The piriformis muscle helps control the stability of the leg at the hip. An increase in the amount of walking, running or an increase in the demand for stability such as moving to off road running can cause its increased use. As the sciatic nerve passes close to or even through this muscle it can impinge on it and cause neural type pain down the back, side or even the front of the leg. (NB Sciatic pain that can be traced back to the lumbar spine and may also be a sign of other medical or chiropractic conditions so further investigation should also be considered).