Molly is jogging on the treadmill. After 20 minutes, her heart rate has not increased, she is able to talk, and she is not sweating. If Molly wants to improve her fitness level, what should she do?

When exercising, especially long-term over a familiar course, you may notice your heart rate not going up as you'd expect. This is because your body has adapted to the workout that you're doing. Molly can increase her heart rate in a few ways: put the treadmill on an incline, run faster, or vary the speeds she is running, so they are always changing.