A phrase we like to us in the rehab world is “sitting is the new smoking.” A lot of information is coming out on the detriments of sitting, both with internal organs and our musculoskeletal system (joints and muscles).
In an ideal world, everyone should be standing or walking most of the day, rarely sitting. Unfortunately, for many people this is not possible. Many people have jobs that require them to drive a lot of the day, most schools and offices have desks in which they need to sit, or they travel a lot and need to be seated on airplanes.
After you have been sitting for long periods, it is a great idea to stretch your hips, especially the hip flexors (muscles on the front of the hip) and external rotators (muscles in the gluteal region, aka buttocks).