If you read enough about fitness and exercise you will come across an article promoting interval training. Wikipedia defines interval training as follows:
Interval training is a type of physical training that involves bursts of high-intensity work interspersed with periods of low-intensity work. The high-intensity periods are typically at or close to near-maximum exertion, while the recovery periods may involve either complete rest or activity of lower intensity.
About a year ago I was training for a triathlon and it was recommended to me to do interval training to help my cardiovascular strength and endurance faster. Fitness experts will always talk about the benefits of benefits of interval training but you never hear about the down sides. When running if you alternate between very high intensity running and a jog you end up running much faster and harder during the high intensity portion than you would if you were running at a constant speed because you have a chance to rest and recover. Unfortunately running (especially on concrete or a treadmill) puts a lot of stress on your knees, shins and back, so very high intensity running puts even more stress on your body. In one 20 minute session of interval training I managed to herniate a disk in my back. A perfectly healthy 25 year old athlete got a serious injury from just running. Unfortunately I did not realize I that I had a herniated disk until I got an MRI last week.
While I would strongly caution against doing any type of interval training on concrete or a treadmill, you can safely do interval training on an elliptical runner.