The pelvic girdle is tightly bound to the axial skeleton, and little relative movement is permitted. In our discussion of the axial musculature, we therefore encountered few muscles that can influence the position of the pelvis. The muscles that position of the lower limbs can be divided into three functional groups: (1) muscles that move the thigh, (2) muscles that move the leg, and (3) muscles that move the foot and toes.

Muscles That Move the Thigh

Table 11-16 lists the muscles that move the thigh. Gluteal muscles cover the lateral surfaces of the ilia (Figures 11-14 and 11-20a, b, c). The gluteus maximus muscle is the largest and most posterior of the gluteal muscles. Its origin includes parts of the ilium; the sacrum, coccyx, and associated ligaments; and the lumbodorsal fascia. Acting alone, this massive muscle produces extension and lateral rotation at the hip joint. The gluteus maximus shares an insertion with the tensor fasciae latae muscle, which originates on the iliac crest and the anterior superior iliac spine. Together these muscles pull on the iliotibial tract, a band of collagen fibers that extends along the lateral surface of the thigh and inserts on the tibia. This tract provides a lateral brace for the knee that becomes particularly important when you balance on one foot.

The gluteus medius and gluteus minimus muscles (Figure 11-20 b, c) originate anterior to the origin of the gluteus maximus muscle and insert on the greater trochanter of the femur. The anterior gluteal line on the lateral surface of the ilium marks the boundary between these muscles.

The lateral rotators originate at or inferior to the horizontal axis of the acetabulum. There are six lateral rotator muscles in all, of which the piriformis muscle and the obturator muscles are dominant (Figure 11-20 c, d).

The adductors (Figure 11-20 c, d) originate inferior to the horizontal axis of the acetabulum. This muscle group includes the adductor magnus, adductor brevis, adductor longus, pectineus, and gracilis muscles. All but the adductor magnus originate both anterior and inferior to the joint, so they perform hip flexion as well as adduction. The adductor magnus muscle can produce either adduction and flexion or adduction and extension, depending on the region stimulated. The adductor magnus muscle may also produce medial or lateral rotation at the hip. The other muscles produce medial rotation. These muscles insert on low ridges along the posterior surface of the femur. When an athlete suffers a pulled groin, the problem is a strain--a muscle tear or break--in one of these adductor muscles.

The medial surface of the pelvis is dominated by a pair of muscles. The large psoas major muscle originates alongside the inferior thoracic and lumbar vertebrae, and its insertion lies on the lesser trochanter of the femur. Before reaching this insertion, its tendon merges with that of the iliacus  muscle, which nestles within the iliac fossa. These two muscles are powerful hip flexors and are often referred to collectively as the iliopsoas  muscle.

Muscles That Move the Leg

As in the upper limb, muscle distribution in the lower limb, exhibits a pattern (Figure 11-21 and Table 11-17). Extensor muscles are located along the anterior and lateral surfaces of the leg, and flexors lie along the posterior and medial surfaces. Although the flexors and adductors originate on the pelvic girdle, most extensors originate on the femoral surface.

The flexors of the knee include the biceps femoris, semimembranosus, semitendinosus, and sartorius muscles (Figure 11-21). These muscles originate along the edges of the pelvis and insert on the tibia and fibula, and their contractions produce flexion at the knee. The sartorius muscle is the only knee flexor that originates superior to the acetabulum, and its insertion lies along the medial surface of the tibia. When the sartorius contracts, it produces flexion at the knee and lateral rotation at the hip--as when you cross your legs.

Because the biceps femoris, semimembranosus, and semitendinosus muscles originate on the pelvic surface inferior and posterior to the acetabulum, their contractions also produce extension of the hip. These three muscles are often called the hamstrings. A pulled hamstring is a relatively common sports injury caused by a strain affecting one of the hamstring muscles.

The knee joint can be locked at full extension by a slight lateral rotation of the tibia.  The small popliteus muscle originates on the femur near the lateral condyle and inserts on the posterior tibial shaft. When flexion is initiated, this muscle contracts to produce a slight medial rotation of the tibia that unlocks the knee joint.

Collectively, the four knee extensors make up the quadriceps femoris: the three vastus muscles, which originate along the shaft of the femur, and the rectus femoris. Together, the vastus muscles cradle the rectus femoris muscle the way a bun surrounds a hot dog (Figure 11-21c). All four muscles insert on the patella; the force of their contraction is relayed to the tibial tuberosity by way of the patellar ligament. The rectus femoris muscle originates on the anterior inferior iliac spine and the superior acetabular rim so in addition to extending the knee, it assists in flexion of the hip. 

Muscles That Move the Foot and Toes

The extrinsic muscles that move the foot and toes are shown in Figure 11-22 and listed in Table 11-18. Most of the muscles that move the ankle produce the plantar flexion involved with walking and running movements. The gastrocnemius  muscle of the calf is an important plantar flexor, but the slow muscle fibers of the underlying soleus muscle are more powerful. These muscles are best seen in posterior and lateral views (Figure 11-22 a,b). The gastrocnemius muscle arises from two heads located on the medial and lateral epicondyles of the femur just proximal to the knee. The fabella, a sesamoid bone, is generally present within the lateral head of the gastrocnemius muscle. The gastrocnemius and soleus muscles share a common tendon, the calcanean tendon, commonly known as the Achilles tendon.

Deep to the gastrocnemius and soleus muscles lie a pair of peroneus muscles (Figure 11-22 b, c). The peroneus muscles produce eversion and plantar flexion at the ankle. Inversion is caused by the contraction of the tibialis muscles. The large tibialis anterior muscle (Figure 11-22b, d) dorsiflexes the ankle and opposes the gastrocnemius muscle.

Important digital muscles originate on the surface of the tibia, the fibula, or both (Figure 11-22b, c, d). Large tendon sheaths surround the tendons of the tibialis anterior, extensor digitorum longus, and extensor hallucis longus muscles, where they cross the ankle joint. The positions of these sheaths are stabilized by superior and inferior extensor retinacula (Figure 11-22 b, d).

Intrinsic muscles of the foot originate on the tarsal and metatarsal bones (Figure 11-23 and Table 11-19). Their contractions move the toes and contribute to the maintenance of the longitudinal arch of the foot.

Which leg movement would be impaired by injury to the obturator muscle?

You often hear of athletes who suffer a pulled hamstring. To what does this phrase refer?

How would you expect a torn calcanean tendon to affect movement of thefoot?

Intramuscular Injections Drugs are commonly injected into tissues rather than directly into the circulation. This method en-ables the physician to introduce a large amount of a drug at one time yet have it enter the circulation gradually. In an intramuscular (IM) injection, the drug is introduced into the mass of a large skeletal muscle. Uptake is generally faster and accompanied by less tissue irritation than when drugs are administered intradermally or subcutaneously (injected into the dermis or subcutaneous layer, respectively). Up to 5 ml of fluid may be injected at one time, and multiple injections are possible. The most common complications involve accidental injection into a blood vessel or piercing of a nerve. The sud-den entry of massive quantities of a drug into the bloodstream can have fatal consequences, and damage to a nerve can cause motor paralysis or sensory loss. As a result, the site of injection must be selected with care. Bulky muscles that contain few large vessels or nerves make ideal targets. The gluteus medius muscle or the posterior, lateral, superior part of the gluteus maximus muscle is commonly selected.The deltoid muscle of the arm, about 2.5 cm (1 in.) distal to the acromion, is another popular site. Probably the most satisfactory from a technical point of view is the vastus lateralis muscle of the thigh; an injection into this thick muscle will not encounter vessels or nerves. This is the preferred injection site in infants and young children, whose gluteal and deltoid muscles are relatively small. This site is also used in elderly patients or others with atrophied gluteal and deltoid muscles.

Compartment Syndrome In the limbs, the interconnections between the superficial fascia, the deep fascia of the muscles, and the periostea of the appendicular skeleton are quite substantial. The muscles within a limb are in effect isolated in compartments formed by dense collagenous sheets (Figure 11-24). Blood vessels and nerves traveling to specific muscles within the limb enter and branch within the appropriate compartments. When a crushing injury, severe contusion, or muscle strain occurs, the blood vessels in one or more compartments may be damaged. These compartments become swollen with blood and fluid leaked from damaged vessels. The connective tissue partitions are very strong; the accumulated fluid cannot escape, so pressure rises within the affected compartments. Eventually, the pressures can become so high that they compress regional blood vessels and eliminate the circulatory supply to the muscles and nerves of the compartment. This compression produces ischemia, or “blood starvation,” known as compartment syndrome. Slicing into the compartment along its longitudinal axis or implanting a drain are emergency measures used to relieve the pressure. If such steps are not taken, the contents of the compartment will suffer severe damage. Nerves in the affected compartment will be destroyed after 2–4 hours of ischemia, although they can regenerate to some degree if the circulation is re-stored. After 6 hours or more, the muscle tissue will also be destroyed, and no regeneration can occur. The muscles will be replaced by scar tissue, and shortening of the connective tissue fibers may result in contracture, a permanent contraction of an entire muscle following the atrophy of individual muscle fibers.