The Psoas muscle connects the legs with the spine. There is one on each side, and each one has two portions: Psoas Major & Psoas Minor. It originates in the spine, from T12 to L5, which means from the last rib till the pelvis. The Psoas crosses the pelvis inside and attaches to the inner head of the femur bone. This muscle contracts every time we forward bend our hip joint, which means every time we seat, walk, run or climb stairs, we are activating the Psoas. Considering this information we could say that since we learnt to walk we have started to use this muscle and we use it so much that we have lost awareness of its existence. Fortunately in Yoga Asana practices, this muscle is re-awaken. It is also connected with another important muscle: the diaphragm, an involuntary muscle which regulates our breath and so, the nervous system. The Psoas is the leader which makes the body running when there is a life threatening situation, and at the same time, while it is contracting so we can run, it also affects the diaphragm which reduces its range of movement, so we breath faster... and it acts the same when we get stressed. We can see the Psoas is very much associated with our survival instinct. It also contracts when we outer rotate our thighs like in postures such as Baddha Konasana or Padmasana. This gives us another insight: this muscle stretches when we do internal rotation and extension of the hips. This is where Asana becomes so important. Most backbend require internal rotation and extension. As more inner rotation we do, as more extension of the Psoas, which combined with the activation of Moola & Uddiyana Bandha, enables the ribs to expand by expanding the diaphragm sideways instead of downward. The opposite is also truth, as more outer rotation les stretch of the Psoas and more pressure we put in our lumbar spine. We tend to outer rotate our legs during Asana , because it feels more comfortable. This is just a result of a shortening of the fibers of the Psoas, which happens due to our sedentary (sitting all day) lifestyle. So if you want to awake this muscle practice backbends with awareness, creating the movements that stretch this muscle. Let your Psoas be more than your survival instinct leader and allow it to become your force and support with flexibility, acting like a bamboo stick: strong but flexible.