The sartorius muscle is the longest muscle in the human body, running from the hip to the knee. The technical origin of the muscle is the anterior superior iliac spine -- the top and front of the hip bone -- and the insertion is the medial aspect of the proximal end of the tibia at the inside bottom of the knee . The sartorius muscle flexes the hip and knee, rotates the hip externally and abducts the hip.
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Flexion of the Hip
Hip flexion occurs when the upper body is lowered or when the leg is raised, which can happen by bending at the waist or lifting the leg with the knee bent. In addition to the sartorius, other muscles that work to flex the hips are th iliacus, psoas, rectus femoris and pectineus.
Flexion of the Knee
Knee flexion is when the knee bends and lower leg is moved toward the thigh . Flexion of the knee happens with the sartorius and six other muscles: the semitendinosus, semimembranosus, biceps femoris, gracilis, popliteus and gastrocnemius.
External Rotation of the Hip
External or lateral rotation of the hip occurs when the thigh and/or pelvis are rotated outward, away from the body. The gluteus maximus, gluteus medius, piriformis, quadratus femoris, obturator internal and external, and gemellus muscles all work with the sartorius to externally rotate the hip.
Abduction of the Hip
Hip abduction is when the leg is kept straight and moved away from the body. The gluteus maximus, gluteus medius, and tensor fasciae latae muscles activate with the sartorius to perform this motion.
Sartorius in Action
The sartorius muscle is biarticulate, meaning it crosses two joints. In this case the muscles crosses the hip and knee joints. The motion of crossing the legs takes the sartorius through all of its actions, and it remains active and engaged in that position. The sartorius muscle derives its name from the Latin word for tailor, which is sartor. Tailors were known to sit cross-legged on the floor while they worked, emulating the the shape of the muscle as well as using one of its functions.