Heart Health

Pushing Your Anaerobic Threshold

March 21, 2011

Pushing Your Anaerobic Threshold

Go to a playground sometime and watch the nearest batch of rambunctious kids.  They mostly spend their time sprinting around in circles and climbing over everything that’s climbable.  Now go hang out with some middle-agers while they engage in recreational activity.  Notice any difference?  The youngsters run and run until they can barely breathe, and then run some more.  The adults sit, gab, text, surf (the web), swig beer, eat lots of food, and pretty much just sit there.

Now let’s say you’re one of those middle-agers.  You’ve done a good job taking care of yourself and you try to exercise on a daily basis by walking a few miles on the treadmill.  Yet, when you challenge your body with the kind of vigorous exertion that kids take for granted, you find yourself sweating like broken sprinkler system and panting like Charlie Sheen on a daytime talk show.  You carry some laundry up a flight of stairs or help a neighbor move some furniture and wonder how you could possibly be out of breath despite your regular exercise.

To get at the answer to this question I’ve got to bore you with a short dissertation on physiology.

Glucose is the fuel your body uses to power its muscles and other organs.  Under normal conditions, your cells employ a highly efficient series of chemical alterations to convert the glucose into mechanical energy with small amounts of exhaled carbon dioxide as the discarded waste.  This process is called aerobic metabolism.  The carbon dioxide never builds up in your blood since your lungs do an excellent job of sensing its presence and accelerating your breathing to whatever level needed to get rid of it.  You should know, by the way, that it is your body’s drive to eliminate carbon dioxide, rather than your need to take in oxygen, that drives how fast you breath—hold your breath for 30 seconds and you’ll quickly realize how much your body doesn’t like even the temporary accumulation of this harmful metabolic byproduct.

As the intensity of exercise increases your heart rate and breathing rise in a predictably linear fashion.  We map this relationship whenever we have a patient do one of our treadmill stress tests.  An interesting thing happens, however, when your heart rate reaches the upper range.   With your muscles demanding more and more energy your cells switch from aerobic to anaerobic metabolism.  This is another series of chemical reactions that produces energy but which is considerably less efficient than aerobic metabolism and which ends up with a dramatic increase in the production of CO2.  At that point your heart rate picks up quickly and your breathing speeds up out of proportion to your level of exercise.  Once you’re in this range you won’t be able to sustain your activity for long.

The point where you switch from the efficient aerobic pathways to the inefficient anaerobic system is called the anaerobic threshold (AT).  When you’re young you have a relatively high AT and can do a lot of hard work and strenuous exercise without stopping to catch your breath.  The typical 20-year-old can get his heart rate up to over 160 beats per minute before switching to anaerobic metabolism.  With age, through, your AT drops in a predictable fashion to the point where, at 65 years of age, you can push your heart rate to only 124 before reaching your AT (see helpful chart here).

The interesting thing about AT is that you have some control over it.  Elite athletes spend a great deal of time exercising right at their threshold in order to maintain or even raise it.  Lance Armstrong, for example, used to design his work-outs to keep him right at his threshold rate of 178 beats per minute for as long as possible with frequent intervals at even higher levels.

While it’s known that AT drops with age it’s also been proven that the rate of decline is not necessarily fixed, but rather depends on how much you train your body to operate at a level near your AT.  A 50-year-old person, for instance, could design an exercise regimen to include two days a week of interval training (e.g. faster walking, running, more resistance on the exercise machinery) so that his heart rate skips up to the 140s or 150s for brief periods of time.  Such training has been proven to retard the decline in AT as you age.

Now back to you.   Your daily exercise is great but you probably never get your heart rate near your AT—never, that is, until you’re climbing stairs or moving furniture.  In fact, the last time you pushed yourself this hard may have been 10 years ago, but at that time your AT was 10 points higher than it is now.  The answer for you, of course, is to incorporate more vigorous exercise into your regimen once or twice a week.  Several months of this and you should find that you’re no longer breathless with a flight of stairs.

As a cardiologist I see this problem pop up just after the first snowfall when I’m asked to evaluate a lot of patients for difficulty breathing when shoveling snow.  “I’ve done this every year but have never been this short of breath.”  For most of these people, shoveling snow may be the only time during the year that they push their bodies to their AT.  Unlike children, for whom maximal exertion is a daily occurrence, most older adults rarely reach that point.  If they had instead spent the summer months pursuing balanced exercise habits they wouldn’t be seeing a cardiologist.

For the rest of us, here’s my advice: Engage in aerobic exercise 30-45 minutes a day for at least 5 days a week.  Devote one or two of your sessions to more vigorous exertion for 5 minutes at a time with 5 minutes of slower rates in between.  As long as you’re relatively healthy you can push yourself fairly hard and try to maintain your pace for at least 5 minutes before slowing down again.

If you have doubts about your safety when exerting to the point of breathlessness just talk to your doctor.  The treadmill stress test is an excellent way to determine if you’re safe to ramp up your exercise.  With this test we push you well past your AT while watching your heart carefully on an EKG monitor and can identify any lurking problems that might put you at risk.  If you’ve not exercised much in the last few years a stress test might be helpful.

At this point you’ve got only 9 more months until the next big snowfall to get yourself in shape for shoveling.  Transplant yourself back to the days of your youth, pick up your exercise intensity and you’ll feel so good by next December that you’ll want to come over and clear my driveway, too.

  1. Eric Van De Graaff, MD

    Billy V, The term MET refers to metabolic equivalent of task, a unit-less number that roughly estimates exertional output. It is simply a ratio of your metabolic output during a particular task to your resting metabolic demand. When you are sitting quietly you are expending 1 MET. Walking is 2-3 METs, running is about 7 METs, and trying to make heads or tails of my dissertations on physiology is over 10 METs. The treadmills at the gym estimate METs from your speed, incline, and body weight. Measuring pulse rate can be tricky. I consider myself somewhat an expert at feeling arterial pulsations but still have trouble measuring my own heart rate at peak exertion. Most heart rate monitors measure your heart rate electrically from a band around your chest and display the number on a watch. That system is the best way to follow your heart rate during exercise. I own a Polar monitor and it works well. Keep up with your exercise and thanks for the questions and feedback. Dr. VDG

  2. Eric Van De Graaff, MD

    Rayray, Thanks for the question. I can't say why your blood pressure would be rising with exercise rather than dropping. This may be a temporary blip and your numbers might return to number in a matter of weeks. If not, don't be discouraged. First off, you should be commended on launching into what sounds like a great exercise program. Keep it up and you'll do great. Don't let this small setback discourage you from continuing with your plans. Second, we have a huge list of excellent BP medications that are effective, inexpensive, easy to take, and nearly side-effect-free. Stay on top of your blood pressure. If you need tighter control there are many good choices that will get you where you need to be without compromising your wallet or your lifestyle. Good luck with everything and thanks again for the comments. Dr. VDG

  3. Eric Van De Graaff, MD

    Jen, Congratulations on your plans to get back in shape and I hope you keep it up. Your question raises an interesting point about the resilience of the human body. I'll start by saying that our bodies are amazing machines that are designed to run, jump, climb, and work nearly ceaselessly. The only thing that limits most of us is that we let our bodies go to pot with years of couch potato-ing and lack of exercise. Thankfully, our bodies are very forgiving and continue to be able to mount a recovery with proper exercise habits. You are young enough that your body has the ability to still function at a high capacity with minimal maintenance. As you can guess, though, at some age your body reaches the point of no return and, if neglected long enough, will not be able to bounce back to full health. I don't know what that point is and it's probably different for each of us. My guess is that you'll be able to whip yourself back into shape in no time. After that happens just be careful to never ever let up. Your goal should be decades and decades of activity and good health. Keep it up! Dr. VDG

  4. Eric Van De Graaff, MD

    Megan, Congratulations on your training for the half marathon. In answer to your question, I think your "block" is probably partly mental and partly physical. In my nonprofessional experience I have come to believe that the progress our bodies make comes in leaps and bounds rather than gradual progression. You are likely at a plateau right now and will just have to push through it until you experience another rapid improvement in performance. Stick with it. It'll get easier and you'll get faster and stronger. Dr. VDG

  5. Jena

    I have made it a point to reach my anaerobic threshold for at least 3 of my 6 workouts a week. I've never been in better shape in my life. I'll never approach working out the same way after training like this. It makes me feel like a rock star every time. In fact, I just got back from a spin class that got my heart rate up to 190 (the point at which I feel like I'm going to throw up).

  6. Woolanda

    We just learned about this concept, cellular respiration, in human phys. Thanks for the review! :-) Note to self: Do some sprints from time to time.

  7. Megan

    I am training for a half marathon in May and I am running 3-4 days a week. I feel at this point I should be improving and becoming a better runner, but it seems just as hard now as it ever has. The first few miles always seem like the worst. Is this a mental block I have or is there something that i can do to improve?

  8. Arie

    I'm confused, Jena, is it the high of exercise that makes you feel like a rock star or the throwing up afterwords (because, I imagine rock stars do a lot of that)?

  9. "pre"

    I wonder if we can achieve the same heart benefits with a glass or two of red wine. This seems much more enjoyable than upping ones AT to the point of emesis.

  10. rayray

    Dr VDG, I had a stress test last year that went well. I take benicar hct, and eat fairly well... most of the time. I had been walking/jogging on the treadmill 3 times a week, with a slight reduction in pulse and BP. Now, I'm in week 5 of a more intense workout, and my BP is up. Any comments or ideas?

  11. Jen

    Thanks for the post! This topic has puzzled me quite a bit recently. Here's why: I'm in my mid-twenties and overweight, due to five years or so of being sedentary and maintaining unhealthy eating (and drinking) habits. But now that I'm starting to exercise more and eat healthier, I'm finding that I still have relatively good cardiovascular fitness (routinely work out with a heart rate between 170 and 185 -- peaking in the low 190s). Now I'm no expert but it seems as though my anaerobic threshold is still on the high end (even for my age) -- despite my current lack of physical fitness. Could this be a holdout from the active lifestyle I used to lead? Or am I still young enough that my years of inactivity hasn't really caught up with me?

  12. Billy V

    I appreciate your AT blog, and would like to learn more about physiology. When I google "exercise and physiology", most of the feedback resembles topics is see in tabloids as I check out at Bag & Save. Rarely do they provide the substance explained in your blog. I hope you continue. Also, I have a couple of questions. First, many exerise machines show distance, speed, elevation, calories, and a few provide heart rate. However, there are few that also measure METS which I do not understand. What are METS? Second, it's easy to get an idea of your heart rate while on an exercise machine, but is there way to guage your heart rate while running outside without stopping in the middle of a run to check your pulse? Or would you recommend a watch that comes with a heart rate monitor? Again, thanks and I didn't find the physiology lesson boring at all.

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