Sitting tightens up your hip flexers because your hips are flaxed. A muscle is tight when it's compressed like a bicep. The further you pull your knee to your chest, the more of a 'hip flexer' you get. When people stand with tight hip flexers, the hip flexers reverse the roll when they stand. So when they stand, based on the orientation, when they contract, they increase the arch in your low back h It is a complex muscle.