A negative feedback mechanism is something that stimulates the organism to stop doing the activity it is doing, to slow down or cease the activity altogether. What happens during normal exercise, is your heart rate accelerates because of the increased need for energy for your cells to conduct cellular respiration. At some point, your body can not get enough oxygen into your body, so a process called fermentation takes over in energy production. One of the by products produced with fermentation is lactic acid, which when accumulating in muscles, causes fatigue. This fatigue causes us to stop the exercise, after which a reasonable time passes, your heart rate returns to normal, your breathing returns to normal, and the process of cellular respiration takes over again. So the accumulation of lactic acid is a negative feedback mechanism that tells your body to stop the extreme physical activity so it can recover and get back to normal.