The systematic study of the relationship between exercise and affective responses dates back approximately three decades. Initially, the emphasis was mainly on "mental health" benefits, such as reductions in anxiety and depression. Now, the focus has been expanded to include the study of the feelings that arise from exercise in general, such as perceptions of energy, of calmness, or fatigue.

My personal interest started from the following observation. I noticed that most people, although they may have the right motivation to exercise, they do not really know how to regulate the intensity of their exercise. This is simply because most people do not have experience in "reading" their bodies during exercise and no one tells them what level of intensity is most beneficial and what level can be detrimental. I think this has a lot to do with the phenomenon we all observe, namely that many people go to the gym a few times (e.g., after New Year's resolutions or in the beginning of summer) and then, because they overexert themselves or do not get the results that they wanted, they stop going back.

This issue has fascinated me for a long time. I realized that, although when you take a medicine, you also get instructions on how much to take, when to take it, for how long, etc, you do not get any instructions when it comes to exercise. People are left to figure it all out by themselves without much clear guidance and, on the contrary, with much confusion and many contradictions surrounding this issue.

The truth is that, as research shows, most people are not particularly good (without previous practice) at monitoring how their body reacts during exercise and adjusting their pace accordingly. As a result, we have many people who either overdo it and experience negative responses or even injuries and many who do not do enough, so after a while they get frustrated with the lack of visible results. The key, in my view, is to find the right balance: what "dose" of exercise is most beneficial without being aversive.

Let's first discuss what the "optimal" intensity of exercise is for people who want to exercise for health. More and more evidence in the exercise science literature shows that the optimal level of exercise intensity is at or near the point at which metabolism goes through the transition from aerobic to anaerobic. Let me explain.

For most people, aerobic activities include walking and similar moderate-intensity activities. For people who are a little more physically fit, even cycling or jogging are also mainly aerobic. In the aerobic range of intensity, the breathing rate is higher compared to resting conditions, but it can remain stable as you exercise. Therefore, in general, an aerobic activity is one that you can do for an extended period of time without feeling that you have to stop due of exhaustion.

Although we can exercise aerobically for an extended period of time, how intensely we can work out while using aerobic metabolism is limited. In other words, depending on how aerobically fit we are, our organism is capable of producing energy aerobically (i.e., using oxygen) only up to a certain rate. If we try to do more (i.e., go faster), we basically exceed our organism's capacity to provide the energy we need through aerobic metabolism. So, to keep up with the higher demand for energy, our organism starts to supplement its energy production by tapping into anaerobic metabolism (i.e., by using energy sources that do not require the use of oxygen). The problem is that the fuel sources that we have available for anaerobic metabolism are limited and very small compared to those available for aerobic metabolism. Therefore, they are depleted very quickly. As a result, once we enter the anaerobic range of intensity, our physiological responses can no longer remain stable. Our breathing rate starts to rise, our heart rate starts to rise, and so on, until we get exhausted. This is why we cannot run very far at a sprinting pace.

Research shows that when people exercise below or at the point of transition from aerobic to anaerobic metabolism, not only is exercise generally safer (associated with fewer injuries, for example), but it also seems to be most effective for accruing health benefits. Walking, for example, has been shown in recent studies to be associated with reduced death rates from various sources, to lower hypertension, to lower "bad" cholesterol (LDL and VLDL) and increase "good" cholesterol (HDL), to help maintain body weight (by helping to maintain a healthy balance between energy intake and expenditure), to help control diabetes, etc. On the other hand, for previously sedentary adults, exercising at an intensity that exceeds this point of transition from aerobic to anaerobic metabolism does not appear to offer any additional benefits, compared to exercise at lower intensity. To the contrary, there is evidence that the risk of injuries and cardiovascular complications increases, the ability of our immune system to fight off infections is reduced, and there can even be a negative effect on our cholesterol profile.

The question, then, is how to detect when we "cross the line" from aerobic to anaerobic metabolism. First, let me say that, although many people take their pulse as a way of keeping track of their exercise intensity, heart rate cannot tell you whether you exercise aerobically or anaerobically.

The most easy-to-detect sign that you have entered the anaerobic range is that your breathing rate increases sharply and you cannot keep it constant. There is something called the "speak test" - that is, if you can maintain a conversation while exercising, then you are probably exercising aerobically. If you are too winded to keep a conversation, then you are probably exercising anaerobically and you should slow down.

In addition, most people, regardless of age, gender, or level of fitness, when you ask them to describe the exercise when the intensity is in the anaerobic range, they say that it feels "hard" whereas they characterize lower intensities as "light".

The transition from aerobic to anaerobic metabolism is also the point at which most people start to say that they feel worse than they did before. However, I want to emphasize that, within certain limits, people differ in the intensity of exercise they prefer and the intensity that they can tolerate without having to stop. Again, this is regardless of age, gender, or level of physical fitness. So, some people may say that they start to feel worse slightly below the point of aerobic-anaerobic transition, whereas some may "hang in there" for much longer. Ultimately, however, the negative feelings will occur, whether it be sooner or later.

Finally, for people who like high-tech gadgets, there are a couple of small handheld devices on the market that measure an acid substance, called lactate, that begins to accumulate in large amounts in the blood when our muscles start to work anaerobically. These devices work exactly like the glucose analyzers used by diabetes patients. They take one droplet of blood from the tip of the finger and give results in 60 seconds. They cost about $300, but the prices are getting lower as the devices are becoming more popular. Using the trial-and-error method, one could experiment for a while until they find the right intensity, one that does not produce large increases of lactate.

The way we collect our data varies, from simple methodologies to some that are quite complex. On the simplest end of the spectrum, we may have individuals walk on a treadmill at a self-selected pace and have them respond to questionnaires about how they feel. On the more complex end, we do multiple sessions, starting with assessments of maximal aerobic capacity, we measure levels of lactic acid or various hormones in the blood, and even brain wave activity.

In many studies, we "manipulate" exercise intensity by increasing the speed and grade of a treadmill or adding more resistance on an exercise bike. In these cases, exercise intensity is expressed as percentages of age-predicted maximal heart rate.

A much better, but more technically difficult and expensive method, also used in some of our studies, is to conduct a "graded" or incremental exercise test to volitional exhaustion. During such tests, we gradually increase (every 1 or 2 minutes) the speed/grade of the treadmill or the resistance on the bike. Most importantly, we collect the exercisers' expired gases and we analyze them using a computerized system on a breath-by-breath basis for their oxygen and carbon dioxide content. This way, we can find each person's maximal aerobic capacity (maximal rate of oxygen uptake), but also the point at which the rate of work exceeds the rate that energy can be produced through aerobic mechanisms and energy production needs to be supplemented by anaerobic metabolism. We call this the point of transition from aerobic to anaerobic metabolism and it can differ substantially between two people, even if they are of the same sex, same age, same weight, and even same maximal aerobic capacity.

In addition to measuring affective responses during such incremental exercise tests, we also do subsequent tests at intensities that correspond to certain percentages of maximal aerobic capacity (e.g., 40%, 60%, 80%) or intensities that are below, at, or above the point of transition from aerobic to anaerobic metabolism.

The way we find out about "affective responses" is by asking people before, after, as well as every few minutes during the exercise sessions to respond to certain standardized questionnaires. Before and after exercise, we use questionnaires that include up to 20 or so questions and measure such things as perceived energy, tension, tiredness, calmness, enjoyment, etc. During exercise, we use single-item rating scales that measure affective valence (feel good versus feel bad), perceived activation (feel "high" versus feel "low"), and perceived exertion (how strenuous the exercise feels). We use paper-and-pencil tests, as well as computerized questionnaires that people respond to by touching a computer screen that is placed in front of the treadmill or the bike.

From our research, we have found that, when walking at their own pace, about 75-80% of people report that they feel better during and for some time after the walk. When they bike at a moderate pace (about 60% of their maximal capacity), about half of the people report that they feel better and half of the people say that they feel worse during the ride, but almost all say that they feel better (more energetic) afterwards. When people do more vigorous activities than that (e.g., vigorous running, particularly at an intensity that exceeds the point of the aerobic-anaerobic transition), about 85-100% say that they feel worse during the activity and for some time afterwards.

Until now, our studies have been with young and healthy volunteers (average age of 22-25 or so). From now on, we are beginning to look into older adults. We are about to finish a study on the effects of walking in older adults (average age a little over 50) and we will shortly begin a study with middle-aged adults.

Although age comparisons cannot yet be made with our own data, other researchers have provided some clues. It appears that, for young and old alike, typically "mild" exercise intensities, such as walking, are pleasant, whereas strenuous intensities, such as a treadmill test, are unpleasant. In between these two extremes, things are less clear, with many differences between people, even of the same age.

I think that, as far as exercise for health is concerned, this kind of mentality, which is a legacy of high-school and college sports, can have devastating effects.

I use my dad as an example. My dad is 69 and, I am sorry to say, has been a chain smoker for over 55 years. He is also a retired PE teacher and track coach, but he has been physically inactive for many years. Like many people his age, he is also a bit overweight.

Although he clearly has a much better understanding of human physiology than the average person, he recently told me that, one day, he decided to go for a run. Needless to say, I was terrified and extremely thankful that nothing bad had happened.

Research shows that exercise, particularly vigorous exercise, is associated with a significantly increased risk of "sudden deaths" due to cardiovascular complications (e.g., blood clots leading to heart attacks) among people who have been sedentary for many years.

The "no pain, no gain" mentality is totally incompatible with exercise for health. It may have some meaning, assuming that one uses scientific principles of training, when one is interested in maximizing the performance of a developing athlete. Applying this principle to exercise for health is clearly misguided and can have deadly consequences.

Moderate-intensity activities, such as walking, have been shown to offer an array of health benefits. In fact, research shows that the largest health benefits occur when one goes from being sedentary to being moderately active. When one goes from being moderately active to being highly or vigorously active, then there are some additional benefits but these gains are smaller by comparison.

Most people are not aware of the current recommendations are for physical activity and may be confused about how much they are supposed to be doing. So, let me quote an excerpt from recommendations from the National Institutes of Health here. They are almost identical to recommendations from the Surgeon General, the Centers for Disease Control, and the American College of Sports Medicine.

"We recommend that all people in the United States increase their regular physical activity to a level appropriate to their capacities, needs, and interest. We recommend that all children and adults should set a long-term goal to accumulate at least 30 minutes or more of moderate-intensity physical activity on most, or preferably all, days of the week. Intermittent or shorter bouts of activity (at least 10 minutes), including occupational, nonoccupational, or tasks of daily living, also have similar cardiovascular and health benefits if performed at a level of moderate intensity (such as brisk walking, cycling, swimming, home repair, and yardwork) with an accumulated duration of at least 30 minutes per day. People who currently meet the recommended minimal standards may derive additional health and fitness benefits from becoming more physically active or including more vigorous activity."

If there is "pain" or discomfort, people are also unlikely to want to continue the activity over the long haul. As adults, we have free will. Nobody can really force us to exercise. So, if we do, the motivation has to come from within. And, naturally, we generally do not like to do things that make us feel bad, we like doing things that make us feel good. I quote from the text of the federal "Healthy People 2010" program:

"Each person should recognize that starting out slowly with an activity that is enjoyable and gradually increasing the frequency and duration of the activity are central to the adoption and maintenance of physical activity behavior."

Likewise, the National Institutes of Health have stated that "moderate-intensity activities are more likely to be continued than are high-intensity activities".

Let me point out, however, that the emphasis on moderate-intensity activities is relatively new. In other words, it has not always been this way. The philosophy on this topic changed dramatically after the mid-1990's. Before that point, the official guidelines assumed that the main way to get health benefits was through the improvement of cardio-respiratory fitness. To that end, what was considered necessary was 3-5 times per week, for 20-60 minutes, at approximately 50-80% of maximal heart rate. At some point, researchers came to the realization that these levels of activity were unattainable for most people and may, in fact, have scared many people away from exercise: "If I cannot do what is required, why bother starting?"

I have not mentioned this so far, but it is also very important to do some strength exercises as well. Particularly as people get older, they lose bone (osteoporosis) and muscle mass. These are the two primary contributors to older people becoming less stable as they walk, increasing their risk of falls and fractures, or becoming incapacitated and losing their independence. Again, I quote from the guidelines of the National Institutes of Health:

"Developing muscular strength and joint flexibility is also important for an overall activity program to improve one's ability to perform tasks and to reduce the potential for injury. Upper extremity and resistance (or strength) training can improve muscular function, and evidence suggests that there may be cardiovascular benefits, especially in older patients or those with underlying cardiovascular disease, but further research and guidelines are needed. Older people or those who have been deconditioned from recent inactivity or illness may particularly benefit from resistance training due to improved ability in accomplishing tasks of daily living. Resistance training may contribute to better balance, coordination, and agility that may help prevent falls in the elderly."

Our next study will be with "usually" sedentary middle-aged adults, people of an average age of about 40 or so who have been sedentary or only sporadically active over the previous year.

We will look into the issue of self-selected exercise intensities. We first want to see what intensity these people will select if asked to choose (rather than us imposing an intensity on them). Where is that level of intensity in relation to their maximal aerobic capacity? Where is it in relation to the point of transition from aerobic to anaerobic metabolism?

We know that people will differ substantially. From previous observations, we have noted that some people go for more intense exercise, some go slow -- even if the people are of the same age, gender, or aerobic capacity. So, we will examine whether we can predict the level of intensity that they will choose based on three factors: (a) trait differences in preference for exercise intensity, (b) trait differences in tolerance of exercise intensity, and (c) self-efficacy, the belief that people have that they are capable to cope with vigorous exercise.

The second question we will examine is what happens when we "force" them to exercise at an intensity that exceeds their self-selected intensity by a small margin (10% of maximal capacity). We want to see if that will cause a deterioration of affect and, again, whether we can predict that change on the basis of preference, tolerance, and self-efficacy.

Our long-term plan is to develop a new method for teaching formerly sedentary adults who start an exercise program to better self-monitor and self-regulate the intensity of their exercise efforts. In general, when asked to estimate the intensity of exercise or reproduce a previous intensity, most people are way off in their estimations. The two methods that are being used now (measuring heart rate and trying to stay within a "target" zone and using a rating scale of perceived exertion) have some serious limitations. Both heart rate and ratings of perceived exertion have a linear relationship to exercise intensity (they go up as intensity goes up).

Using affect may be a better option. Affect has a curvilinear relationship to intensity, so people may be able to detect when something that felt "good" starts to feel "bad". Remember that the "turning point" for going from "good" to "bad" in most people is very close to the point of transition from aerobic to anaerobic metabolism, which also happens to be the optimal exercise intensity for accruing health and fitness benefits for previously sedentary adults.

The data that we have collected from several studies have been published in some of the best academic journals in public health, behavioral medicine, health psychology, and exercise science.

The next step will be to try to influence the public health recommendations that are being issued to the public. Right now, how exercise is prescribed to the public is being guided almost exclusively by physiological considerations, as if the body did not have a head. Fortunately, we do have heads, and psychological considerations maybe just as important. If a person has had negative experiences from exercise in the past, that person is unlikely to have the motivation to exercise. So, all the physiology in the world will not make one bit of difference as long as that person remains seated in front of the TV screen.

I am hoping that in a few years the instructions given to exercise leaders, fitness professions, health care workers, etc. will be revised to emphasize that the amount of exercise we ask people to do may have not only physiological, but also psychological and motivational consequences. At some point, I hope to help put the whole "no pain, no gain" mentality to rest.