Model Twisting when applying armature, human

Hey,
I am just wondering why my human model starts twisting and deforming when I use any type of parent to armature. I sort of rig, but I have never seen this happen and I want to know why and how to prevent it if possible
Thanks

Without a screenshot or a blend it’s hard to guess, but the most common reasons for “weird” mesh behavior are

  1. you have un-applied locations, rotations or scale on your mesh and/or armature
  2. different points of origin between your mesh and armature
  3. bones with messed up axes
  4. combinations of the above three

Good luck!