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Why sleeping with straight legs results in sore back?

I have a herniated L5-S1 for years. For the past few years, as long as I sleep on my sides I'm fine.

Starting a month or two ago, I now have to put a rolled-up yoga mat or something under my knees. Otherwise I wake up with a sore back.

Can someone explain why a straight or bent knee would affect my back?


  • Having a pillow under your legs takes pressure off your back, allowing it to flatten whereas without the pillow there tends to be a sway or curve in the lower back, keeping it tight.
    4 level ACDF C4-C7 5-2-11, laminectomy & discectomy L4-L5 1/26/12, ALIF L4-5, L5-S1 12/10/12.
  • Keeping legs straight while laying puts a lot of pressure on the lower back. My PT told me this in 2008. She suggested the same thing that you are doing- sleeping with a roll of something (pillow, towel, whatever) under the legs. I've done it since then and it really helps. I'm glad you found this out and can sleep well like that!
    Chronic pain since 2007. Have scoliosis. Had ACDF surgery for C5 Dec 2011. Sick of dealing with pain. I just turned 32 and struggle through but work full-time in IT.
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  • SpineyKD23SSpineyKD23 IllinoisPosts: 88
    I am recovering from surgery now and I have found that a foam wedge pillow (the ones designed to make you sleep upright for problems like acid reflux) works very well for supporting your legs. You lay on your back with the tallest/thickest part of the pillow under your knees and then the sloping part of the pillow allows your lower legs to relax and be angled down. You can scoot the pillow farther away from you if you want to flatten your back less or you can scoot the pillow all the way to your butt and your thighs will be at a 90 degree angle for maximum relief of pressure. Pretty nifty.
    25 years old: Herniated L4-L5, L5-S1 December 2008. L4-L5 microdiscectomy Sept 2010. L5-S1 microdiscectomy March 2012. Redo L4-L5 microdiscectomy Sept 2013. Redo microdiscectomy Oct 2015.
  • Most of my pain is in my neck & upper back, but when I started having pain in my lower back I changed my matteress, it was very firm went less firm and that took care of my lower back, For my neck I have a cervical memory foam pillow, that is great.
  • I sleep with a pillow between my knees and it makes an incredible difference to my pain levels in the morning. I learnt it from someone on this site - thank you whoever you were. Wish I had paid you $350 for 15 minutes instead of the specialist.
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  • a millennium queen pillow in large [tempur} for my head and a generic memory foam pillow in between my knees and I lay on my right side with knees slightly bent .I also have a dormeo octaspring mattress on an electrically adjustable base and with all that comfort I still only get two hours sleep the pain wakes me ..my wife slept in my bed when I was in hospital and she said my bed was the best bed by a mile it so comfy she slept for 11 hours and did not move all night she woke in the same position she went to sleep in ..bending at the knee take the stress of the lower back .
    1997 laminectomy
    2007 repeat laminectomy and discectomy L4/L5
    2011 ALIF {L4/L5/S1}
    2012 ? bowel problems .still under investigation
    2014 bladder operation may 19th 2014
  • I lay my stomach with a body pillow tucked down my right side, with my right leg over the body pillow, bent at a 90º angle at the hip and a 45º angle at the knee. Left leg straight. Another pillow under my head while I face toward the right. This gets pressure off most of my problems.

    A pillow or something under the knees essentially forces you to straighten your lower back (L3ish to S1)
    Without the support, your lower back stays curved all night, possibly resulting in your muscles tightening up.
  • The only thing that I can think of is that there will be less pressure. Otherwise I am not sure why that is.
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