Deconditioning, also called disuse atrophy (due to sedentary lifestyle and physical inactivity) is the most common cause in north america.
Other causes include vascular (blockages of arteries down the leg) or spinal stenosis
(tightening around spinal nerves). If not better with daily running
- walking - stationary bike - rowing machine use for 6-8 weeks, have your family doc start the workup.