There are three muscular layers of the abdominal wall, with a fourth layer in the middle anterior region. The fourth layer in the midregion is the rectus abdominis, which has vertically running muscle fibres that flex the trunk and stabilize the pelvis. To either side of the rectus abdominis are the other three layers of abdominal muscles. The deepest of those layers is the transversus abdominis, which has fibres that run perpendicular to the rectus abdominus; the transversus abdominis acts to compress and support the abdomen and provides static core stabilization. The internal oblique layers run upward and forward from the sides of the abdomen, and the external oblique layers, which form the outermost muscle layers of the abdomen, run downward and forward. The internal oblique layers act in conjunction with the external oblique on the opposite side of the body to flex and rotate the trunk toward the side of the contracting internal oblique (“same-side rotator”).

The hip

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The hip joint is a complex weight-bearing ball-and-socket joint that can sustain considerable load. The socket of the joint is relatively deep, allowing for stability but sacrificing some degree in range of motion. The movements described in this section include flexion, extension, abduction, and adduction.

Hip flexion is the hip motion that brings the knee toward the chest. The major muscles of hip flexion include the iliopsoas, which is made up of the psoas major, psoas minor, and iliacus. Together, those muscles act mainly to flex the hip, but they also contribute to abdominal flexion and hip stabilization. Other hip flexors include the sartorius, the rectus femoris, the pectineus, and the gracilis. The sartorius also contributes to external hip rotation and knee extension and abduction, and the rectus femoris also acts in knee extension. The pectineus is also involved in hip adduction and internal rotation.

Hip extension is accomplished primarily by the muscles of the posterior thigh and buttocks, which when contracted serve to move the thigh from a flexed position toward the midline of the body or the trunk of the body from a bent position toward a more-erect posture. Hip extension is accomplished mostly by the gluteus maximus, the biceps femoris (which is divided into two heads, the long head and the short head), the semitendinosus, and the semimembranosus. A minor contribution is also provided by the adductor magnus and other small pelvic muscles.

The movement of adduction is used to describe a direction of limb motion that serves to take the limb from a lateral position to its more-axial alignment. During a jumping-jack exercise, for example, abduction of the leg occurs when it is moved away from the midline and adduction when it is moved back toward the midline. The main abductors of the hip are the gluteus medius, gluteus minimus, and tensor fascia lata. Those three muscles also serve to internally rotate the thigh in an extended position and externally rotate the thigh in the flexed position. Another minor contributor is the piriformis. The main hip adductors are the adductor magnus, the adductor brevis, and the adductor longus. A minor contribution to hip adduction is performed by the pectineus and the gracilis.

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The upper leg and knee

Extension of the knee is accomplished by a group of muscles collectively referred to as the quadriceps femoris, which increases the angle of the knee, bringing the lower leg into a straight position. Knee extension is used in the forward, swing phase of the gait and is integral in movements such as kicking. The quadriceps femoris group includes the vastus medius, vastus lateralis, vastus intermedius, and rectus femoris. A minor contribution to knee extension is provided by the sartorius.

Knee flexion refers to bending of the knee from the straight position. The muscles that perform that action oppose those of knee extension and are generally referred to as the hamstring muscles. The hamstring muscles are situated in the back of the thigh and include the biceps femoris, the semitendinosus, and the semimembranosus. Small contributions to knee flexion are made by the gastrocnemius muscle in the back of the calf and by several small muscles that cross the knee joint posteriorly.

The lower leg and foot

The muscles of the lower leg and foot are complex and work in many planes. Their actions depend on whether the person is bearing weight, as well as on the position of the foot. The following paragraphs provide a brief overview of the actions of the muscles of the lower leg and foot.

Dorsiflexion refers to ankle flexion in the direction of the dorsum, or anterior surface of the foot (the surface of the foot viewed from above). Dorsiflexion is accomplished by several muscles, including the tibialis anterior, which in addition to dorsiflexion also inverts the foot (tilts the foot toward the midline), stabilizes the foot when striking the ground, and locks the ankle when kicking. The extensor digitorum longus (EDL) also acts in dorsiflexion and functions to extend the last four toes. In addition to the EDL, some individuals also have a muscle called the peroneus tertius (fibularis tertius), which participates to a limited extent in dorsiflexion and eversion of the foot (tilting of the foot away from the midline). The extensor hallucis longus primarily acts in big toe (hallux) dorsiflexion, but it also acts to dorsiflex, as well as weakly invert, the ankle.

Plantarflexion refers to flexion of the ankle in the direction of the sole of the foot. That is most easily demonstrated by having a person stand on his or her toes. The majority of ankle plantarflexion is performed by the large calf musculature, including the gastrocnemius and the soleus, which lies just behind the gastrocnemius. It is generally accepted that those are two distinct muscles; however, there is some debate as to whether the gastrocnemius and the soleus are two parts of the same muscle.

Other muscles of the lower leg and foot include the plantaris, which runs obliquely between the gastrocnemius and the soleus; the flexor hallucis longus, which contributes to ankle flexion but is involved primarily in big toe flexion; the flexor digitorum longus, which also flexes the second to fifth toes; the peroneus longus, which flexes the ankle and everts the foot; and the peroneus brevis, which is involved in plantarflexion and eversion of the foot.

Intrinsic muscles of the foot arise in the foot and do not cross the ankle joint. Hence, their action is confined to the foot. The intrinsic muscles of the foot include the abductor hallucis, which abducts the big toe; the flexor digitorum brevis, which flexes the second to fifth toes; the abductor digiti minimi, which abducts and flexes the fifth toe; the quadratus plantae, which assists in toe flexion; the lumbricals, which flex the metatarsophalangeal (MTP) joints and extend the distal IP and proximal IP joints of the toes; the flexor hallucis brevis, which flexes the big toe; and the adductor hallucis, which flexes and contracts the big toe. The adductor hallucis has two heads, the oblique head and the transverse head, which share an insertion on the lateral (outer) side of the base of the proximal phalanx of the big toe. The oblique head arises from the base of the second to fourth metatarsal bones, and the transverse head arises from the ligaments of the MTP joints of the third to fifth toes. The flexor digiti minimi brevis extends and adducts the fifth toe. The dorsal interossei abduct the toes, and the plantar interossei adduct the toes.