Apply the above concept to your hips. When you sit, your hips are in a "flexed" position. Therefore, the muscles that flex your hips are in a shortened state. You probably spend at least a third of your day sitting down. Think about how much time those hip flexor muscles stay shortened. A lot. Over time, they become tighter and tighter until you look like the old man in the picture. So unless you want to look like that, perform the stretches shown below.
The sartorius originates at the ASIS and proceeds to traverse obliquely and laterally down the thigh to eventually insert at the anterior surface of the tibia, just inferomedial to the tibial tuberosity, as part of the pes anserinus. In addition to flexing the hip and knee, the sartorius aids in the abduction of the hip. It is innervated by the femoral nerve (i.e., the posterior division of L2 and L3).
Of course, you know what it feels like to have a tight muscle. But tight hips aren't just uncomfortable—they can lead to all sorts of other aches and pains, especially in the lower back. "People focus on the hips and say their hips are tight, but we don't always think about the fact that the lower back connects to our legs at the hip," Charlee Atkins, C.S.C.S., instructor at Soul Annex in New York City and creator of Le Stretch class, tells SELF. Tight hip flexors make it harder for your pelvis to rotate properly, which can cause your lower back to overcompensate, "and this can be a setup for lower-back injury," Teo Mendez, M.D., an orthopedic surgeon at NY Orthopedics who focuses on operative and non-operative management of sports-related injuries, musculoskeletal injuries, and arthritis, tells SELF.