Tight hip flexors can result in lower back pain, hip pain and injury.  A lot of strain is put on those muscles during activities that involve sprinting and kicking.  For example, runners are more prone to hip flexor injuries because of the small, repetitive movement during running.   But even if you’re not an athlete, hip flexor injuries can occur during everyday activities (for instance, slipping and falling or running to catch a bus).  When those tight muscles are suddenly stretched beyond what they are accustomed to, you might also experience pain in the upper groin region, typically where the hip meets the pelvis.   
You can strain or tear one or more of your hip flexors when you make sudden movements such as changing directions while running or kicking. Sports and athletic activities where this is likely to occur include running, football, soccer, martial arts, dancing, and hockey. In everyday life, you can strain a hip flexor when you slip and fall, for example.
The lower back where most back pain occurs includes the five vertebrae (referred to as L1-L5) in the lumbar region, which supports much of the weight of the upper body. The spaces between the vertebrae are maintained by round, rubbery pads called intervertebral discs that act like shock absorbers throughout the spinal column to cushion the bones as the body moves. Bands of tissue known as ligaments hold the vertebrae in place, and tendons attach the muscles to the spinal column. Thirty-one pairs of nerves are rooted to the spinal cord and they control body movements and transmit signals from the body to the brain.
Transcutaneous electrical nerve stimulation (TENS) involves wearing a battery-powered device consisting of electrodes placed on the skin over the painful area that generate electrical impulses designed to block incoming pain signals from the peripheral nerves. The theory is that stimulating the nervous system can modify the perception of pain. Early studies of TENS suggested that it elevated levels of endorphins, the body’s natural pain-numbing chemicals. More recent studies, however, have produced mixed results on its effectiveness for providing relief from low back pain.
There is a significant overlap of nerve supply to many of the discs, muscles, ligaments, and other spinal structures, and it can be difficult for the brain to accurately sense which is the cause of the pain. For example, a degenerated or torn lumbar disc can feel the same as a pulled muscle – both creating inflammation and painful muscle spasm in the same area. Muscles and ligaments heal rapidly, while a torn disc may or may not. The time course of pain helps determine the cause.
Degenerative disc disease. At birth, intervertebral discs are full of water and at their healthiest. As people age over time, discs lose hydration and wear down. As the disc loses hydration, it cannot resist forces as well, and transfers force to the disc wall that may develop tears and cause pain or weakening that can lead to a herniation. The disc can also collapse and contribute to stenosis.
Women may have acute low back pain from medical conditions affecting the female reproductive system, including endometriosis, ovarian cysts, ovarian cancer, or uterine fibroids.[28] Nearly half of all pregnant women report pain in the lower back or sacral area during pregnancy, due to changes in their posture and center of gravity causing muscle and ligament strain.[29]
Antidepressants may be effective for treating chronic pain associated with symptoms of depression, but they have a risk of side effects.[13] Although the antiseizure drugs gabapentin, pregabalin, and topiramate are sometimes used for chronic low back pain evidence does not support a benefit.[79] Systemic oral steroids have not been shown to be useful in low back pain.[1][13] Facet joint injections and steroid injections into the discs have not been found to be effective in those with persistent, non-radiating pain; however, they may be considered for those with persistent sciatic pain.[80] Epidural corticosteroid injections provide a slight and questionable short-term improvement in those with sciatica but are of no long term benefit.[81] There are also concerns of potential side effects.[82]
Sleeping in an awkward position can cause you to be in pain from the moment you wake up. The best sleeping position for lower back pain may be sleeping on your side with your knees drawn up close to your chest (also known as the fetal position). Placing a pillow or two between your legs, while sleeping on your side, helps to reduce stress on your lower back. Sleeping on a too soft mattress can also cause lower back pain. A firmer mattress is best.

Hamstring squeeze. Use the machine that works your hamstrings; you will either lie on your stomach or sit with a pad behind your knee. Push against the pad, moving your knee up toward the ceiling or backward (depending on which position you’re in). “In other words, bend your knees,” Pariser says. But to avoid cramps in your hamstring muscles, don’t bend your knee so much that your heels are too close to your buttocks.

Spondylolisthesis. This condition occurs when one vertebra slips over the adjacent one. There are 5 types of spondylolisthesis but the most common are secondary to a defect or fracture of the pars (between the facet joints) or mechanical instability of the facet joints (degenerative). The pain can be caused by instability (back) or compression of the nerves (leg).
Long periods of inactivity in bed are no longer recommended, as this treatment may actually slow recovery. Spinal manipulation for periods of up to one month has been found to be helpful in some patients who do not have signs of nerve irritation. Future injury is avoided by using back-protection techniques during activities and support devices as needed at home or work.
In the elderly, atherosclerosis can cause weakening of the wall of the large arterial blood vessel (aorta) in the abdomen. This weakening can lead to a bulging (aneurysm) of the aorta wall. While most aneurysms cause no symptoms, some cause a pulsating low back pain. Aneurysms of certain size, especially when enlarging over time, can require surgical repair with a grafting procedure to repair the abnormal portion of the artery.