Muscles of the Hip and Lower Limbs

Unlike the upper limbs, the hip and femur require muscles to provide stability and strength rather than agility and precision, for walking, running, standing, and maintaining balance. The hip's os coxae is the point of origin for most muscles acting on and inserting at the femur. Major muscles on the anterior hip are the iliacus in the pelvis and the psoas major, which originates primarily at the lumbar vertebrae. Together they form the iliopsoas, which inserts by a single tendon at the femur. The iliopsoas flexes the hip joint, an action that raises the thigh, moves the leg forward, and permits bending at the waist. Four muscles are prominent on the hip's lateral and posterior sides. These include the tensor fasciae latae and three gluteal muscles—gluteus maximus, gluteus medius, and gluteus minimus. The most significant of these is the gluteus maximus, which largely makes up the buttocks. An extensor of the hip joint, the gluteus maximus rotates the femur and provides lift in climbing motions.
Six lateral rotator muscles lie deep in the pelvic region: the gemellus superior and inferior, the obturator externus and internus, the piriformis, and the quadratus femoris. Together, these muscles abduct (move outward), adduct (move inward), and laterally rotate the femur. Abductors enable walking by allowing shifts in body weight each time a foot is lifted. Thigh muscles are enclosed and bound by a connective tissue sheath called the fascia lata. Tendons of the gluteus maximus and the tensor fasciae latae muscles join the fascia lata to form the iliotibial band on the lateral side of the hip. The iliotibial band can tighten, which braces the knee. The fascia lata also divides thigh muscles into three compartments: anterior, or extensor; medial, or adductor; and posterior, or flexor. Five muscles in the medial compartment—adductor longus, adductor brevis, adductor magnus, gracilis, and pectineus—act on the hip joint by adducting the thigh.

Some muscles that act on the femur cross both the hip and knee joints to act on the leg. Other muscles are recognized as more exclusively affecting knee motions. The body's most powerful muscle, the quadriceps femoris, is a tensor muscle that plays the primary role in extending the knee. A tensor muscle is a muscle that stretches or tightens a part of the body. In the knee, this mechanism of action is critical to running, kicking, and standing up. Located in the thigh's anterior or extensor compartment, the quadriceps femoris has four heads: the rectus femoris, the vastus lateralis, the vastus medialis, and the vastus intermedius. These meet and connect to the knee's patella through the quadriceps (patellar) tendon. The connection continues as the patellar ligament, which inserts on the tibial tuberosity. The patellar ligament is the structure physicians tap with a reflex hammer in order to test the knee-jerk reflex. The sartorius muscle, also in the thigh's anterior compartment, is the body's longest. The strap-like muscle crosses over the quadriceps, arranged between the hip's lateral side and knee's medial side. The sartorius enables crossing the legs by laterally rotating the thigh and flexing the hip and knee.

The group of muscles known as the hamstrings are contained in the thigh's posterior or flexor compartment. These three muscles—the biceps femoris, the semimembranosus, and the semitendinosus—flex the knee. Together with the gluteus maximus, they also contribute to walking and running by extending the hip. All the hamstrings originate at the ischial tuberosity; a second head of the biceps femoris originates at the femur's posterior midshaft. The biceps femoris inserts at the head of the fibula, the semimembranosus at the tibia's medial condyle, and the semitendinosus near the tibial tuberosity. Injuries to the hamstring are common in many athletes. The femoral nerve innervates muscles acting on the knee that are situated in the thigh's anterior compartment. Hamstring muscles, which are in the posterior compartment, are innervated primarily by the tibial nerve.

Connective tissue fasciae divide the muscles that make up the fleshy region below the knee into three compartments: anterior, posterior, and lateral. Called the crural muscles, these muscles act on the foot. Four anterior compartment muscles dorsiflex (bend upward) and evert (turn outward) the foot: the extensor digitorum longus, the extensor hallucis longus, the fibularis tertius (also known as the peroneus tertius), and the tibialis anterior. These also raise the toes during walking. The posterior compartment of the leg further divides into superficial and deep muscles. The three superficial group muscles—gastrocnemius, soleus, and plantaris—plantar flex the foot (point it downward with toes below the ankle). The gastrocnemius and soleus together make up the triceps surae, which inserts on the calcaneus or heel bone by the calcaneal or Achilles tendon. Three of four deep posterior muscles are also plantar flexors: the flexor digitorum longus, the flexor hallucis longus, and the tibialis posterior. Plantar flexion provides lift and forward thrust during walking and enables standing on tiptoe. The fourth deep muscle, the popliteus, medially rotates the tibia and unlocks the knee to allow it to flex. The lateral compartment below the knee includes two muscles, the fibularis brevis and the fibularis longus. Each originates at the fibula. They act as plantar flexors and evert the foot. The deep peroneal nerve innervates the anterior compartment muscles, the tibial nerve innervates the posterior compartment muscles, and the superficial peroneal nerve innervates the lateral compartment muscles.
Intrinsic muscles of the foot are those not arising elsewhere. These layered muscles aid locomotion by collectively extending, flexing, abducting, and adducting the toes and supporting the arch of the foot. They bear similarities in name and action to muscles of the hand. The extensor digitorum brevis, which extends the toes, is the only intrinsic foot muscle on the foot's dorsal (upper) side. The remaining muscles are found on the foot's ventral side (bottom) or between the metatarsal bones. These are categorized into four layers, from top to bottom. The first, or most superficial, layer includes the flexor digitorum brevis, which is connected by tendons to all the digits except the hallux (large toe). It is flanked by muscles connected by tendons to the little and large toe, respectively—the abductor digiti minimi and the abductor hallucis. The quadratus plantae muscle, which flexes the toes, and the four lumbrical muscles make up the second layer. The lumbricals flex metatarsophalangeal joints and extend interphalangeal joints. Deeper still, the third layer includes the adductor hallucis, which adducts the hallux or big toe, the flexor digiti minimi brevis, and the flexor hallucis brevis. The latter two muscles flex the little toe and big toe, respectively. The plantar aponeurosis, a broad, fan-shaped tissue between the plantar skin and muscles, is the point of origin for many muscles in the first three layers. The fourth and deepest layer comprises seven muscles. Four dorsal interosseous muscles abduct toes 2 to 5, and three plantar interosseous muscles adduct toes 3 to 5. Innervation of the foot's intrinsic muscles is achieved primarily by the medial and lateral plantar nerves.