There is a difference between lengthening a contracting muscle and stretching the same muscle. When a muscle stretches, it is lengthening. The bones that it attaches to are moving apart or away from each other. If the muscle is relaxing then you are stretching. If it is contracting, you aren’t stretching the muscle. When you raise your leg and then let it go down, you are creating an eccentric contraction of the muscle. In an eccentric contraction, the muscle is lengthening but it’s working hard, burning calories, using oxygen, getting tired and getting warm. In a stretch it isn't contracting, burning a lot of calories or using much oxygen. In fact, it cools off. For example, in a restorative practice, you start to cool off and need to put on a sweater because you aren't burning calories and creating heat.
So, when you’re want to stretch a muscle, you need to put it in a position where it can relax, let go and get longer. If not, the muscle can actually be contracting and then it’s not going to feel relaxed and released afterwards.
Julie Gudmestad says that if you sit a lot, the psoas is being left in a shortened position for hours every day. And, driving home you sit in the car, then sit at the dining table, and sit on the couch for TV. Then, if you sleep with your knees pulled up, the psoas is also in a shortened position with the thigh towards the torso for all those hours. If the psoas is left in a shortened position for hours and days and weeks and years, the entire structure becomes shorter, not just the contracting part of the muscle, but all the connective tissue fibers that wrap around it. Consequently, if you don’t take the psoas through its normal range of movement, you leave it in a shortened position and eventually it will lose its range of motion. The psoas will shorten and can stay in a ninety degree hip position. Then, when a person stands up, the psoas is going to try to stay in a shortened position. So, when you’re standing, the psoas is going to pull down and forward on the lumbar spine and the iliacus is going to tilt the pelvis forward. This puts more weight on the facet joints of the spine which sit between every two vertebra. Their job is to guide movement and not bear weight. This contributes to arthritis in and around these tiny joints. The lower back hurts and the abs become long and weak.