Sitting seems pretty harmless but you may not realize what hours and hours of sitting every day does to your body. There is one specific body part that gets strongly affected. It's called the hip flexor (the psoas for you more medical folks). This muscle and tendon structure connects your upper leg to your lower back. It's job is to allow you to flex or raise your leg up towards your chest. When you are standing, this muscle relaxes to allow it to stretch and lengthen in order for you to stand up straight. When you sit, it is flexed and shortened. What happens when you sit too much? That muscle sort of gets used to staying short. It gets so used to it that it doesn't want to stretch out when you stand up. So, when you stand up, it literally pulls your lower back and hip bones forward to tilt and make up for the loss of flexibility. Your low back ends up arching more and your stomach stick out a little more. This leads to low back pain and tight hips. All this because we sit too much!
So how can we fix it? Get up and get moving! Try not to sit too long and take stand and stretch breaks during work. Go on a walk at lunch time. Hit the gym several times per week to work on strength and flexibility. Do yoga stretches. All these things will help to keep you flexible, strong, and free from low back pain and tight hips.