Running on your treadmill, eyes wandering over the flashing numbers and blinking lights, your gaze falls upon a small chart tucked away to the side of the console display. The chart is labeled “Target Heart Rate Zones” and lists a bunch of percentages and age ranges that are supposed to tell you where your heart rate should be when performing cardiovascular-specific, i.e. aerobic, exercise. You wonder which percentage range you should be in for someone of your age with your particular set of goals, but the only explanation you can find are vague labelings such as “fat loss zone”, “cardiovascular zone”, or similar phrases. Does this mean that you can’t train for fat loss and cardiovascular endurance at the same time? Does it mean that if you happen to slip out of your determined zone then you will not reach your goals? This article will help to clarify the topic of appropriate heart rate zones and what they mean for you.
Let’s start with defining what is meant by the terms Heart Rate (HR) and Cardiovascular Exercise Intensity and expand upon that. Your HR is the number of times your heart beats in one minute. Obviously the harder you work, the more work the heart must do in order to increase the blood flow throughout the muscular system. If your actions (the exercise you are performing) demand a significant increase in blood flow in order to allow your body to continue, then your HR must elevate. The higher your HR, the harder you are working and here is where we come to the definition of cardiovascular exercise intensity, which is the percentage of your Maximum Heart Rate (an estimation of the maximum number of times your heart can beat in one minute). Someone exercising at 85% of their MHR is working harder and placing more of a demand than somebody working at 65% MHR.
So all this talk about heart rates is really a measure of how much work you are asking your cardiovascular system (heart and lungs) to perform in order to adequately support the current demands of your muscular system. Now this begs the question, where should your Training Heart Rate (THR) be in relation to your MHR? In other words, where should your HR be during exercise? And the answer to that lies in your particular set of goals and in how fit your heart and lungs are.
Research has shown that training at 50-60% MHR forces the body to utilize oxygen and fatty acids as the primary energy sources in order to continue the activity (hence the term “fat loss zone”). It is at this point in the cardiovascular intensity spectrum that your body begins to tap into the existing fat stores to fuel the activity. 60-85% MHR is oftentimes referred to as the “cardiovascular training zone”, where the body now begins to expend more glucose in order to continue the exercise in conjunction with fatty acids and oxygen. This zone produces a much greater training effect upon the heart and lungs, encouraging them to adapt and become fitter. 85-90% MHR and above is what is commonly referred to as the “anaerobic training zone” and utilizes a breakdown in glycogen (the storage form of glucose) to fuel the activity. This level of energy substrate usage is responsible for higher-intensity shorter-duration muscular contractions and this training zone is touched upon most often by athletes and those looking for more intense conditioning protocols. At this level glycolysis increases as the body places more of a priority on breaking glycogen down into glucose in order to help fuel these higher-intensity muscular contractions through a temporary increase in ATP production. As a result lactic acid production increases as a natural byproduct of the kind of intensity required by the muscular contractions to push one’s heart rate this high up on the spectrum.
The mistake often made when trainees look at these percentage charts is in looking just at the individual zones and not at the complete picture. Yes, training at the 50-60% range does burn fat however all of these effects are cumulative as you move up the cardiovascular intensity spectrum. If you move into the “cardiovascular zone” (60-85%), you may not be using existing fat stores as the primary fuel source but you will be burning more calories. Caloric expenditure is of prime importance for those looking to lose bodyfat. It therefore makes much more sense to train at the higher intensity level if fat loss is your goal, given that you will be burning more calories and achieving greater cardiovascular conditioning at the same time (increasing the fitness level of your heart & lungs).
Choosing which range is right for you depends on your goals and fitness level. Somebody just coming out of cardiac rehabilitation will not be able to safely exercise at the higher intensity levels until they build up to that. On the other hand, somebody who has no cardiovascular complications can exercise more intensely with little risk. Anyone with cardiovascular limitations should consult with their physician to determine their appropriate training range. For those with no limitations, the following guidelines apply for the corresponding goals:
- Fat Loss: Train at 70-85% MHR 3-6 days/week for 30-60 minutes. This allows for a moderately high calorie burn overall with some cardiovascular conditioning thrown in.
- Cardiovascular Endurance: 70-85% MHR 3-6 days/week for 20-60 minutes.
- Sports Conditioning: Depends on the sport (what muscles & movements are dominant). Consult a nationally-certified trainer or coach.
- Muscle Mass Gain: Train at 60-70% MHR 3 days/week for 20 minutes. This allows for the minimum level of exercise appropriate for maintaining aerobic fitness without burning too many of the calories which are necessary for weight gain.
Now that we have defined the terms which are pertinent to this topic and briefly touched upon what each zone does, the next point to establish is what your own individual MHR is, for you cannot determine your THR without knowing what maximum value you are basing these percentages off of. The most common method, and the one used by most of those charts you find on cardiovascular equipment, is 220-Age (in years). So a 35-year old would use the formula 220-35=185. The estimated MHR for the 35-year old would be 185. This person would then multiply 185 by their desired training range percentage (listed above) in order to arrive at their particular HR range for training. Is this the most accurate formula one can use? No, for there are other factors to take into account besides one’s chronological age, such as fitness level.
One of the indications of an individual’s particular level of aerobic fitness is the Resting Heart Rate (RHR). Your RHR reflects how much work your heart does when your body is at rest. The best time to take this is in the morning before you get up from bed as this is when your body is most “at rest”. Take your index and middle fingers and lightly touch the base of your opposite thumb. Slowly slide the fingers down where the thumb joins the wrist (you can oftentimes feel a little “canal”) and keep searching until you feel a pulse. Count the number of pulses in one minute (or count the pulses for 10 seconds and multiply that number by 6) – that is your RHR. Let us now make the previous formula a little more accurate to the individual by plugging in the RHR: (220-Age)-RHRxTraining Percentage+RHR. This is known as the Karvonen Formula and is more accurate than the Age-Based Only Formula.
So let us assume 35-year old Susan wants to lose fat and build some endurance. She counts her RHR in the morning before rising at 72 beats per minute. Her goals to lose fat and build endurance are complementary to each other so she needs to be in the 70-85% range. 185-72=113x.70=79.1+72=151.1~(rounded to) 152 lower value. 185-72=113x.85=96.05+72=168.05~169 upper value. Therefore her THR is 152-169. She needs to keep her HR at 152-169 for 30-60 minutes 3-6 days/week in order to help reach her goals.
If during training you happen to go a little over your THR range or dip a little lower, do not despair. Remember, the cardiovascular exercise intensity spectrum is just that, a spectrum. 59% MHR will not dash your hopes to build endurance and 91% MHR will not generate so much lactate that you will “overwork” your system. Just keep constant track of your current HR when doing aerobic exercise and adjust the level as appropriate to keep your HR as close within your range as possible. Do not be surprised as well if over time you find that you have to push harder in order to keep your HR elevated – this is a positive sign as it shows your heart and muscles are becoming increasingly capable of performing more work with less effort, i.e. becoming fitter.
One thing I did not touch upon was the particulars of the anaerobic zone (85-90% and above) and when/when not to train at that intensity. That is a more complicated topic which goes beyond the scope of this article and is best discussed with a qualified fitness professional who can help determine if and how it should be best utilized as part of your fitness program. What is commonly referred to as “interval training” or HIIT (high intensity interval training) is a method of training that requires the trainee to push to or past that anaerobic threshold for brief intervals in order to facilitate a greater spike in metabolism and hence a greater overall training effect upon your systems (both the muscular and cardiovascular systems as opposed to primarily the cardiovascular).
In conclusion, I hope you have come away from this article with a better understanding of what the heart rate zones mean for you and in what range to stay into for your particular goals. As always, if you feel as if the exercise is too much (excessive shortness of breath, particularly noticeable if you cannot talk at all during your aerobic workout) slow it down and work your way up to that level of intensity over time. Remember, safety comes first and consult with your physician if in doubt as to your ability to exercise at a certain level.