Specifically, the gluteus maximus, a big ass (pun intended) muscle and one of the main hip extensors. (The other muscle group involved in hip extension is the hamstrings, but their location and action is distinct enough to warrant them getting their own segment.) Hip extension is basically the movement of drawing the thigh backward behind vertical and/or a neutral pelvis alignment.
While the glutes function as hip extensors more during running and jumping more than they do during walking, they are well and truly all over the hips — which is a claim the hamstrings cannot make and why I am going about things ass first. (Also, for the butt jokes. Can you blame me?) Of the three gluteal muscles, the gluteus maximus is the closest to the skin’s surface. It has its origins on the sacrum, the top of the iliac crest (that curvy bone at the top of the pelvis), and the fascia (connective tissue) near the lumbar spine and the gluteus medius. On the lower side, parts of it attach to the femur and to the IT band.
Hip extension is the complementary action to hip flexion. Since there’s a tendency for contemporary people to have hip flexors on the tighter side, it can help to have strong hip extensors in order to counterbalance the pull on the pelvis. Weak glutes can result in some instability in the lumbar spine, which can then sometimes lead to back pain. Similarly, an inflexible gluteus maximus can increase the risk of lower back injury during some activities.
So. That’s the hip flexor most directly attached to the hips themselves. That’s also the top layer of the butt. I’ll be getting to the other hip flexors later as well as some of the deeper butt muscles. But up next with hip yoga: gentle ass stretches.