There are 2 reasonably unique muscles in the lower body- the hamstrings and calves (gastrocnemius).
What makes them unique is that these two muscles cross 2 major joints, which makes them hard to keep stretched out. When we sit, these 2 muscles tighten. Then, when we stand to walk, we are too tight to be comfortably upright. You have got to stay loose throughout the day. Stretch 3-5x each day.