What Happens to Your Hip Flexors When You Sit for Hours The hip flexors are a group of muscles located at the front of your hips. Their job is to lift your knees and bend your hips. When you sit for extended periods, these muscles stay in a shortened position. This constant contraction causes them to become tight and less flexible. Tight hip flexors pull on your pelvis, tilting it forward. This tilt affects the natural curve of your lower back, often increasing the arch and l