IN SCIATICA, WHY DOES THE TIGHTNESS TENDS TO MOVE DOWN THE LEG?

That’s because the sciatic nerve is the longest in the human body running from your lower spine through the buttock and down the back of the leg. The tightness tends to move along the sciatic nerve and at times the pain can be felt all the way to the toes. https://www.nhs.uk/conditions/sciatica/