Thursday, December 9, 2010

Aerobic exercise: How hard should I work out?

"How hard should I be working out?" This is one of the most common questions people ask me during fitness training. This is a good question and the answer is different for everyone. This is because an individual's exercise intensity level, or how hard they need to be working, is directly related to things like their fitness level, age and health. Everyone should ask themselves this important question before beginning an exercise program and when reassessing a current workout. Knowing your exercise intensity level is important in determining how best to bring about improvements in your health.

You may have heard the expression "in the zone" with regard to exercise intensity. This can be used to describe target heart rate (THR) training levels which are benchmarks that are measured during aerobic exercise. You may have seen this range visually depicted in a chart on your gym's treadmill control panel. Some use the misleading phrase "fat burning zone", more on this later, where others just say training zone. In any case, the theory of training in a target heart rate zone can seem a little perplexing.

Adding to the confusion, especially for women, is the emerging scientific data about a gender specific formula for maximum heart rate, which directly impacts how a woman knows if she's in "the zone." Additional information is available here, but before we get into that let's take a look at several factors that are helpful in determining whether or not you're working hard enough during your aerobic workout.

What is an aerobic workout? When we perform aerobic exercise, or aerobics as coined by Dr. Kenneth Cooper way back in 1968, we are increasing our need for oxygen by performing an activity that engages the larger muscles of the body.  This sustained activity, in turn, causes the heart and lungs to work harder than when the body is at rest in order to provide more oxygenated blood to fuel the working muscles (1). There are many health benefits to aerobic exercise (3), and the Mayo Clinic lists ten of them here.  One reason we are focusing on in this post is heart health. 

Heart rate or pulse is the rate at which blood is squeezed out of the heart per minute (BPM). We can use our heart rate to measure how hard our heart is working at rest or during aerobic exercise. This can be done by taking a pulse or heart rate and determining our BPM number. Pulse points are places where an artery passes close to the skin and makes it easier to feel blood pulsing through it. There are several pulse point locations on the body, but 2 of the more commonly used sites are found on the wrist (radial pulse), or on the neck (carotid pulse).

Once you've found your pulse, count the number of times you feel the blood pulsing under your fingers for 10 seconds. Then multiply that number by 6 and you will have your heart rate for one minute. For a more precise measure, you can also use a blood pressure monitor that counts your pulse for you. To learn more about manually taking your pulse, just follow the link here to the Cleveland Clinic's website.

Resting heart rate (RHR) is another useful indicator of heart health. This is the number of times our heart beats per minute while we aren't engaging in any activity and is best taken in the morning before we get out of bed (3). I suggest measuring it for 3 mornings and then taking the average. According to the National Institutes of Health, a normal RHR range is:
Newborn infants - 100-160 BPM
Children 1 to 10 yrs. - 70-120 BPM
Children over 10 and adults (including seniors): 60-100 BPM
Well-trained athletes - 40-60 BPM (5)

Maximum heart rate (MHR) - Knowing your MHR will help you in calculating your target or training heart rate (THR).  The gold standard for determining your MHR is to take a doctor supervised treadmill test. However, for many of us this may not be practical, so exercise physiologists have developed several formulas to help estimate MHR.  One of these is called the estimated maximum heart rate formula:
MHR = 220 BPM - age
At the beginning of this decade, research conducted on the estimated MHR formula found that it "underestimated MHR in older adults" (7). These scientists proposed a new MHR formula:
MHR = 208 - 0.7 x age
This year, more recent research has found that this formula overestimates the MHR for age in women, especially those over the age of 35. See footnote (8). As a result of this finding a new gender specific MHR formula is being proposed, however it is not yet being used in a clinical setting because the research is considered preliminary. The gender specific formula is:
MHR = 206 - .88(age)
After we have determined our MHR we can calculate our training heart rate.

Training heart rate (THR) and finding the right intensity for you - THR is the desired heart rate range which will provide us with the most benefit from our aerobic workouts. Calculating a THR zone helps us determine the level of sustained exercise intensity that best challenges our heart and lungs safely. When calculating THR, I prefer to use the Karvonen or Heart Rate Reserve Formula.  This formula is more personalized because it factors in the resting heart rate (RHR) to determine heart rate reserve (HRR). The formula for calculating HRR is:

Now we move to the next step in determining THR. We multiply HRR by the desired training intensity range at which we want to workout. The American College of Sports Medicine recommends healthy adults exercise at a range beginning at 55% and up to no higher than 90% of MHR.

Let's put all this information into practice. A 30 year old healthy female who has a RHR of 70 and is of average fitness level wants to determine her THR zone.  Here is how she would calculate her THR range:
Begin by calculating MHR. Using the new gender specific MHR formula -
MHR = 206 - .88(age)
179.6 = 206 - 26.4
MHR = 180
Next we determine HRR using the formula HRR = MHR - RHR
HRR = 180 - 70
HRR = 110
Now we plug the HRR number into the rest of the Karvonen equation to determine the THR range.  We begin with the lower end of her training range (light to moderate) or 65%.
THR = 110 x .65 + RHR
THR = 71.5 + 70
THR  lower range = 141
Now we find the THR for our sample female of average fitness level at the top end of her training range (moderate to vigorous) or 85%.
THR = 110 x .85 + 70
THR  higher range = 164

So, this individual would monitor and strive to maintain for the duration of her aerobic activity a target heart rate zone between 141-164 BPM.  These numbers would be adjusted as her fitness level changed.

How much aerobic exercise is enough?   According to the American College of Sports Medicine healthy adults under the age of  65 should shoot for at least 30 minutes of aerobic activity, 5 days a week.  The Center for Disease Control (CDC) suggests healthy adults get at least 150 minutes of moderate intensity or 75 minutes of vigorous intensity aerobic activity per week. This activity should be sustained for bouts of at least 10 minutes. For even greater health benefits the CDC urges us to shoot for 300 minutes of moderate intensity or 150 minutes of vigorous intensity aerobic activity per week, or an equivalent mix of both, again in bouts of at least 10 minutes.

What's light, moderate or vigorous intensity activity?  According to the CDC, "moderate intensity means "you're working hard enough to raise your heart rate and break a sweat." Examples would be:
  • walking fast
  • doing water aerobics
  • riding a bike on level ground or with few hills
  • playing doubles tennis
  • pushing a lawn mower (CDC)
Vigorous intensity aerobic activity means "you're breathing hard and fast, and your heart rate has gone up quite a bit" (CDC). Examples include:
  • jogging or running
  • swimming laps
  • riding a bike fast or on hills
  • playing singles tennis
  • playing basketball
So, having plowed through the formulas and worked through the calculations, we now know how to determine our THR zones.  As important and useful as this information is, it should be noted that it is all an estimation.  One research study found that when monitoring aerobic exercise in a group setting, perceived rate of exertion, or how the person exercising is feeling, was as good an indicator of training intensity as THR calculations.  Plus, the exercisers didn't have to stop to take their pulse. Whatever method you choose to monitor your exercise intensity,it is always best to work within your comfort level and not to exceed 90% of your MHR because of increased risk for injury.

The short video below from the American College of Sports Medicine's Exercise is Medicine Library talks more about using Rate of Perceived Exertion as a way to monitor THR. The take away from all of this is to just get moving because exercise really is good medicine.

Don't forget: It is important to check with your physician before beginning any exercise program. Next post will take a look at the misleading concept of the fat burning training zone, and will be much shorter I promise!

(6) Physical activity & health: an interactive approach, Thomas, D.Q., Kotecki, J.E.
(7) Tanaka et al., Age predicted maximal heart rate revisited
(8) St. James women take heart study 
American College of Sports Medicine
Centers for Disease Control

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