If you cross your legs while sitting, you will cause your pelvis to shift and you will not be sitting equally on your gluteal bones. The end result of this posture is the shortening and development of trigger points in the muscle located on either side of the spine. These muscles, (QL) when contracted, will tilt (laterally flex) your spine to one side and extend your back. One or both sides may be affected depending on the posture held. These muscles will tire easily when sitting slouched over a desk similar to the erector spinae muscle group.
To stretch the QL:
Lie on your back with both legs straight to start. Take your right arm and with palm facing the floor extend it to your right side. Take your left hand and with right knee bent, pull the right knee towards the floor on the left side. Hold the stretch for 30 seconds and repeat 3 times. Perform stretch on both sides. During any stretch you should feel mild discomfort, if at any time you feel pain, do not pull the knee as far over. If this does not relieve the symptoms of pain, discontinue the stretch.