Rectus abdominis muscle

Lore & Background

The rectus abdominis is a long, flat muscle contained in the rectus sheath, formed by aponeuroses of the lateral abdominal muscles. It is traversed by tendinous intersections that subdivide it into distinct muscle bellies, and tensing causes expansion between these intersections. The muscle has multiple blood supplies: the inferior epigastric artery and vein serve the lower part, the superior epigastric artery supplies the upper portion, and the lower six intercostal arteries provide segmental contributions. Innervation comes from thoraco-abdominal nerves (T7–T11 intercostal nerves), with sensory supply from the 7–12 thoracic nerves.

Reader's Guide

The rectus abdominis is an important postural muscle that flexes the lumbar spine, bringing the rib cage toward the pelvis or vice versa, and assists in forceful exhalation and creating intra-abdominal pressure during activities like heavy lifting, defecation, or childbirth. Clinically, it can be affected by abdominal muscle strain (pulled muscle) or rectus sheath hematoma, an accumulation of blood often caused by rupture of the epigastric artery or muscular tear, with causes including anticoagulation, coughing, pregnancy, abdominal surgery, and trauma. The muscle is similar in most vertebrates, though the number of tendinous intersections varies in animals. Its role in core stability and common injuries makes it a key focus in sports medicine and general health.

Did You Know?

• The rectus abdominis is separated at the midline by a band of dense connective tissue called the linea alba.
• It is typically around 10 mm thick, but some athletes can have a rectus up to 20 mm thick.
• The muscle is innervated by thoraco-abdominal nerves, continuations of the T7–T11 intercostal nerves.
• A rectus sheath hematoma may be caused by rupture of the epigastric artery or by a muscular tear.

Regional Anatomy and Terminology

The human lower limb is a complex structure comprising thirty bones in each leg, with the femur, tibia, and fibula serving as the principal long bones. Anatomically, the limb is divided into distinct regions: the thigh spans from the hip to the knee, while the segment between the knee and the ankle is designated the lower leg, shank, or crus. Within the lower leg, the anterior surface is called the shin (formed by the tibia and the smaller fibula), and the posterior surface is the calf. The patella, the body's largest sesamoid bone, sits in front of the knee joint. It is worth noting that anatomists reserve the word "leg" exclusively for the crus region, whereas the broader term "lower limb" or "lower extremity" encompasses the entire structure from hip to foot. Most of the skeletal framework presents palpable bony landmarks, though the hip joint and the femoral shaft remain hidden beneath soft tissue.

Bipedal Adaptation and Proportional Evolution

Evolution shaped the human lower limb into a mechanism finely tuned for efficient upright walking, a specialization that distinguishes us from other primates. While chimpanzees and orangutans can briefly adopt a vertical posture, they expend considerable energy doing so, whereas humans walk upright with remarkable economy. This adaptation reshaped the entire body: the center of gravity shifted, internal organs were reorganized, and the trunk's biomechanics were restructured. The double S-curve of the vertebral column now functions as a shock absorber, channeling trunk weight down through the feet. Leg length relative to the trunk tells a striking story of evolutionary divergence: in orangutans the ratio is 111 percent, in chimpanzees 128 percent, and in humans a full 171 percent. Muscles such as the gluteals, knee extensors, and calf musculature were all remodeled for this task. Sex-based differences also exist, with females tending toward greater hip anteversion and tibiofemoral angles, while males typically possess longer femoral and tibial segments.

Skeletal Alignment and the Mechanical Axis

In a normally aligned leg, the major joints form a straight mechanical line known as the Mikulicz line, running from the femoral head through the intercondylar eminence of the tibia to the center of the ankle mortise. In the tibial shaft the mechanical and anatomical axes coincide, but in the femoral shaft they diverge by approximately six degrees, producing a femorotibial angle of 174 degrees. A leg is considered straight when, with the feet together, both the medial malleoli and the medial condyles of the knees make contact. Deviations from this norm create clinical conditions: genu varum occurs when the knee center falls lateral to the axis (intermalleolar distance exceeding three centimeters), while genu valgum arises when it falls medial (intercondylar distance exceeding five centimeters). Both impose uneven joint loading. The collodiaphysial angle between the femoral neck and shaft also shifts with age, declining from roughly 150 degrees at birth to 120 degrees in old age, with pathological extremes producing coxa vara or coxa valga.

Hip Musculature and Multi-Axis Joint Mechanics

The muscles surrounding the hip joint can be organized in several ways: by their innervation source (ventral versus dorsal plexus divisions), by their developmental insertion pattern (a two-layered posterior group and an anterior group), or by their functional role as extensors, flexors, adductors, or abductors. Because many of these muscles span extensive origin and insertion areas, a single muscle often participates in several distinct movements, and some even influence the knee or vertebral joints. The hip itself permits motion along multiple axes: lateral and medial rotation occur along the limb's longitudinal axis, flexion and extension swing along a transverse axis, and abduction with adduction rotate about a sagittal axis. A key example is the iliopsoas complex, a group of two or three muscles sharing a common insertion on the lesser trochanter of the femur, with the psoas major originating from the lowest thoracic vertebra and running along the lumbar spine before descending into the pelvis.

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