Probably the most-restricted movement in modern humans' bodies is hip extension, that is taking the leg behind the body. The reason? The muscles at the front of the hip – the hip flexors – and the front of the thigh – quadriceps – are tight. In our experience, one of the muscles in this complex – rectus femoris, which crosses both the hip and knee joints – is the tightest/most restricted; we begin there.
In the absence of other causes, one tight hip flexor, or both hip flexors being tight, are the most common source of back pain in the modern population, partly because we spend most of our life sitting, and partly because tight hip flexors guarantee an anterior pelvic tilt (so increased lumbar tension and curve). Read more about hip flexors on the ST Back Pain Relief Wiki.