The lower limb (lower extremity) is the anatomical region extending from the hip joint to the toes, consisting of four main segments: the thigh (femur), the leg (tibia and fibula), the foot (tarsals, metatarsals, phalanges), and the kneecap (patella, at the junction between thigh and leg). The lower limb is specialised for weight-bearing and locomotion (walking, running, jumping), and its bones are correspondingly more robust and less mobile than those of the upper limb. The lower limb is connected to the axial skeleton through the pelvic girdle, which consists of two hip bones (os coxae, each formed by the fusion of ilium, ischium, and pubis) joining the sacrum posteriorly. The total number of bones in one complete lower limb including the hip bone is approximately 31 (or 30 if counted from the thigh down). The lower limb's characteristic bone arrangement — single thigh bone (femur) → kneecap (patella) → paired leg bones (tibia and fibula) → ankle (tarsals) → foot (metatarsals) → toes (phalanges) — provides a stable, weight-bearing column that can absorb the substantial forces generated during locomotion.

The femur is the single bone of the thigh, connecting the hip joint above to the knee joint below, and is the longest, heaviest, and strongest bone in the human body. In a person of average height, the femur is approximately 45-50 cm long and constitutes roughly a quarter of total body height. At its proximal end, the rounded femoral head articulates with the cup-shaped acetabulum of the hip bone to form the ball-and-socket hip joint, which allows movement in multiple planes (flexion, extension, abduction, adduction, rotation). The femoral neck connects the head to the shaft at an angle and is a common fracture site in elderly people with osteoporosis (a "broken hip" usually refers to a femoral neck fracture). The distal femur broadens into two condyles (medial and lateral) that articulate with the tibia below to form the knee joint, the largest joint in the human body. The patellar surface between the condyles is where the patella (kneecap) slides during knee flexion and extension.

The patella is the largest sesamoid bone in the human body — a sesamoid bone being a bone that develops within a tendon rather than from typical embryonic cartilage like most bones. The patella develops embryologically within the quadriceps tendon (the tendon of the large four-headed quadriceps femoris muscle of the anterior thigh) and serves two primary mechanical functions: protecting the knee joint anteriorly from direct trauma (it serves as a natural shield over the anterior surface of the knee), and improving the mechanical efficiency of the quadriceps muscle by increasing the perpendicular distance from the line of force of the quadriceps tendon to the axis of knee rotation (the moment arm), effectively amplifying the extension force the quadriceps can generate at the knee by approximately 30-50%. The inferior patellar tendon (also called patellar ligament) connects the patella to the tibial tuberosity on the front of the tibia, transmitting the pulling force of the quadriceps to extend the lower leg. The patellar reflex (knee-jerk reflex) — tapping the patellar tendon causing involuntary knee extension — is a fundamental neurological examination test for the integrity of the L3-L4 spinal nerve segments.

The leg (anatomically defined as the segment between knee and ankle) contains two bones: the tibia (medially placed, larger, weight-bearing) and the fibula (laterally placed, slender, primarily non-weight-bearing). The tibia is the second-longest bone in the body after the femur and bears virtually all the body's weight transmitted from the femur — it articulates superiorly with the femoral condyles at the knee joint, and inferiorly with the talus at the ankle joint. The tibial tuberosity (a roughened prominence on the anterior upper tibia) serves as the attachment point for the patellar tendon, and its subcutaneous anterior edge (the "shin") is vulnerable to direct trauma. The fibula runs parallel to the tibia on its lateral side, articulating with the tibia at both its upper and lower ends, and participates in the ankle joint with its lateral malleolus. Although the fibula bears very little weight, it is important for stabilising the ankle joint (the lateral malleolus forms the outer ankle prominence) and provides attachment for numerous leg muscles. The fibula is commonly fractured in ankle injuries (lateral malleolus fracture).

The tarsals are a group of seven short, irregular bones forming the proximal portion of the foot (the ankle and heel region): the calcaneus (heel bone, largest tarsal, forms the heel of the foot and provides attachment for the Achilles tendon), the talus (the only tarsal that articulates with the leg bones — its trochlea surface articulates with the tibia and fibula to form the true ankle joint allowing dorsiflexion and plantarflexion), the navicular (boat-shaped bone on the medial midfoot), the cuboid (lateral midfoot, articulates with calcaneus behind and 4th and 5th metatarsals in front), and three cuneiforms — medial, intermediate, and lateral — which articulate with the first three metatarsals anteriorly. The arrangement of tarsal bones forms the medial and lateral longitudinal arches and the transverse arch of the foot, which function like springs to absorb shock during walking and running and to store and release elastic energy, reducing the metabolic energy cost of locomotion compared to a flat, rigid foot. Flat feet (pes planus) represent a failure of the medial longitudinal arch and can cause pain and biomechanical problems with prolonged standing or walking.