A large majority of the population sits for eight hours a day, and with that comes tension in many parts of the body. Some of the most stressed parts of the body reside in the hip flexor muscles, which actually shrink when they are not stretched for a long period of time. This is why even if you do work at a desk I always recommend taking frequent breaks to stretch!

Your hip flexor muscles are responsible for flexing the hip; lifting the thigh up to the torso. They are comprised of two main muscles: the Psoas and the Iliacus. The Iliacus is nestled right near your hip while the Psoas curves upwards near the spine. (See picture below).

When we sit for too long, these two muscles have a tendency to shrink and tighten up, causing us to lean forward a little. This constant shortening of the muscles can cause major pain and irritation in our lower back, which can eventually stunt your ability to walk upright.

Luckily there are many easy ways to stretch out and loosen up hip flexor muscles. You can use a foam roller, practice yoga, get daily exercise, or do some simple hip stretches at home. One easy way to stretch out your hip flexors is to take one step forward, as if you are going to prepare for a lunge. Next lower yourself down into a lunge position and slide out that back leg until it straightens. This is one of the most common types of hip flexor stretches, and will give you a nice, deep stretch in the front part of your hip. Give it a try at least once every hour or so during your work day. Trust me when I say your body will thank you. Back pain is the worst!

References:
http://www.abc-of-fitness.com/leg-stretch/hip-flexor-stretch.asp