Got this from an article:
Sitting alot causes your hip flexors to "shorten in a way" f you have short hip flexors and stand up, your hip flexors will pull on the femur on one end and the hip, as well as the lumbar spine (lower back). This will cause the hip to tilt forward and the lumbar curvature to increase (excessive lordosis).
So basically, anterior pelvic tilt potentially leads to lower back pain, hip pain (because of the rotated femurs), knee pain (because of the knock-knee position) and flat feet. Further up the body anterior pelvic tilt will get you a stronger curvature of the thoracic spine, because otherwise you’d be leaned backwards all the time. That overemphasized kyphosis will put you at risk for neck pain, shoulder pain, headaches and many other fun adventures.
Additionally, this can hurt your deadlift and squat numbers a lot. The good thing is that its reversible. It is also a lot more common than you think if you go to gym you will notice it if you pay attention to peoples posture. We live in a day and age where everyone sits ALOT so it has become pretty common.