You may not know what or where your psoas muscles are, but I can assure you you’ll feel them stretching when you do a lunging pose or any full back bend. The psoas are part of the hip flexor group which can either lift your leg 
towards your torso (when the torso is fixed) or pull the torso toward the leg (when the leg is fixed). The psoas major actually starts from the lower back, goes through the abdomen and over the brim of the pelvis to the inner part of the upper thigh. It’s because of this attachment to the low spine that tight hips can cause low back pain.
Most of our day to day activities, such as sitting, shorten these muscles. That’s one of the reasons why back bending and lunging can be so intense, but it’s still very important to do so the sake of your lower back and hips. Trust me — nothing feels better after a long day of sitting in front a computer than sinking into a deep lunge.
Since the psoas is also a major postural muscle, it’s equally important to strengthen it. So with all that in mind here are a few great poses to stretch and strengthen the psoas muscles:
Stretch:
- Low/High Lunge
- Setu Bandha Sarvangasana (Bridge Pose)
- Virabhadrasana I (Warrior I Pose)
- Eka Pada Rajakapotasana (One Legged King Pigeon Pose)
Strengthen (Bonus! These poses will also work your abs!):
- Uttitha Hasta Padanguthasana (Extended Hand to Big Toe Pose)
- Navasana (Boat Pose)
- Ubhaya Padangusthasana (Both Big Toe Pose)
