How to Start a New Exercise Habit that Can Last (and Extend) a Lifetime: Unpacking the Physical Activity Guidelines for Americans, 2nd Edition. | Part 2 of 4

By Kevin Stahl, PT, DPT

Last week, we opened a discussion about what would be an appropriate (i.e. helpful, realistic, and actionable) set of New Year’s Resolutions around increasing your physical activity. For a full recap, be sure to follow the link above! 

I’m not going to spend much time going over the benefits of exercise. I am somewhat expecting (and hoping) that if you’ve read the previous blog post and have lived within reach of a smartphone or sight of a television in the past decade that you are aware of the national health crises surrounding rising rates of preventable chronic disease—and that you are more interested in reading the how-to’s of being physically active rather than the why-you-shoulds.

So, this week we’re going to explore the following:

  • What is aerobic exercise?
  • How do you know if you’re actually performing aerobic exercise?
  • What is the difference between moderate and vigorous exercise?
  • How do you know if aerobic exercise is alright for you?

First on the list:

Aerobic Exercise: What is it?

Have you ever been walking up a long flight of stairs, got to the top, and realized that you were breathing a little harder and your skin felt a little hot under the collar of your shirt? Perhaps a bead of sweat was winding its way down your spine, too. 

If you think back, you might have noticed a similar set of sensations when going for a bike ride as a kid, or even during PE class while completing a weekly mile and a half run around the campus of your middle or high school. 

Maybe—since we live in gorgeous San Diego—you have even chanced to wade out into the ocean and take a swim on a hot day in August. Imagine plunging into the waves headfirst, then breaching the surface of the water only to find that the tide has pulled you down the beach from where your belongings await you on your striped towel. 

Suddenly, you notice that your picnic basket had caught the attention of some unsupervised toddler holding a sand shovel, with a set to their shoulders that says they might have plans to bury something. 

You charge back into the waves, arms and legs flailing to outmatch the current trying to push you further out to sea. Once you feel your knees scrape sand, you rise to your feet and jog up the dunes to defend your chilled sandwiches and soda (*cough* YES, SODA). 

You begin to towel off, huffing and puffing from exertion, not even cold from the wind brushing your wet skin, and notice your heartbeat thumping in your ears. Within a few minutes, your heartbeat and breathing have slowed and quieted, and you’re relaxed. Without you noticing, however, your heart beats a little faster the rest of the day, and when you lie down in bed that night, sleep is faster to find and deeper than usual, all thanks to your heroic efforts to preserve your lunch. 

Essentially, each of these instances describes a bout of aerobic exercise, each of varying intensity and duration, but all beneficial given that you are free from certain chronic systemic diseases (which we’ll discuss soon). 

The American College of Sports Medicine (ACSM) defines aerobic exercise as “any activity that uses large muscle groups, can be maintained continuously and is rhythmic in nature.”

Put another way: it’s doing something that involves using your arms and/or legs in large, repetitive, motions for a while. 

It’s also generally accepted that this form of activity should involve increasing the number of breathes you take in a minute and also the number of times your heart beats per minute, too.

You have—no doubt—seen some form of aerobic exercise in your life. Here are some examples:

  • Walking (briskly or uphill)
  • Jogging
  • Running
  • Swimming
  • Dancing (except for the slow, romantic stuff—although that might raise your heart rate, too…)
  • Step aerobics
  • Jumping rope
  • Kick boxing
  • Rowing
  • The choices go on and on!

Alright, but now it’s good to ask…

What isn’t Aerobic Exercise? And How Can You Tell if You’re Doing it?

Well, one of the things that make aerobic training unique is that they rely on your aerobic metabolism, which produces ATP (cellular energy) from glucose using oxygen, rather than… 

Wait-wait-wait! Don’t leave! I promise no return to high school nightmares about the Kreb’s Cycle and the dreaded Power House of the Cell, mitochondria, are forthcoming.

Let’s just say that aerobic exercise forces your cardiovascular system (your heart, lungs, arteries, and veins) to work harder to meet the demands of your muscles energy demands instead of the other systems—specifically the anaerobic system—which will be discussed next week

Your heart, your lungs. Sound familiar? In the examples of aerobic exercise that I listed above: climbing stairs, jogging, and swimming in the ocean, you might have noticed that I mentioned breathing and heart rate quite a bit—and even sweat. 

You know you’re engaging in aerobic exercise when two very specific things are happening: 

  • Your heart rate increases (this usually happens first)
  • Your breathing gets a bit harder and faster (this happens second on account of your body moving past it’s anaerobic system to tax its aerobic system).

“Why not sweat, Kevin? Isn’t that important?” you may ask.

Good question! Generally speaking, yes, you will also begin to sweat more during aerobic exercise. However, since you might choose to exercise in a pool, I don’t want to have you worry about measuring the volume or pH level of the water before and after a swim to tell if you were exercising or not. Heartrate and breathing are sufficient.

Also, from the ACSM’s definition, we can pull out the further specification that aerobic exercise must be sufficiently low in intensity that it can be performed continuously. 

But, for how long?

How Long Does a Bout of Aerobic Exercise Have to Be to Count?

There have only been two editions to the Physical Activity Guidelines for Americans.2 This second edition comes ten years after the first was published in 2008. It includes a number of revisions and modifications—the majority of which do not so much refute the previous guidelines as much as refine them specifically to make it easier for people to start making healthy changes.

For someone who has not engaged in exercise regularly since high school, the prospect of carving out 150 minutes of time just for aerobic exercise alone is daunting. (Not to mention that next week, I’m going to encourage you to build muscle and strength, too. How rude, right?)

The Department of Health and Human Services took this into account when publishing the new guidelines. They reexamined the research surrounding exercise and found two very important things: 

  • People were worried that if they didn’t do everything, it wouldn’t be enough—so they didn’t even start.
  • AND, they reaffirmed that even some exercise is better than none.

These findings led them to make the following addendums:

  • Aerobic exercise can come in bouts of as little as 5 minutes.
  • People were encouraged to reduce their time sitting, even if they could not formally exercise, because this inadvertently meant that they would engage in many more small, but still meaningful bouts of activity.3

That leads us to our next topic:

What is the Difference Between Moderate and Vigorous Exercise?

I’m bringing this up here rather than earlier because now that we know how short exercise can be and still be useful, we can work on building the intensity of our exercise to further ease our task of accumulating adequate physical activity.

The guidelines also specify that either 150 minutes of moderate activity or 75 minutes of vigorous activity can be performed to achieve our weekly aerobic goals. We can also use a combination of moderate and vigorous activity minutes (at an exchange rate of 2 minutes of moderate activity for 1 of vigorous) to achieve our threshold of exercise.

For those of us on a time crunch, this is useful information.

How then, can we select activities that are perhaps more vigorous from the list above?

Well, the easiest way to decide whether you’re doing moderate or vigorous exercise is something called the Talk Test. 

First, I must mention that, like sweating, this is not something to be used to evaluate your exercise inside the pool since I do not want you to gulp chlorinated water. 

Essentially, if, while exercising sufficiently hard enough to increase your heart rate and your breathing rate AND you can speak in short sentences while exercising, then you are likely performing moderate exercise.

For example, you might be speed walking with your dog and say “Can I pass the talk test?” The dog doesn’t say anything constructive because, of course, they are unable. So, you pause for a breath, then answer yourself, “Yes, I can pass the talk test.” More than likely, you’re exercising at a moderate, or even low-intensity pace. Try to work hard enough that you can’t speak in whole paragraphs before breathing.  

If, however, you find yourself gasping for air between the words, “Pass,” and “The,” then you are likely performing vigorous exercise. Great job!

This is arguably the most low-tech way of determining your exercise intensity. It is also scalable to each person. We all have different levels of fitness, and we might require different levels of activity to reach the same benefits of exercise. 

Luckily, there are also several resources available to help determine whether specific activities meet the ACSM’s, CDC’s, and Department of Health and Human Services’s definition of moderate or vigorous activity. Here is a short list.4

As I said, exercise intensity varies between activity, but it also varies from person to person depending on their current health. This leads us to our last question of the week:

How Do You Know if Aerobic Exercise is Alright for You?

The short answer is: if you’re alive and breathing, you are currently undergoing some amount of aerobic activity. Still, it probably isn’t enough to make you healthier. 

However, there are certain conditions that might prohibit you from engaging in vigorous aerobic exercise. In fact, there are some who might find putting on their clothes in the morning to be a strain on their breathing. This is the case for someone with advanced chronic obstructive pulmonary disease (COPD). They would not benefit from running or kickboxing because they are already taxing their cardiovascular system to its limit with self-care activities.

It is for this reason, to reduce the risk of such chronic conditions, that we make the conscious effort to exercise and improve our health. 

Still, individuals with unmanaged chest pain, unmanaged high blood pressure, unmanaged diabetes mellitus, restrictive lung disease, obstructive lung disease, or osteoporosis would definitely wish to consult their primary care physicians before engaging in any rigorous exercise regimen. These are just to name a few. If you have ever been treated or diagnosed with ANY chronic condition, you should talk to your doctor to be cleared for exercise. 

There are also several considerations to be made when choosing your best form of aerobic exercise, especially if you are attending physical therapy for an injury. 

Be sure to consult your physical therapist and your primary care physician to discuss the potential risks and benefits of a particular form of exercise for you!

That said, aerobic exercise has been shown to be of such benefit that it should definitely be high on your list of new healthy habits to adopt in the new year! There is almost certainly one—if not several—forms of aerobic exercise that you would find rewarding and enjoyable while accumulating your 150 or so minutes of weekly activity.

If you need any ideas, just ask one of our excellent physical therapists!

Now, I’ve kept you from your holiday revels for long enough. We have a few more topics to cover in the next week before New Year’s, so hang in there!

Next week we’ll explore:

  • What resistance training is according to the guidelines
  • How you can build strength and muscle using common household items
  • How you can combine resistance training with aerobic activity to save time
  • AND how you can begin to incorporate both aerobic exercise and resistance training into your weekly routine.


  1. Patel et al. Aerobic vs anaerobic exercise training effects on the cardiovascular system. World J Cardiol. 2017 Feb 26; 9(2):134-138. Doi: 10.4330/wjc.v9.i2.134 
  2. Physical Activity Guidelines for Americans, 2nd Edition. 
  3. Joshua Schuna Jr. PhD. Changes in Guidance: Implications of the Physical Activity Guidelines for Americans, 2nd Edition.