If you have high blood pressure, congestive heart failure, or an irregular heartbeat, you might be on a medication that controls your heart rate. This medication may be a beta blocker. Beta blockers help your heart work more efficiently by reducing the amount of blood and oxygen that it pumps. Ordinarily, when you start to exercise, your heart rate increases. But, if you are taking this type of medication, your heart rate does not increase.
Your heart rate is determined by the amount of work you are doing. When you are sleeping, you require the least amount of oxygen. When you first wake up, your heart rate is the lowest rate of the day. If your muscles start calling for more oxygen because you are working out, your heart rate increases to meet this demand. As your muscles start to cool down, your heart rate returns to normal.
As you go about your day, your heart rate increases and decreases to meet the demands of your body. When you start to work out, your muscles start demanding a steady oxygen supply for fuel. Your heart rate increases to pump blood faster to your muscles so they can continue to work. As you work out harder, your heart rate reaches your target training rate and you know you are at the right intensity.
A beta blocker limits how fast your heart beats. Your heart rate will still increase because of the demands of the muscles, but you probably will not be able to reach your target heart rate. This makes it difficult to determine the intensity of your workout. Your heart is not circulating blood as fast as the muscles are demanding it, so you get tired faster. Lack of fresh oxygen to your muscles stops the production of fuel for your muscles to burn. You feel as if you are working hard, but you may not reach your target heart rate.
The rate of perceived exertion, or Borg rating of perceived exertion, is an alternative to using your target heart rate to measure the intensity of your workout. During your workout, you assign a number rating to how hard you feel you are working. The perceived exertion scale runs between 6 and 20, with 20 being maximum exertion. Once you get comfortable using this scale, you can begin to challenge yourself to reach a higher number. The exertion scale is a reliable way of measuring the intensity of your workout.