Our work with elite athletes serves as the proving grounds for our methods. Most people don’t play professional sports. But if our methods help athletes at the highest levels, they can work for anyone.
Speaking generally, in sitting your hip flexors (iliacus, psoas, TFL) are shortened and your glutes are lengthened.
That doesn’t paint a full picture. Some of it will depend on how you sit, too. For example, if you sit with your knees splayed out wide you’ll probably find that your hip external rotators (glute med, etc.) become short and stiff, which could go hand in hand with your internal rotation deficit.
According to this article by Donald Neumann (2010), hip flexion is when the muscles that generate hip internal rotation work their best. These would be the anterior fibres of the glute med and glute min and the TFL.