Today I sat with my legs spread on the ground and realized that I couldn't keep my lower back straight (perpendicular to the ground) without feeling significant strain in my thighs.

I realized this has been affecting my posture. I feel a strain to keep my lower back upright even when I sit down, evidently the result of spending too many years slouched on a chair.

How do I cure this inflexibility?

Happy New Year you basement dwelling, woman hating virgin neck beards.