Spine Difference
Your spine curves, causing you difficulty and pain to straighten it directly upright, if at all. This gives you a −(IE) penalty on all Dexterity (Acrobatics) checks and Strength (Athletics) checks to jump. (Roll 1d12 for the direction of the curve, 12 being straight forward, then clockwise around.)
Real-world Examples
Scoliosis, Kyphosis, Lordosis
Assistive Options
A brace is helpful to prevent curvature from getting worse, although braces can be painful, and some choose to have their back assume whatever position it would naturally move into.