The abdominals do much more than bend and twist the torso. They hold us up when we stand and walk.
When the pelvis moves out of alignment this function of the side bending abdominal muscles (internal obliques and deep abdominal wall) becomes weak. This causes the hips to collapse and shift too far to one or both sides (like a waddle).
When the hips shift to the side too much outer hip strain/pain (think IT or iliotibial band), inner knee strain/pain and an increased risk for ankle sprains.
Remember to strengthen this area as a part of your abdominal function, pelvis support and overall health.
The Abdominal Series