heart rate is controlled by the oxygen demand your body needs... and by the amount of carbon dioxide your body needs to get rid of... as you exercise you are producing waste products of metabolism... Oxygen levels in your red blood cells is low (which creates a higher oxygen demand so cellular respiration can take place) so your heart speeds up to carry the same amount of oxygen through your cardiocascular system...
Also your central nervous system (autonomic) comes in to play with your epi and norepinephrine which will create a higher o2 demand and increase breathing and heart rate... MEDITATION or controlled breathing will work the best... Deep breathing in through the nose and out through the mouth in a contolled manner will help your bodies fight or flight hormones not to work against you...
Anyways these tests just want to make sure your heart rate does not stay elevated at a certain number of beats pers minute for a peroid of a minute ... so if you are in descent shape just relax and you will do just find my friend... hope my ramblings helped you out... DETROIT FIRE