One of the common ailments suffered particularly by runners, and triathletes particularly because of the time spent on the bike, is tight and over-active hip flexors. This can mechanically restrict movement into extension of the hip and put extra stress on the lower back in response.
A less widely appreciated effect of having tight and over active hip flexors is that on a neural level, this overactivity can cause reciprocal inhibition of the gluteal muscles. When the glutes stop activating during functional movement, the risk of injury is significantly greater, as they have such an important stabilizing role in the overall movements of the body.
Try the stretching exercise in the video above on a regular basis and feel the movement in your hips improve.