By Dr. Brian McDonough, Medical Editor

PHILADELPHIA (CBS) - You may have noticed that this happened to you in the past. You begin an exercise program, perhaps you are running, you have a good workout and feel good about the effort but the next day your legs feel heavy. They may even hurt. The problem is the result of the development of lactic acid.

When lactic acid builds up it leads to tightness and muscular pain. The reason you get this lactic acid is because you undergo a process called anaerobic metabolism when you work out –especially when you are just starting out. Anaerobic metabolism means that you have used up oxygen in metabolism and go to a process without it.

But don’t worry. As you get in better condition it is harder to, as they say, ‘go anaerobic’.