Cross or do not cross your legs when sitting?
Circulation
When you sit, your legs fight gravity to keep blood flowing as it normally should. Crossing your legs makes it even more challenging for blood to circulate to different areas of the body, causing vein inflammation and potentially putting you at greater risk for a blood clot.
Back pain
Sitting cross-legged can increase lower back pain and lead towards a poor posture if you sit incorrectly (which most of us tend to do). If you choose to sit cross legged … avoid hunching your back while sitting and keep your spine in a neutral position. Distribute your weight on your hips instead of your feet.
As well as the potential vascular problems outlined above, crossing knee over knee shortens the muscles around the hip of the leg on top. Unequal muscle length around your hips can cause a wide variety of hammer effects on the mechanics of your hips, pelvis and spine and is a significant cause of back pain.

Hip strain
If you’re inflexible to start with, the muscle groups and joints responsible for supporting your body while you’re sitting like this might not have been used for a while so a cross-legged position will place strain on your hip flexors.
Effected muscles
The glutes; in particular, the gluteus medius—an often overlooked and under-trained muscle found on the outer surface of the pelvis: The glutes “deactivate” during prolonged sitting, especially when crossing your legs, this leads to instability in the pelvis.
Break the habit
If you habitually sit with one leg crossed over the other, especially if it’s the same leg position rather than alternating, you’re placing continuous pressure on your hip joint and sacroiliac joint. … If you experience hip or groin pain when crossing your legs mostly on one side, this symptom points to sciatica.
Researched by Shaun Spencer