Longer duration lower intensity exercise made up the majority of early human activity.³^–⁵ Walking was the chief means of movement for millions of years. Early humans walked most of the day and did this everyday. Their survival was dependent on it. Research suggests that early Homo sapiens foraging on the plains of Africa covered a range of over 10 miles daily hauling weapons, game, and tools.³^–⁶ Analysis of modern day hunter–gatherers confirms this degree of walking activity.

In addition to this lower intensity activity, survival also depended on higher intensity shorter duration activity. Sprinting away from danger or after wild game as well as lifting and hauling were essential. Both women and men engaged in these more anaerobic bouts of exercise. Although in most cultures women did not participate in the hunt, they did lift and haul wood, help construct shelters, and carry babies. The average baby was carried almost 1000 miles in the first 2 years of life in more traditional cultures.⁴ The more intense activities of hunting, hauling, and foraging were done in what is termed the “paleolithic rhythm,” where more intense activities were spaced out over 2 to 4 nonconsecutive days per week.^4,⁶

Moderate intensity exercise, much like aerobic exercise today, was also a part of life. However, this exercise did not take the shape of going for a jog or leisure run. Aerobic exercise was mostly in the form of sporadic running after food and dancing, which occupied an important place in the culture of ancient peoples and still does in modern day hunter–gatherer societies. These dances can last from one to a few hours and take place several times during the week.⁴^–⁶

The movement patterns of our historical ancestors were a necessary part of life and were fully integrated into the lifestyle. This movement was what could be best described as “cross-training,” with elements of lifting, jumping, running, walking, throwing, climbing, hauling, hiking, or whatever movement pattern was required to procure food, stay protected from the elements, and defend territory. Our species is adapted from an environment where food was not guaranteed and a nomadic hunter–gatherer way of living dominated. This understanding is essential for the appropriate prescription of exercise. Hunter–gatherer societies in the past and present have been analyzed through a number of different scientific tools, which show that they do not experience the same degenerative and chronic diseases of modern westernized peoples.^3,^7,⁸

After taking into account body weight, the modern day human energy expenditure is estimated to be only 38% that of our hunter–gatherer ancestors.⁴ It has been shown that for modern day humans to approximate the level of activity of hunter–gatherers, they would have to walk 12 miles/day in addition to other activity.⁴ Policy leaders on appropriate levels of human activity, such as the American College of Sports Medicine, recommend movement patterns estimated to be 44% less than that of our prehistoric ancestors and modern day hunter–gatherers.⁴

Several studies have shown that total energy consumption from exercise may be the primary health promoting property of movement.^4,^9,¹⁰ Given the drastically different environment in which humans now reside, we are left with a serious dilemma. How do we get people to do an activity not required for their survival, that takes time, and is not an enjoyable activity for many? It is useful to remember that the number one reason given for not participating in exercise is time.^11,¹² These facts, as well as the historical frame of reference provided by evolutionary movement patterns, create a problem. Exercise prescriptions must deliver acceptable caloric expenditure, should mimic the cross-training attributes of our ancestors, but also be time conscious and “doable” for the average population.

A New Perspective on Movement

It is easy to look at the data presented and quickly move to a “more is better” approach to exercise. However, movement quality is an important consideration before simply instructing more exercise. Quality in this segment is discussed in terms of more functional and efficient movement. In other words, movement that provides the perfect balance of range of motion and stability leads to more efficient animation of the body. The body is a chain of parts that are completely integrated. Individual exercises, although viewed in isolation by us, are actually complex and highly entrained patterns of neurologic connections translated into muscular action. These “movement patterns” are the building blocks on which all exercise is constructed. Without a strong foundation, the benefits of exercise can sometimes become a liability, increasing the risk of injury, pain, and dysfunction.

It is both irresponsible and shortsighted to focus exercise prescriptions solely on delivering greater metabolic stimulus if there is not a strong and appropriate base of mobility and stability through basic movement patterns. In other words, doing more activity may not be beneficial if it simply reinforces poor movement mechanics. We must keep in mind that our ancestors were able to move more, because they also moved better.

Human development and survival from infancy to adulthood required mastery of movement. The neurologic and muscular systems are built as a unit and cannot be separated. Movement should not be thought of in terms of isolated anatomy but rather as an integrated system that moves in patterns. A squat, for example, is not simply a matter of contracting certain muscles that move joints. It is instead a coded and rehearsed pattern of neurologic firing translated into physical movement.

Gray Cook, physical therapist and the foremost expert on functional movement, is the author of two of the most foundational pieces of work in movement science. In his books, Athletic Body in Balance: Optimal Movement Skills and Conditioning for Performance and Movement: Functional Movement Systems, Cook described movement science as analogous to the relationship between computer software and hardware.^13,¹⁴ The muscles and joints act as the hardware, but the neuromuscular firing pattern is the software. Software obviously is what controls and informs the computer, and so it is with the nervous system controlling the muscle. To address movement, he rightly pointed out that we need to work movement patterns and not simply isolate parts.

The brain does not view exercise as individual muscles working. Instead, the brain has a code or a pattern for each movement. This pattern consists of thousands of different neurons firing in very specific ways to elicit a coordinated action, resulting in a movement. Each movement is to the brain like a computer software program; movement science calls this a motor program. It is these motor programs that need to be attended to in terms of movement quality.

Although exercise science has developed core principles and protocols to be followed, movement has not developed such protocols. Up until very recently, there has been no standardized understanding of what constitutes normal functional movement. This gap has now been filled by the work of Gray Cook and his colleagues. They have put together a system of screening and analyzing movement so it can be objectively measured, tracked, and corrected. If we are to use movement as medicine and stay true to the natural medicine axiom of “first, do no harm,” we must establish a standard of practice in movement medicine.

Standards of Movement

The Functional Movement Screen and Selective Functional Movement Assessment

The Functional Movement Screen, or FMS, as it is popularly known, is “a ranking and grading system that documents movement patterns that are key to normal function.” By acting as a screen for movement, the FMS pinpoints movement dysfunctions that may predispose to injury and/or inefficiency.

The FMS has a scoring system that allows for a hierarchical ranking of the most limiting patterns or asymmetries unique to an individual. This in turn provides direction to clinicians as to what corrective strategies should be implemented and when. The FMS score can then be used to systematically address underlying movement dysfunctions and restore a base of movement from which conditioning can be built.

The FMS is complimented by the Selective Functional Movement Assessment (SFMA). The SFMA is the diagnostic system of functional movement. For the clinician concerned with movement, the two work hand-in-hand. The FMS is both the precursor to and the follow-up from the SFMA. The FMS looks at seven key movement patterns, including a squat, lunge, and hurdle step. Its purpose is to ascertain a baseline of mobility and stability as well as to determine if movement is pain free. The SFMA is the clinician’s tool for diagnostic determination of when pain is present. Together, the screen and the assessment provide a movement system that allows tailored movement plans that can find, address, and track changes in movement from pain and dysfunction to recovery and optimal movement.

The FMS is a screening tool for the clinician; it is not diagnostic, but rather prognostic. The SFMA is diagnostic and helps the clinician pinpoint the issue and further prioritize treatment. Once a patient has been discharged from treatment and no longer has pain with movement, movement quality must still be measured. This is where again the FMS is used.

Several recent studies showed the FMS to be a reliable and repeatable screening tool for movement quality, but also proved its ability to predict injury in several high population groups. Minick et al ¹⁵ showed that the FMS demonstrated substantial inter-rater reliability by both novice and expert users. This demonstrates an important quality of any clinical tool, that of repeatability and objective evaluation in the hands of different users.

When the FMS was put to the test in very active population groups its usefulness became clear. American football players were analyzed in a 2009 study in the Scandinavian Journal of Medicine, Science and Sports; they demonstrated reliable improvements in FMS scores from a standardized intervention based on the screening examination and established a FMS score cutoff predictive of increased injuries.¹⁶

Firefighters benefitted from a FMS guided functional exercise program.^16a The FMS guided intervention resulted in a 62% reduction in time off of work due to injuries and reduced injuries by 42% during a 12-month period.

Movement pattern performance is an important precursor to increased exercise participation and is an important addition to the exercise world. The FMS and SFMA are new and essential tools for the use of exercise as medicine. They provide a much needed “standard operating procedure” for movement quality. Gray Cook and colleagues have a clinical website (www.functionalmovement.com) that teaches and instructs on movement systems and provides valuable clinical tools for clinicians.

New Insights on Exercise for Weight Loss

Once movement quality is attended to, exercise quality needs to be addressed. The current approaches of exercise for weight loss and health are still centered in the low intensity “aerobic zone,” calorie-burning paradigms. Recent understanding is redefining this narrow approach to health and fitness. A caloric focused model of metabolism may not be the most useful way to view exercise. If we look at track athletes, both elite marathoners and sprinters have very low percentages of body fat. Sprinters, however, have less body fat and higher amounts of muscle mass, yet they exercise in a very different way.¹⁷^–¹⁹ Sprinters engage in short bursts of anaerobic effort lasting seconds, whereas marathoners run in an “aerobic zone” for hours and utilize large amounts of caloric energy. If the aerobic model of exercise truly is the best way to gain fitness and fat loss, why is there a discrepancy between these two groups of athletes? In analyzing recent data, we can see that anaerobic contributions to energy expenditure now need to be considered in exercise prescription.

There is a common misunderstanding of fat burning percentages in exercise. Many still perceive exercise done in the aerobic zone, usually defined as between 65% and 80% of maximum heart rate (MHR), burns the most fat per unit time. This is not the case. Lower exercise intensities burn a higher proportion of fat compared with sugar, relatively speaking. However, exercise beyond the aerobic training zone burns more total energy and fat because it is a higher intensity.

Suppose two people exercise for 30 minutes. Person A jogs at an intensity of 60% MHR, whereas person B jogs at an intensity of 60% MHR and then, every few minutes, sprints for a short period, reaching over 90% MHR before returning to a lower intensity. Assume person A burned 200 calories, 60% coming from fat and 40% from sugar. Person A therefore used 120 total units of fat and 80 units of sugar (200 calories × 0.60 = 120 fat calories). Person B exercised at a higher intensity, burning a lower percentage of fat. Assume Person B used 50% fat and 50% sugar, but burned 100 more calories due to the higher workload. Person B therefore burned 150 units of fat and 150 units of sugar (300 × 0.50 = 150 fat calories). When the energy burned is totaled, person B burns more energy (300 calories) and more total fat (150 units compared with 120 units) than person A, despite using a lower percentage of the energy as fat. This example shows that higher intensity exercise of the same duration surpasses lower intensity exercise in fat calorie use. There are further metabolic consequences created through higher intensity exercise that make its application in exercise prescription for fitness and fat loss more intriguing.

Does Aerobic Exercise Work for Weight Loss?

A 2009 review by Melanson et al ²⁰ looked at the impact exercise had on metabolic stimulation. The study primarily looked at moderate intensity aerobic exercises like jogging, biking, or swimming while including a small sample of anaerobic exercise studies. It showed that aerobic exercise of moderate intensity did not provide a metabolic advantage aside from the calories burned during activity. A previous meta-analysis done over a 25-year period came to a similar conclusion.²¹ This study analyzed the data from over 400 studies comparing diet alone, aerobic exercise alone, or diet plus aerobic exercise on weight loss. The results showed that aerobic exercise did not provide a significant advantage to weight loss over diet by itself. Although aerobic exercise has been shown to be a reliable tool in the maintenance of weight loss,^22,²³ these studies suggest it may not be enough to elicit significant fat loss effects.²¹

At the same time, new lines of research show potentially promising effects from more anaerobic modalities. High-intensity interval training (HIIT) is a method of alternating highly anaerobic activity with more relaxed aerobic movement. This method allows the benefits of intense exertion under more tolerable conditions. Rest is required to sustain intense activity for any length of time. Weight training is another anaerobic-centered activity. These more anaerobic-dominated exercise approaches have unique benefits that may compliment the calorie burning effects of aerobic exercise.

Aerobic Versus Anaerobic Exercise

In very simple terms, aerobic metabolism takes place in the mitochondria and requires the use of oxygen. Anaerobic metabolism proceeds through a different pathway and requires neither the involvement of mitochondria or oxygen. It is well known that as exercise intensity increases, anaerobic metabolism dominates; unfortunately, the exact anaerobic contribution to energy production is exceedingly difficult to measure. The standard way to approximate calorie expenditure and substrate utilization during exercise is through the measure of respiratory gases. The ratio of carbon dioxide expelled to oxygen consumed can give a predictable evaluation of not only energy use but also fuel utilization—glucose versus fat.

However, this method is only valid at lower exercise intensities. At higher intensities, the relationship is less clear. To help address this error, researchers also measure excess postexercise oxygen consumption (EPOC). This is a measure of the recovery energy expenditure after exercise and it has been thought to consist of anaerobic contributions to exercise as well. There is some argument as to how meaningful this EPOC effect can be. Many researchers claim the impact does not last long, only several hours, and amounts to at best 15% of total calories burned.²⁴^–²⁶ However, these approximations come largely from studies with lower exercise intensities involving standard aerobic exercise protocols.

Studies utilizing highly anaerobic protocols including cardiovascular interval protocols and weight training showed a much different picture. In 2001, Schuenke et al ²⁷ showed that circuit resistance training, utilizing heavy weights and short rest periods lasting only 31 minutes, were able to generate an EPOC that persisted for 48 hours. The results showed that metabolism 24 and 48 hours after the exercise session was increased by 21% and 19%, respectively. The researchers pointed out that for a typical 180-lb individual “this equates to 773 calories expended post exercise” while resting and as a result of the workout. This is far from insignificant and greatly exceeds the 15% many researchers quote for EPOC. Similar findings were shown in women using a similar resistance training protocol. In women, the elevation in metabolic rate lasted 16 hours.²⁸ The same findings were seen with HIIT protocols with significant EPOC values lasting up to 24 hours.^29,³⁰

Exercise Burn and “After-Burn”

Dr. Christopher Scott of the University of Southern Maine published extensively in this area and is the author of one of the authoritative textbooks in this field, A Primer for Exercise and Nutritional Sciences: Thermodynamics, Bioenergetics, and Metabolism.³¹ In his works, Dr. Scott pointed out that EPOC did not fully explain anaerobic energy use and that the anaerobic contributions to exercise might be even greater than originally thought, especially where lactic acid production is concerned. Dr. Scott emphasized that to fully account for calories burned during exercise, three components must be measured: (1) calories burned aerobically during exercise, (2) calories burned aerobically after exercise (EPOC), and (3) anaerobic calories burned from exercise.³²^–³⁶ EPOC and the anaerobic lactic acid measurements for exercise should be considered separately according to Dr. Scott.

A 2005 study by Dr. Scott demonstrated the potential ramifications of anaerobic exercise. This study compared a 3.5-minute aerobic exercise challenge with 3 work-equivalent 15-second sprints.³⁷ Calorie use during the exercise bout was calculated to be 29 kcal for the aerobic exercise and approximately 4 kcal for the sprinting. However, when the EPOC contribution was added to the two exercise bouts, energy use rose to 36 kcal for the aerobic bout and 39 kcal for the sprint exercise. Finally, when the purely anaerobic contribution was added to the calorie totals, the numbers for the anaerobic sprint exercise rose significantly. The final tally was 39 kcal for the aerobic exercise compared with 65 kcal for the sprint exercise. By adding both EPOC and the anaerobic contribution to the original calorie total, the sprint exercise was shown to far surpass the aerobic exercise in calories burned. This is interesting when one considers the aerobic exercise session took over four times longer to complete (210 vs 45 seconds). Without including both the EPOC and anaerobic energy use, a full 94% of the calories used during the sprinting would go uncounted.

In studies published in 2006 and 2009 in the Journal of Strength and Conditioning Research, the same researchers quantified anaerobic energy use during weight lifting.^32,³⁶ Using the method of measuring and quantifying all three components of calorie burn (aerobic metabolism during exercise, EPOC, and anaerobic contributions), these studies showed that weight training exercise burned 70% more calories than originally thought.

Hormonal Versus Caloric Exercise

To understand the full ramifications of this new exercise science, the discussion must move to hormonal metabolism. Hormones as we describe them in this chapter refer to all signaling molecules in the body, including steroid hormones, muscle-derived cytokines (myokines), and other signaling molecules. Hormonal messengers directly influence the fat burning stimulus during exercise and may be responsible for part of the EPOC after-burn as well.

Brief bouts of anaerobic exercise appear to adjust hormones for greater caloric burn during and after exercise.³⁸ This increased energy use is partly explained by EPOC. This is a measure of how much oxygen the body consumes in the hours and days after a workout. An example of EPOC in the acute sense is climbing a long flight of stairs. While walking up the stairs breathing is labored, but respiration becomes most difficult after reaching the top. The body does this to recover the “debt” of oxygen used during activity. The EPOC created by climbing a flight of steps is an example of the much larger metabolic effect created from intense movement.

Exercise of sufficient intensity elevates stress hormones like adrenaline, noradrenaline, and cortisol. Together these hormones ensure the switch to glucose metabolism, which historically supplied the energy to fight or flee. As exercise intensity is elevated further, anaerobic contributions to metabolism increase. This produces lactate (lactic acid), which, contrary to common belief, is not a waste product, but a physiologic buffer and signaling molecule.³⁹^–⁴¹ As lactate rises, it is correlated with, and some studies suggest actually induces, the anabolic steroids testosterone and human growth hormone (HGH).⁴²^–⁴⁴ This “hormonal soup,” catecholamines along with cortisol, HGH, and testosterone, acts synergistically to increase postexercise fat loss and lean muscle tissue production, thus creating a more fit and functional physiology.

Aerobic and anaerobic exercise have different hormonal effects. Hormones do not work in isolation, and like people they behave differently depending on the social environment. Interestingly, the hormone cortisol, often seen as a fat storing hormone due to its insulin desensitizing effect, behaves differently when combined with growth hormone and testosterone.^41,^45,⁴⁶ When cortisol is “socializing” with testosterone and growth hormone, its catabolic action on muscle is blocked, fat storing at the belly is attenuated, and the three may synergistically enhance fat burning.⁴⁶^–⁴⁹ Attempting to blunt the cortisol response to high intensity exercise may be counterproductive for fat burning and not necessary in the context of growth hormones.⁵⁰^–⁵⁴

Long duration lower intensity cardiovascular exercise behaves differently in regards to cortisol. There are also key changes in hunger hormones that are different between lower intensity aerobic exercises compared with higher intensity anaerobic workouts. With aerobic zone exercise, there is a danger of cortisol increasing unopposed by the growth promoting hormones.⁴⁵ This, along with a lowering of leptin and an increase in ghrelin, can lead to compensatory eating with specific cravings for fatty, sugary, and salty foods.⁵⁵^–⁶¹ Higher intensity shorter duration activity has the opposite impact on ghrelin, with a more balanced ratio of catabolic versus anabolic hormones.^58,⁵⁹ This may explain why standard aerobic prescriptions have not been shown to be as effective for optimal body composition.^20,^21,⁶²^–⁶⁵

Aerobic Zone Versus Interval Training

A 2001 study compared standard aerobic zone training and anaerobic interval exercise in women.⁶⁶ The anaerobic interval group exercised for 2 minutes at a highly intense 97% of MHR. They then rested by doing 3 minutes of low intensity activity. The more aerobic group performed moderately intense activity at close to 70% of MHR. The researchers made sure that each group burned 300 calories. Despite exercising longer and burning the same amount of calories, the aerobic group lost less body fat at the end of the study compared with the interval group. In addition, fitness in the interval group was substantially greater than in the aerobic group.

A similar study published in the same journal in 1996 showed that an anaerobic trained interval group burned significantly more fat than their aerobically trained counterparts.⁶⁷ Not only did the interval group burn greater amounts of fat during exercise, but they also exhibited increased fat burning effects that persisted for 24 hours after the exercise had stopped. The interval group was able to accomplish this with an exercise session that was a full 15 minutes shorter than the aerobic group.

A 1994 study ⁶⁸ tracked two groups of people, one group doing aerobic training for a period of 20 weeks, whereas a second group was followed for 15 weeks and engaged in HIIT. The researchers wanted to see how each program would impact body composition. The aerobic group burned 48% more calories than the interval group (120.4 vs 57.9 MJ) during exercise. The interval group, however, enjoyed a ninefold greater loss in subcutaneous fat. At the conclusion of the study, muscle biopsy analysis showed resting levels of 3-hydroxyacyl coenzyme A dehydrogenase, a marker of fatty acid oxidation, were significantly elevated in the interval group, but not in the aerobic group.

A 2008 study looked at intense intermittent exercise compared with steady-state aerobics.⁶⁹ Forty-five healthy women between the ages of 18 and 30 were recruited for the study, divided into 3 groups, and studied for 15 weeks. One group did HIIT, where they sprinted on a bike for 8 seconds followed by a 12-second rest. This was repeated for 20 minutes. Another group did moderately intense peddling that was sustained for 40 minutes. The final group did no exercise. At the end of the 15 weeks, the high intensity interval group lost 2.5 lbs of fat, whereas the aerobic group actually gained 0.6 lbs of fat. A measure of the fat-related hormones leptin and insulin were also positively affected in the HIIT group compared with the steady-state group. This was accomplished with a workout that was half as long (20 vs 40 minutes) as the steady-state’s group.

Resistance Training Studies

Research hints that resistance training may also provide unique benefits for fat loss and fitness. Recent analyses showed that the clear distinctions once set for aerobic exercise and resistance training are no longer delineated so clearly. Circuit training routines have been shown to provide an aerobic stimulus great enough for cardiovascular benefit while providing strength-training benefits.^70,⁷¹ Resistance training may also have great applicability for not just fitness and fat loss but also diseases of insulin resistance.^72,⁷³ Weight training, compared with aerobic exercise, greatly attenuates the natural loss of muscle mass that occurs with aging and dieting,⁷⁴ and improves body composition and self-esteem better than aerobic exercise modalities.⁷⁵^–⁷⁷

Resistance training may also have a much greater impact on EPOC. Two studies already discussed ²⁸ showed significant metabolic elevations in men lasting up to 48 hours and in women lasting up to 16 hours. These workouts used exercise regimens that were more intense than most studied resistance-training programs, but they were also shorter. Combining the benefits of resistance training with cardiovascular exercise seems to have the most benefit.⁷⁸ This type of training is called concurrent exercise and involves resistance-training workouts that are followed immediately by aerobic exercise or vice versa. Studies showed that these approaches afforded the benefits of both aerobic and resistance-training workouts.⁷⁹^–⁸² This “cross-training” approach falls in line with historic movement patterns and saves time.

Concurrent exercise approaches follow two patterns, serial concurrent exercise (SCE) and integrated concurrent exercise (ICE). In SCE, the two modalities (aerobic and resistance training) are done one right after the other, whereas in the integrated format the two modalities are alternated; a weight training movement is done followed by a cardiovascular “burst” of exercise. Based on studies, it appears the benefits of this type of approach can be amplified further using the ICE approach. In a series of three studies in 2008, all published in the Journal of Strength and Conditioning Research, Davis et al ⁸³^–⁸⁵ showed the potential benefit of ICE protocols. An ICE workout consisted of brief 60 second bursts of “cardioacceleration” placed between traditional weight training sets. This protocol was able to dramatically amplify multiple fitness parameters over and above the same workout volume done in an SCE format.⁸⁴ Fat loss in the ICE group was close to 10 times greater than in the SCE group over an 11-week period. This constituted a time commitment of just over 4 h/week. Because the work was equivalent in both groups, it was the combination of exercises that made the difference. This same workout protocol showed the ability to improve cardiorespiratory and cardiovascular parameters even in well-trained athletes.⁸³ It also decreased the delayed-onset muscle soreness, the feeling of soreness lasting 24 to 48 hours after the workout, in the ICE group.⁸⁵ Similar protocols showed the same promising results in body composition change and cardiovascular benefits.^86,⁸⁷

Safety of Interval Exercise

HIIT, as well as intense weight training, can be used effectively and safely when combined with the appropriate use of HR monitoring, perceived exertion rate (PER), and the use of intervals: periods of exertion followed by rest. Studies show this type of activity is manageable in several illness models, including chronic obstructive pulmonary disease,^88,⁸⁹ post coronary artery bypass patients,⁹⁰ congestive heart failure,⁹¹ and heart transplantation patients.⁹² This type of anaerobic stimulus more realistically mimics real-world challenge and allows for self-paced exercise that is safe, tolerable, and more beneficial for many heart and lung patients.⁸⁸^–⁹⁶ Cardiac patients also may have less risk with this type of activity, since it has more favorable affects on ST-segment changes and heart rate variability (HRV).⁹⁴^–⁹⁶

Monitoring HR is useful for any health care provider prescribing exercise. It is important to understand that HR equations are merely estimates based on age; there is much variability. HR equations, in general, underestimate HRs in the very fit. The old HR equation of 220 − age is an inferior HR equation that underestimates HR in the old and overestimates in the young.⁹⁷ Newer equations allow for better predictive value. Based on current understanding, women and men should use separate equations for predicted HR percentages. For men, the MHR equation should be 208 − age (0.7).⁹⁷ For women this equation should be 206 − age (0.88).⁹⁸ It is also useful to know the equation for translating percent of MHR to percent of oxygen uptake (VO₂) and vice versa. That equation is %MHR = (0.64) %VO₂ + 37.

Because multiple drugs and disease states can interfere with HR, exertion ratings used alone or combined with HR are a more useful clinical tool. In exercise research, a 16-point exertion scale called the Borg scale is used. It is a cumbersome tool to use and teach in clinical practice. A simple 1 to 10, with 10 being maximum exertion and 1 being at rest is a far more useful tool clinically. It is easy to teach and implement. Traditional aerobic exercise scores between 6 and 8 on the 1 to 10 scale. HIIT will reach between 8 and 10 on the work phase and between 1 and 4 on the rest phase. In clinical practice, this is a useful aid in working with exercise intensity. Combining HR percent monitoring with PERs and heart rate recovery (HRR) allows for tight control of workout safety.

Two other useful clinical tools for health care practitioners are the “talk test” and HRR. Exertion and respiration are closely linked. The ability of a person to talk during exercise is a direct indication of whether they have crossed into the anaerobic zone.⁹⁹ For the unfit, this usually occurs around 55% of VO₂ max and for the very fit it occurs around 85% VO₂ max. This corresponds to 72% MHR and 91% MHR, respectively, leaving the average healthy exerciser right around 80% of MHR for the anaerobic threshold, at which point their ability to talk will be compromised. Correlating this “talk test” with an individual’s HR allows practitioners to closely monitor exercise. HRR is a measure of how fast the heart recovers from exertion and is an indication of sympathetic and parasympathetic tone. A healthy heart should recover at least 25 beats/min or more within 1 minute after exertion. A heart that recovers 10 or less beats in 1 minute is a concern that should constitute a referral to a cardiologist.¹⁰⁰

Muscle–Body Messengers and Inflammation

Every time the body moves, muscles release signaling molecules that communicate to the rest of the body. The endocrine properties of muscle, like fat, have been confirmed.¹⁰¹^–¹⁰³ In the case of muscle, compounds called myokines are released in response to voluntary contraction. Myokines are cytokines, yet are derived specifically from muscle. These myokines give instructions to the body about how to function and adapt as well as hold the key to controlling chronic inflammation.

Interleukin-6: The Exercise Factor

The most important myokine related to muscle and inflammation is interleukin-6 (IL-6). When muscle contracts, IL-6 is released. IL-6 is a well-known cytokine that has long been thought to be inflammatory in nature and part of what is known as the inflammatory triad: tumor necrosis factor-α (TNF-α), IL-1, and IL-6. However, like people, IL-6 seems to behave differently depending on its origin, amount, and other cytokines around it. When released from muscle and in high concentrations without TNF-α and IL-1, IL-6 is anti-inflammatory.^104,¹⁰⁵ IL-6 acts to reduce the amount of TNF-α and IL-1 in circulation by increasing the cytokine inhibitors, IL-1 receptor antagonist (IL-1ra), and soluble TNF receptors (sTNFR).¹⁰⁶^–¹⁰⁸ IL-1ra antagonizes the IL-1 receptor, decreasing IL-1 effects, whereas sTNFR binds up TNF-α before it can react at its target cells. At the same time, IL-6 triggers the release of the major anti-inflammatory cytokine, IL-10.^107,¹⁰⁸ It appears that exercise-induced IL-6 has unique action as opposed to TNF-α mediated release of IL-6.¹⁰⁴ Exercise causes a huge increase in IL-6, far above TNF-α levels. This is in sharp contrast to infection or sepsis, which shows an exponential rise in both. It may be the ratio of IL-6 to TNF-α that is the real concern with regard to chronic inflammation. Epidemiologic studies on TNF-α and IL-6 genetic polymorphisms support this, showing that those with the highest TNF-α and lowest IL-6 levels have the greatest risk of diabetes.¹⁰⁹

Other researchers support TNF-α as the real inflammatory culprit.¹⁰⁴ They speculate IL-6 levels may be a marker of whole body TNF-α levels and could be acting in direct opposition to the more inflammatory cytokines. The IL-6 effect implicates exercise as a first line defense against inflammation and may explain the “counter intuitive” findings on the benefit of resistance training in highly inflammatory diseases like rheumatoid arthritis.¹¹⁰ For some time, science has been searching for a molecule that could account for the acute metabolic effects of exercise. Exercise reduces “all-cause mortality” due to its effects on the leading killers: heart disease, diabetes, and cancer.^111,¹¹² IL-6 is beginning to be shown as protective against diseases like diabetes.^109,¹¹³^–¹¹⁵ These same diseases have strong links to inflammation, which is now suspected as a major underlying cause. It has long been thought that exercise’s impact on weight loss was the reason behind this. However, IL-6 also plays a role as a mediating factor in exercise’s effects on fuel metabolism.¹⁰¹^–¹⁰³ ^,114 ^,116 The broad effects IL-6 has on inflammatory cytokines, fuel metabolism, plus its ability to “talk” to the brain, liver, and adipose tissue, has some researchers thinking it is the best candidate for the elusive exercise factor.¹⁰⁶

As muscle contracts, the genes controlling IL-6 production are turned on. The degree of IL-6 released from muscle is directly proportional to the amount of muscle being contracted; the more muscle used, the greater the response.^106,^107,¹¹⁷^–¹¹⁹ IL-6 also shows a tight relationship to muscle glycogen and exercise intensity. When muscle sugar stores begin to decrease, an intensity threshold is breached and much larger amounts are released.¹²⁰ Increasing exercise intensity, full body muscle contraction, and muscle glycogen depletion are the major exercise elements enhancing IL-6 release from muscle.^102,^117,^120,¹²¹ These factors together can induce an increase of plasma IL-6 that is 20- to 100-fold over resting levels.^102,¹²¹ At these levels, IL-6 begins to exert influence over the body, relaying messages about the metabolic needs of the muscle. In this way, IL-6 acts more like a hormone than a cytokine by sending communications from muscle to adipose tissue, immune cells, and the liver. These messages instruct the body to burn fat, control glucose regulation, inhibit the production of the proinflammatory cytokines, and ultimately generate a fully anti-inflammatory effect through the release of IL-10.¹²² IL-10 is a potent reducer of TNF-α and IL-1 in its own right.¹⁰⁷

From the previous scenario, it should be apparent that the ability to harness IL-6 through exercise could have a significant effect not only on inflammation, but also on whole body fuel usage and tissue repair. This process is far different than the usual chronic inflammatory scenario. A situation of chronic inflammation is one where TNF-α is elevated along with IL-6 and IL-1. Exercise-induced, muscle-derived IL-6 shifts the balance, causing a reduction in TNF-α and IL-1 with a simultaneous increase in IL-10.

In addition to its more direct effect, exercise-induced IL-6 has other secondary effects that account for increased benefits. 11β-Hydroxysteroid dehydrogenase type 1 (HSD1) is an enzyme that should be on the radar of physicians. It is responsible for the conversion of cortisone into active cortisol. This cortisol/cortisone ratio is important in determining the possible detrimental effects of cortisol. This enzyme is present in visceral adipose and is overly active in the overweight and obese.¹²³ This is an important revelation, since it points to visceral adipose tissue as a new site of cortisol production. TNF-α and IL-1β have both been shown to upregulate HSD1 and contribute to total glucocorticoid production.¹²³ IL-6 is a potent inhibitor of both TNF-α and IL-1β, and the largest amounts are released through exercise. Intense exercise potentiates these effects by increasing sympathetic stimulation of α-2 receptors as well as adrenocorticotropic hormone; all of these have independent effects in suppressing HSD1 activity. The ability to blunt HSD1 is beneficial in controlling obesity and diabetes, and intense exercise may be the best way to effect these changes.

In addition to the cytokine effects, exercise-induced IL-6 crosses over into hormonal action and allows the muscle to “talk to” the adipose tissue and the liver.¹⁰¹ Its major action at these sites is to release energy substrate to fuel continued movement. IL-6 is a potent stimulator of adipose tissue fatty acid oxidation¹⁰⁴ and is a major factor in liver glycogenolysis.¹⁰¹ Although the mechanism for this action has not yet been fully elucidated, studies have confirmed that IL-6 has direct effects on the expression of adenosine monophosphate kinase^117,¹¹⁸ and hormone sensitive lipase,¹¹³ two chief fuel regulating enzymes in human tissue.

Finally, IL-6 has the ability to cross the blood–brain barrier, having direct effects on the brain. The brain produces IL-6 in response to exercise as well. This sparks curiosity as to what brain IL-6 is doing. Animal studies showed that IL-6 had a direct and important effect on the brain, thus playing a role in appetite regulation, fuel regulation, and body composition.¹⁰¹

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