There is so much confusion regarding exercise intensity that I could probably write a column on this one topic every other day for a month, and not clear it up for everyone. I have to constantly remind myself that i’ve chosen to understand this topic whereas most people have no desire to. Then again, I understand how important calculus is to the modern world, and am grateful I don’t have to know it. So lets break this down.
There are two main types of Fitness Energy Zones: the Aerobic and the Anaerobic. Here are dictionary definitions.
Anaerobic Exercise: relating to or denoting exercise that does not, or is not intended to improve the efficiency of the body’s cardiovascular system in absorbing and transporting oxygen.
Aerobic Exercise: relating to or denoting exercise that improves or is intended to improve the efficiency of the body’s cardiovascular system in absorbing and transporting oxygen.
Now it seems to me that the general public and the media seem to privilege aerobic exercise over anaerobic exercise based on the premise that aerobics “make the heart healthier” and weight lifting is for certain athletes and other men who are trying to overcompensate for something. So lets dig a little deeper.
First, about those definitions. Most pundits and the general public seem to misread and stop reading the definitions at a certain point:
Anaerobic exercise “…does not…improve the efficiency of the cardiovascular systems…”
We’ve all read quotes that look like the above in a variety of contexts. When a writer decides to edit a quote from the beginning, in the middle, and again at the end, you should become immediately suspicious. Heres the second quote the way popular magazines tend to put it:
Aerobic exercise “…improves…the efficiency of the body’s cardiovascular system…”
Written like that, who wouldn’t say that aerobics is more important? But it’s a lie based on misreading and selective editing. All either definition is describing is whether or not the energy system involved involves the absorption and transportation of oxygen through the blood stream. It says nothing about whether one has greater overall health benefits. It doesn’t even say what the relative health benefits of each might be. If these readings were valid, all U.S. Marine, Army Ranger, and SEAL’s special forces must be very unfit.
The assumption is that anything that is “good” for cardio, is good for life, and by selectively editing the definitions presented above it seems apparent that aerobic is more beneficial. But did you know that anaerobic training has been proven to improve cardiovascular health? It improves heart action! Your heart becomes more powerful, just like it does with conventional cardiovascular aerobic training. This is referred to as General Adaptation Syndrome (GAS) to progressive overload. You do remember that the heart is a muscle, and like all muscles, will respond and adapt to what you put it through. A person is even training their heart; as well as every skeletal muscle, and even the integrity of the bones in their bodies; when they sit on a couch for 4 hours. The difference is, when you sit on the couch, your training consists of telling your body you don’t need a strong heart, you don’t need strong muscles, and you don’t need strong bones. Welcome to slugville (a negative response of specific adaptation to imposed demand).
Now, I teach Spin. I’ve been a body builder, a strength competitor (both strictly amateur) and a ranked racquetball player. I love athletic exercise, even though I don’t compete in any of those things, anymore. I’m not a dancer, so Zumba, cardio jam, the modern step class are not my cup of tea. That doesn’t invalidate these activities at all. We each need to find those things that get us going, get us excited, and get us wanting to put everything we have into them. The problem isn’t whether or not this class or that method is worthwhile, the problem is whether your effort is worthwhile.
So lets take our discussion of intensity to a deeper level. We talk about intensity zones like the fat burning zone (60-70% max heart rate). This is the zone where 85% of all the calories you burn will come from stored body fat, after the first 12-15 minutes of activity (how long it takes fat metabolism to kick in). At this level, the intensity of your activity is below the threshold necessary to improve actual fitness of the participant. You need to train in this zone for a significant length of time (90+ minutes) to get any appreciable caloric burn off.
The Aerobic training zone (70-80%) is where you get a well-rounded workout. You will still burn significant fat calories (50% of total), your calories/minute will be more significant, allowing you to get real results in about 60 minutes (the average length of most group exercise classes), and your heart and lungs will start getting enough stimulation to actually improve its functioning over time.
The anaerobic zone (80-90%) is the next step up. Your body will consume very high amounts of calories/minute. 85% of those calories will come from stored carbohydrates, and only 15% from fat stores, as fat metabolism requires oxygen, and your burning energy too fast to use the oxygen delivery system efficiently. This intensity level severely limits the duration of an exercise session. Most people will become utterly exhausted within 10-20 minutes. Ever worked out so hard you felt like throwing up (or did)? Welcome to the anaerobic zone.
Max Zone (90-100%) Nausea, dizziness, light headedness, even fainting, can occur in an unprepared exerciser. This level should generally be avoided unless you know your fitness level. The problem is, no one knows their true fitness level or heart rate zones. All our tools at the consumer level are no more than good estimates. Get a stress test to determine your true max heart rate and then you can get reliable personal zones (mine is 177 bpm).
Where does this leave us? As intensity goes up, fat burning seems to go down, but this is misleading. At the fat burning intensity you burn so few calories/minute that you have to train for an exceptionally long time to get any significant benefit. If you want results in this zone, train for 2 or 3 hours without stopping. This is the zone competitive marathoner’s spend most of a marathon in, pushing up into higher intensities only as the race nears the end. A runner who was leading most of the way and faded at the end miss timed their kick. They pushed their intensity up too soon, or their opponents were better trained, or both. Of course, this is equally true of a Tour de France or Ironman triathlete competitor.
So why bother with anything besides the Aerobic zone? Based on those original definitions, you’d think higher zones are counter productive or a waste of time. What’s happening is you’re consuming energy (calories/minute) faster than your aerobic capacity can produce it, and your body starts switching over to the lactic energy system. This doesn’t mean that your aerobic system has shut down however. It is desperately trying to keep up with the demand, as it is the energy system your body is most efficient at using. Here’s why you want to include the anaerobic energy zone.
Your body is really just a conveyor belt of energy (calorie) distribution. During fat burning you start burning glycogen (starchy sugars) stored in the muscle directly. As these start depleting, your body sends spare energy stored in your liver to your muscles, replenishing them so they can continue to move, or do more reps. After 12-15 minutes of continuous movement without rest, fat metabolism starts up and fat is converted to glycogen and sent to the muscles and the liver to replenish them. Move up the intensity 1 step (aerobic zone), the process accelerates a little. Move into the Anaerobic zone and fat metabolism still occurs, but the workout duration will be shorter because the energy is being burned faster than the body can convert and transport energy to your muscles. Remember, fat metabolism begins 12-15 minutes after you begin exercising. Most people become exhausted 10-20 minutes in. See the problem? Even if you’re very conditioned, and you can go 30 or 40 minutes, you will only get 15 or 20 minutes where fat metabolism is even happening at all. But it doesn’t matter. At the anaerobic intensity level, your body is burning large amounts of calories/minute, your liver is desperately trying to replace the muscle glycogen so you can keep going, and your aerobic energy (fat metabolism) can’t keep up. When you are finished, your body still needs to replenish the muscle and liver sugars (glycogen) that you depleted, as quickly as possible, because the body does not like deficits (homeostasis). This forces your body to stay at an enhanced metabolic rate for 4-6 hours to quickly convert fat into sugar (glycogen) to get the liver and muscles full. At the end of traditional aerobic exercise, the human body returns to its original pre workout metabolic rate within 30 minutes.
We’ve seen how fat burning percentages go down as intensity goes up. The inverse is equally true. The less intense your activity level, the greater percentage of fat your body can use as an energy source. Want the highest fat metabolism rate? Sit on your ass. Go to sleep. There’s a great weight control strategy.
Here’s a great secret for the gym, whether the aerobic class, Spinning, or the weight room. The more you adapt your body to train in higher intensity zones, the better your body gets at training at every intensity below that threshold. If I spend my time in the weight room doing leg press with 75 lb, 3 sets, 15 reps every other day. I will reach a certain level of fitness and then my body will stop responding. If I leave the weight the same and increase the number of reps, I’ll get better at doing more reps, but will eventually plateau. On the other hand, if I start increasing my weight to 100 lb., even if I can no longer complete 15 reps initially, my muscles will get stronger, and my ability to push lighter weight (75 lb.) for higher reps will still improve; and at a faster rate with less risk of plateauing. As the heart is a muscle; in arguably the most important one we have, it responds to all these intensity zones in just the same way. For the athlete, it’s just a question of knowing when to apply each level of intensity during training and competition. For everyone else, it’s all about physically manipulating your body through them to help you achieve your goals. In the Spin room, use a heart rate monitor if possible. The more time you spend in the higher intensity zones, the longer you will be able to train at the lower intensity zones, when the circumstances dictate. And we want to prevent plateau’s as much as possible. That’s why official spin classes are 45 minutes, not 60 like other classes.
Good luck.
Fitnessgroan's Blog 
I cannot believe I have reduced my exercise level to stay within the “fat zone”. I’ll not make that mistake again. I guess the gyms buy equipment with “fat zones” included on the meters so that users can feel like they are doing something but only exerting a bit more than couch-level-amounts of energy.
The “fat zone” is what we like to refer to as the active recovery zone. After a really intense segment where your body has pushed into either the high aerobic zone or higher anaerobic/max zone we tend to give you a rest (or you take one). During the rest, though your movement slows down and the resistance decreases, your cardiovascular and energy systems are still trying to pump elevated amounts of energy back into your muscles. It can take 60-180 seconds for someones 85% HR to return to a 60% HR. therefore, more energy is being pumped back into your muscles than you’re burning while “resting”, even though you’ve stayed in your fat burning zone. If you actually allow your heart rate to drop below 60% for more than 60-90 seconds, fat metabolism will shut off completely, and you have to re-start it from scratch (12-15 minutes).
Wow, your comment that fat metabolism will have to start over from scratch if heart rate drops below 60% for more than 60-90 seconds is shocking! I guess I’ll have to remind myself of this during spin class and circuit training (and definitely get my hands on a HR monitor).
Also, I didn’t think anything could be more strenuous than your spin classes, but your circuits proved me wrong! I haven’t been so sore after a workout in a long time…. I like it.
I might’ve overstated a bit. Fat metabolism never really shuts off completely. But burning at a meaningful rate shuts down. As for how long it can take after you drop below 60%, I’ve read this in almost every certification I’ve ever had or seen, but have never been able to track down the original research. The amount of time is probably more individualized. What is established is that specific hormones regulate whether a human body is better at fat burning or fat storing. This hormonal response is controlled by a combination of genetics and a general adaptation response to physical activity. It can take as long as 12 months for a lifelong overweight sedentary person to flip this hormonal switch to fat burning. And typically it works in the reverse. A “naturally” skinny person/physically active person can enter into an extremely sedentary state with poor diet and usually maintain a certain leanness for 6-12 months before their hormonal switch flips to fat storage mode. The bottom line is, to get a meaningful aerobic workout, try not to let your heart rate drop below 60%. For older adults who are newer to exercise the low heart rate could be closer to 50% max.
To finish the point above, someone who is “switched” to fat burning probably stays in and gets back into their fat burning/aerobic zone more quickly since their bodies have better adapted to that biochemical state, while someone who is “switched” into fat storage probably takes a longer time to get back into the fat burning/aerobic zone. Ironically, because these people tend to be less physically fit, their heart rates drop much more slowly, so they can usually rest longer than a fit person while maintaining their zone.
[…] Intensity is all about understanding your goal. It is the most important factor that will determine whether you actually achieve any measurable progress, whether its increasing you distance on a bike ride, speed in the 400 meter run, or your 1 rep max doing an olympic bench press. Trying to lose weight requires a certain intensity. Building muscle requires its own level of intensity. Improving athletic performance…has its own special intensity demands. Not understanding what intensity you need to achieve will undermine everything you try to accomplish. Too much for intensity will undermine a marathoner, and too little will be futile for the sprinter or weight lifter. If you haven’t read my posts on intensity, or want to refresh your memory click here and here. […]
[…] if you do it fervently every other day till the day you die, assuming you follow all the rules of intensity that I’ve laid out innumerable times and you are otherwise physically healthy with no major […]