Lactic acid build up is the result of anaerobic (no oxygen is being used to complete the energy cycle) use of muscles common with weight lifting and sprints (if you breath heavily afterward, you are using your anaerobic system). Once this builds up in the muscles, time is what is needed to release the acid from the cells and "flush" it out of the area. It has been theorized that heat applications, sports massage, trigger point massage, light-warming exercise and stretching can help hasten the clearing process because all of these approaches encourage additional blood flow and an internal warming of the cells and vessels. When you press on a tender point, initially the pain will more than likely increase first, then over the next several moments (5 seconds to 2 minutes) of continued pressure and/or "press and release" maneuvers, a host of your body's natural pain killers will flood the area, deaden the pain, vastly increase the local blood flow and begin to release the affected muscle group. If you continue to push yourself in your exercises to a point where you are challenging your anaerobic system, then you can expect to feel residual pain as a result.