My PsoAs, My AsPso

Your psoas stabilizes the lower spine when sitting. Hence when you try to stand, you have a tight back. It also flexes and rotates the lumbar spine. 

When this muscle (psoas) is shortened/contracted for hours a day due being seated, can cause a stiff lower back as well as a very tight gluteus Maximus because now the glutes, (a powerful hip stabilizer) is working to extend the hip which in turn pulls on the lower back as well as the hips, legs, pelvis (essentially all the same thing when you think about it).

This can lead to pain in the knees because the hamstrings stabilize the knee. The ankle and feet can also be affected do to altered biomechanics. You see, everything is connected