior angle of the scapula during their oblique course. There is no osseous attachment of the latissimus fibers to the inferior angle of the scapula.

The scapulothoracic “articulation” differs from the other joints of the shoulder complex, as there is no articular cartilage, synovium, or capsule but a series of bursal and muscular planes, which allow sliding. The scapulothoracic joint is defined by soft tissue apposition. The subscapularis muscle spreads across the concave ventral scapula, lying over the serratus anterior muscle and the convex thoracic cage.

From a muscular functional perspective, the movements of the scapulothoracic joint are facilitated by three layers of muscles and bursae. The superficial layer includes the trapezius, and latissimus dorsi which is not attached to the inferior angle of the scapula. In some cases a well-defined bursa is present between the superior fibers of the latissimus dorsi and the inferior angle of the scapula. When absent, the space is filled with areolar tissue. The middle layer is formed by the rhomboid and levator scapulae muscles. The scapulotrapezial bursa is always present between the fibers of the middle and inferior trapezius and the superomedial scapula. The deep layer includes the serratus anterior and subscapularis muscles. The scapulothoracic bursa is always present between the thoracic cage and the deep surface of the serratus anterior. A bursa is not always present between the superficial surface of the serratus anterior and the subscapularis [24, 53]. This functional complex enables the scapula to slide on the external surface of the thorax, due to the scapulothoracic articulation, and to perform rotation and translation movements which are not independent of one another.

Instability of this system can result in scapular winging, an abnormal scapulothoracic posture and motion. In high-level athletes, periscapular weakness resulting from overuse may manifest as this type of dysfunction. Loss of motion of the scapula in the different planes can affect a thrower’s power and place a strain on the posterior shoulder capsule [14, 29].

2.3.3 The “Deltoid–Pectoralis” Complex

Morphological classification of muscular mass of the human body by the first anatomists was based on purely topographical, even aesthetic, criteria. The potential function of the different muscle bundles has not always been taken into account. This is particularly the case of the clavicular heads of the deltoid and pectoralis major muscles.

The deltoid muscle is a large, thick, triangular muscle that superficially envelops the shoulder joint. The deltoid muscle is divided into three anatomical parts. The anterior deltoid originates from the lateral third of the clavicle. The middle deltoid originates from the acromion and the posterior deltoid originates from the scapular spine. The muscle fibers of the deltoid converge into a V-shaped pattern, inserting on the humeral shaft.

The pectoralis major muscle is a large muscle of the anterior chest wall. The muscle is divided into a clavicular head, inserted on the medial portion of the clavicle, and a sternal head, inserted on the sternum and the adjacent sterno-costal cartilages. From its broad medial insertions, muscular fibers transform into a tendon inserted on the humeral diaphysis.

From a functional point of view, the clavicular head of the pectoralis major muscle and the anterior deltoid are very similar muscles. They act as arm flexors. The anterior deltoid is only slightly involved in abduction, the major function of the deltoid muscle. The clavicular head of the pectoralis major muscle is not really involved in adduction of the upper limb, the main function of the pectoralis major muscle.

These two heads are moreover very similar in structure, which clearly differs from that of the muscles to which they are attached. The deltoid muscle has a complex anatomical structure with intramuscular tendons which should be taken into account when assessing the function of the muscle. The structure of the clavicular portion of the deltoid muscle is however different and more simple than that of other segments of the muscle [26, 41]. Similarly, the intramuscular structure of the pectoralis major muscle is complex. Whereas the sternal head is divided into six or seven segments, the clavicular head is compact and unsegmented [15]. This more simple structure is similar to that of the anterior portion of the deltoid muscle.

Furthermore, the tendons corresponding to these two muscular heads are very close to one another. Concerning the deltoid muscle, the tendons of the three different parts (anterior, middle and posterior) insert individually into the humeral shaft and form three discrete lines [39, 41]. The anterior insertion line corresponding to the clavicular portion of the muscle is well individualized and separated from the lines of the middle and posterior parts that are closer to one another and far from the anterior line. Organization of the tendon of the pectoralis major muscle is also complex. Medially, the tendon consists of two distinct layers, anterior and posterior, that are separated by fat tissue. Laterally, the tendon fuses prior to its insertion at the lateral lip of the intertubercular sulcus of the humerus. The two layers are broad and flat and are continuous inferiorly. The anterior layer courses in an inferolateral direction, whereas the posterior layer courses superolaterally. The anterior layer corresponds to the clavicular head, even if in some cases bundles of the sternal head project onto the anterior layer. At the musculotendinous junction, the clavicular head overlaps all the sternal segments deep to it [15].

There is a marked anatomic proximity between the tendon of the anterior part of the deltoid muscle and that of the clavicular head of the pectoralis major muscle [23]. Proximity and these distal interconnections further enhance the common function of both muscles and focus their muscle power.

This anatomical knowledge of these two muscle bundles is important to consider in clinical practice. It is fundamental for surgical approaches and repair of the deltopectoral region. This information is essential in rehabilitation of the shoulder, especially in athletes.

2.4 Organization of Tendons

Muscles produce force that is transmitted via connective tissue networks and tendons to the skeletal system in order to propel movements. This sometimes complex intramuscular network organization has been previously discussed (cf. muscular structure).

The tendon is the fibrous structure necessary for the muscle to insert on the bone. The surface of tendinous insertion causes bone relief, defined as ridges, tuberosities or even processes. There are tendons of origin and termination. Tendons can be flat and thin, thus defined as an aponeurosis. Within the same muscle, a tendon can develop gradually into an aponeurosis.

In some cases, muscle fibers are inserted directly onto a bone structure without an interposing tendon. Moreover, some muscular fibers are inserted directly on structures other than bone, such as on a septum or fascia. A fascia is defined as connective tissue sheets enveloping muscles or muscle groups and which do not contain tendon fibers of muscle origin.

The myotendinous junction corresponds to the macroscopic insertion area of muscle fibers on a tendon or aponeurosis. It depends on the configuration and structure of the muscle. For the same muscle, it may be subjected to variations (Fig. 2.5).

Fig. 2.5

Variations of levels of the soleus myotendinous junction. (a) Low myotendinous junction. (b) High myotendinous junction. (1) Soleus muscle – (2) Flexor digitorum longus muscle – (3) Extensor digitorum longus muscle – (4) Flexor hallucis longus muscle

Tendon structures may sometimes have a complex organization that is important to consider in pathology, especially in athletes. To illustrate these concepts of organization, we will present the anatomy of the proximal hamstring muscle complex and the femoral portion of the iliopsoas muscle.

2.4.1 The Proximal Hamstring Muscle Complex

The hamstring refers to the three muscles located in the posterior compartment of the thigh: semimembranosus, semitendinosus and biceps femoris (long and short heads). These muscles are bi-articulate, extending the hip joint and flexing the knee joint.

The ischial tuberosity is the site of origin of the hamstring muscles, except for the short head of the biceps femoris, which arises from the middle third of the linea aspera and the lateral supracondylar ridge of the femur. The proximal region of the hamstring muscles is characterized by complex architecture with overlapping tendons and inter-relations between muscles. The raphe, a tendinous inscription dividing the semitendinosus muscle into two distinct parts, belongs to this proximal complex. The semitendinosus muscle is considered to be a digastric muscle with opposite bellies due to this raphe.

The areas of origin of the semitendinosus, semimembranosus and long head of the biceps femoris muscles are clearly divided into two parts on the ischial tuberosity. The anteromedial part is occupied by semitendinosus and the long head of the biceps femoris muscle which are highly adjoined. The posteromedial part is occupied by the origin of semimembranosus. These two parts are divided by a vertical ridge delimiting the anteromedial and posterolateral facets. A small portion of semitendinosus is inserted on a small inferior facet of the ischial tuberosity. The adductor magnus muscle has also a tendinous slip originating from the infero- and anteromedial aspect of the ischial tuberosity, but is usually not considered a component of the hamstring muscle complex [4, 52].

The close relationships between the long head of the biceps femoris muscle and the semitendinosus muscle have led to the description of a common head and a common tendon. Biceps femoris and semitendinosus usually separate from their common tendon 9 cm away from the ischial tuberosity. Most of the common head is constituted by semitendinosus. In this common tendon, the long head of the biceps femoris muscle consists of the tendinous part and semitendinosus mainly consists of the muscular part. The tendinous portions of the proximal parts of these muscles can be delimited. The long head of the biceps femoris muscle is composed of a thick and long tendon and semitendinosus contains a thin and short tendon. The tendon of the long head of the biceps femoris muscle is widely connected to the sacrotuberous ligament [4, 43, 52].

The semimembranosus muscle originates just laterally to the common insertion of the semitendinosus and biceps femoris muscles, posterior to the origin of the quadratus femoris muscle. The most proximal part of semimembranosus tendon is joined to the common tendon of semitendinosus and the long head of the biceps femoris and separates around 2.5 cm away from the ischial tuberosity [4, 31, 43].

Knowledge of the anatomy of this tendinous complex is important in understanding pathologies of this region. Hamstring muscle strain is one of the most common injuries in sports medicine. The long head of the biceps femoris muscle is the most commonly injured muscle of the hamstring. The most vulnerable part of the tendon-muscle-bone unit is the musculotendinous junction. Strain on the musculotendinous junction is usually caused by eccentric load during either acceleration or deceleration. Distinction can be made between two mechanisms of injury: hamstring injuries sustained during high speed running, usually affecting semitendinosus, and hamstring injuries sustained during stretching with a combination of extensive hip flexion and knee extension, usually located in semimembranosus. Most hamstring strains or tears can be treated conservatively, but proximal hamstring avulsions can cause significant disability and may need surgery. Surgical intervention must respect the myotendinous organization of the proximal part of the hamstrings (Fig. 2.1).