Sartorius:

external image sartorius200.jpg
Origin: Anterior superior iliac spine and the region just below it.
Insertion: Proximal tibia, medial to tibial tuberosity. Two joint muscle.
Action(s): Flexes, abducts, and laterally rotates thigh at the hip, flexes leg at the knee. This muscle is affective as a hip flexor or as a knee flexor. It is weak when both actions take place at the same time.
Strengthening: To strengthen this muscles you should perform exercises with resistance that go through various ranges of motion. Excersises involving strengthening through hip abduction or hip flexion, for example, will strengthen the Satorius. Stretching those same areas is also very important and will help strengthen.
http://www.livestrong.com/article/254019-exercises-to-strengthen-the-sartorius-muscle/
Interesting/Fun Fact: A single cell in the Sartorius can be as long as 12 Inches.