The prime movers in hip flexion are the psoas, which originates on the lumbar vertebrae and inserts on the inner upper femur (thighbone), and the iliacus, which originates on the inner bowl of the pelvis and attaches on the upper femur. (These muscles are often grouped together and referred to as the iliopsoas, because they perform the same action and converge to attach to the thighbone via the same tendon.) The psoas and iliacus are assisted in hip flexion by several other muscles, most notably the rectus femoris. Part of the quadriceps, the large muscle group on the front of the thigh, the rectus femoris originates on the pelvis near the ASIS and attaches with the other three parts of the quadriceps just below the knee.
The hip flexors fold the thigh and torso closer together. They are in a shortened position when you're seated, and they will shrink and tighten if they're never stretched out to counteract long hours of daily sitting. If your hip flexors have become short and tight, your iliopsoas and rectus femoris will continue to pull forward and down on the front pelvis and lumbar (lower) spine even when you are standing. This pull creates an anterior pelvic tilt, which in turn contributes to an increased curve in the lower back.
If the hip flexors and extensors are balanced in terms of both strength and flexibility, they will support the pelvis in a neutral position, which helps maintain the normal spinal curves and keeps the weight of the upper body centered over the hips when you sit and over the legs when you're on your feet. If the hamstrings are comparatively short and the hip flexors comparatively long, the pelvis will be pulled into a posterior tilt and the lower back curve will be flattened, which can contribute to back strain and, more seriously, disk injuries. On the other hand, if the hip flexors are comparatively short and the hamstrings comparatively long, the anterior tilt contributes to an overarched, compressed lower back. This compression can cause not only short-term discomfort but also a wearing down of the cartilage of the facet joints along the back of the lumbar vertebrae, contributing to arthritis of the lower spine.
Undoing Bad Habits
Any imbalance between your hip flexors and hip extensors can negatively affect your yoga poses in many ways. Fortunately, conscious practice of pelvic and lumbar alignment during your practice can improve the balance between your hip flexors and hip extensors.
Tadasana is a good pose for increasing your pelvic awareness and balance. If you have tight hamstrings, a posterior pelvic tilt, and a tendency toward a flat lower back, you need to release any gripping in the hamstrings and buttocks in Tadasana; this will allow your tailbone to lift up and move back slightly. Moving your inner upper thighs backward will also help release the hip extensors' downward pull on the sitting bones. Also soften any gripping in your abdominal muscles and allow your breath to move gently in the belly. All of these actions and releases will help reestablish the normal lumbar curve.