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Leg — Grokipedia Fact-checked by Grok 3 months ago Leg Ara Eve Leo Sal 1x The leg, in human anatomy, is the segment of the lower limb extending from the knee joint to the ankle joint , comprising the tibia and fibula bones along with their associated muscles, tendons, ligaments, nerves , and blood vessels that facilitate weight-bearing , propulsion, and balance during locomotion. [1] [2] This region, also known as the crural region, is distinct from the thigh (upper leg) and foot, and it plays a critical role in bipedal movement by transmitting forces from the body to the ground. [3] The skeletal framework of the leg consists of two parallel long bones: the robust tibia , which serves as the primary weight-bearing structure and articulates proximally with the femur at the knee and distally with the talus bone of the foot at the ankle, and the slender fibula , which lies laterally and primarily provides attachment sites for muscles while contributing to ankle stability through its distal articulation. [1] [2] Surrounding these bones are four fascial compartments—anterior, lateral, superficial posterior, and deep posterior—each containing specific muscle groups innervated by branches of the sciatic nerve (tibial and common fibular divisions) and supplied by the posterior tibial and fibular arteries. [4] The anterior compartment houses the tibialis anterior, extensor hallucis longus, extensor digitorum longus, and fibularis tertius muscles, which primarily function in dorsiflexion of the ankle and extension of the toes to prevent foot drop during walking. [4] In contrast, the posterior compartment's superficial layer includes the gastrocnemius, soleus, and plantaris muscles, responsible for plantarflexion of the foot to propel the body forward, while the deep layer features the tibialis posterior, flexor digitorum longus, and flexor hallucis longus, which invert the foot and flex the toes. [5] The lateral compartment contains the fibularis longus and brevis muscles, aiding in foot eversion and stability on uneven terrain. [6] Collectively, these structures ensure coordinated gait , shock absorption, and postural control, with clinical significance in conditions like compartment syndrome where increased pressure can impair blood flow and nerve function. [4] Overview and Terminology Definition The leg, or hindlimb , in vertebrates refers to the posterior paired appendage primarily adapted for supporting body weight and enabling locomotion, such as walking, running, or jumping . [7] This structure typically attaches to the pelvic girdle and extends distally to the foot, contrasting with the anterior forelimbs that connect to the pectoral girdle. [8] Legs are distinguished from forelimbs (or arms) by their position and specialized functions, despite sharing evolutionary homology through common developmental pathways and genetic controls that establish serial homology between anterior and posterior appendages. [9] In many vertebrates, including tetrapods, forelimbs have evolved for diverse roles like grasping, swimming, or aerial propulsion, whereas legs emphasize stability, propulsion, and load-bearing during terrestrial movement. [10] The basic classification of legs in tetrapods follows the pentadactyl limb plan as the ancestral model, featuring a single proximal bone (e.g., femur ), two parallel distal bones (e.g., tibia and fibula ), wrist- or ankle-like elements, and a five-digited foot that provides a versatile template for adaptation. [11] Key Terms In human anatomy, the lower limb is divided into proximal and distal regions for descriptive purposes. The proximal region, known as the thigh , extends from the hip joint to the knee and contains the femur as its primary bone . The distal region, known as the leg (or lower leg, shank, or crural region), spans from the knee to the ankle and includes the tibia and fibula . [12] The femur is the longest and strongest bone in the human body , forming the structural core of the thigh and articulating proximally with the pelvis at the hip joint and distally with the tibia and patella at the knee joint . [2] The tibia , commonly referred to as the shinbone, is the larger and weight-bearing bone of the lower leg, connecting proximally to the femur at the knee joint and distally to the talus at the ankle joint. [13] Parallel and lateral to the tibia , the fibula serves as a slender bone that primarily provides muscle attachment sites and contributes to the ankle joint's stability, though it bears little weight. [1] The patella , or kneecap, is a small sesamoid bone embedded in the tendon of the quadriceps femoris muscle, located anterior to the knee joint where it enhances leverage for knee extension. [14] Key joints include the hip joint, a ball-and-socket synovial joint formed by the femoral head and the acetabulum of the pelvis ; the knee joint , a hinge-type synovial joint involving the femur , tibia , and patella ; and the ankle joint, a hinge synovial joint between the tibia , fibula , and talus bone of the foot. [15] In medical contexts, "leg" specifically denotes the distal region (crus or shank) between the knee and ankle, emphasizing the tibia and fibula , whereas in broader zoological usage, it may describe the entire hindlimb or analogous structures in non-human species . [16] The term crus, derived from Latin for "leg," is used in anatomical nomenclature to precisely refer to the shank or lower leg portion. [17] Anatomy and Physiology Skeletal Components The skeletal framework of the human leg provides structural support, leverage for movement, and protection for underlying tissues. The leg, defined as the segment from the knee to the ankle (crural region), consists of two parallel long bones: the robust tibia and the slender fibula . [3] The tibia , also known as the shinbone, forms the medial, weight-bearing pillar of the leg. It articulates proximally with the femur at the knee joint via its tibial plateau and condyles, and distally with the talus bone of the foot at the ankle. The fibula lies laterally to the tibia , primarily providing attachment sites for muscles and contributing to ankle stability through its distal end, the lateral malleolus, which forms part of the ankle mortise. The proximal tibiofibular joint is a synovial plane joint that allows slight gliding, while the distal tibiofibular syndesmosis—a fibrous joint —binds the tibia and fibula to maintain alignment during weight transfer. These bones also serve as attachment sites for leg muscles, such as the tibialis anterior on the tibia and fibularis muscles on the fibula . [1] [3] Key joint articulations involving the leg include the knee and ankle. The knee joint functions primarily as a hinge ( ginglymus ) synovial joint between the distal femur , patella , and proximal tibia , permitting flexion and extension with limited rotation, reinforced by menisci and ligaments for shock absorption. At the ankle, the talocrural joint is a hinge-like synovial articulation between the distal tibia and fibula (forming the malleolar mortise) and the talus, enabling dorsiflexion and plantarflexion. [14] The development of the leg's skeletal components occurs through endochondral ossification , where mesenchymal precursors form cartilaginous models that progressively mineralize into bone . Primary ossification centers appear in the diaphysis (shaft) during fetal development—around the seventh week for the tibia and fibula —expanding via vascular invasion and osteoblast activity to replace cartilage with trabecular bone . Secondary ossification begins postnatally in the epiphyses (ends), leaving growth plates (epiphyseal plates) as zones of persistent hyaline cartilage that facilitate longitudinal growth through chondrocyte proliferation and hypertrophy . These plates, rich in type II collagen and proteoglycans, close via ossification in late adolescence —typically ages 14-16 for females and 16-18 for males—converting to epiphyseal lines and halting further elongation. Disruptions to this process, such as fractures through the growth plates, can lead to angular deformities if not managed promptly. [18] [19] [20] Muscular and Soft Tissue Elements The muscular system of the human leg comprises groups organized into four fascial compartments—anterior, lateral, superficial posterior, and deep posterior—that facilitate movements such as dorsiflexion, eversion, and plantarflexion. The anterior compartment includes the tibialis anterior, extensor hallucis longus, extensor digitorum longus, and fibularis tertius muscles, which primarily function in dorsiflexion of the ankle and extension of the toes. The lateral compartment contains the fibularis longus and brevis muscles, aiding in foot eversion and stability. The superficial posterior compartment features the gastrocnemius, soleus, and plantaris muscles, responsible for plantarflexion, while the deep posterior compartment includes the tibialis posterior, flexor digitorum longus, and flexor hallucis longus, which invert the foot and flex the toes. The gastrocnemius and soleus form the triceps surae complex, enabling propulsion at the ankle. [4] [5] [6] Ligaments in the leg provide passive stability to the knee and ankle joints by limiting excessive motion and resisting shear forces. The knee joint is reinforced by four primary ligaments: the anterior cruciate ligament (ACL), which prevents anterior tibial translation relative to the femur ; the posterior cruciate ligament (PCL), which restricts posterior tibial displacement; the medial collateral ligament (MCL), stabilizing against valgus stress on the medial side; and the lateral collateral ligament (LCL), countering varus stress laterally. These ligaments work in concert to maintain knee integrity during weight-bearing. The ankle is stabilized by the medial deltoid ligament and lateral ligaments (anterior talofibular, calcaneofibular, and posterior talofibular), which prevent excessive inversion and eversion. T