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Fitness Science Vol 2--Cardiovascular and Musculoskeletal Adaptations to Exercise Training

The first diary in this series discussed the body's stress response reflex, and how that stimulus initiated physical changes leading to improved fitness. In this diary, changes to the cardiovascular and musculosketal systems in response to exercise training will be described in greater detail. The goals are to provide more information for those who are interested and give some practical tips on applying this information to your workout routine.

To summarize, physical adaptations to exercise can be grouped into two categories: central and peripheral. As a rule, central changes are short-term and peripheral changes are long-term.

Cardiovascular System
The role of the cardiovascular system is to deliver energy to living tissues. It includes the heart, the lungs, the vascular system, the enzymes and cellular structures necessary for metabolism, and the central nervous system. The system is set up to extract oxygen from ambient air and deliver that oxygen to specific muscle cells to generate adenosine triphosphate (ATP) aerobically.

The circulatory system is a 2-loop system: the right side of the heart pumps blood to the lungs where it is oxygenated and returns to the left side of the heart. The left side of the heart then pumps blood to the rest of the body.

The amount of blood that the left ventricle can pump with each heart rate is called stroke volume. The volume of blood pumped each minute is called cardiac output, which is stroke volume x number of heartbeats. The average cardiac output for a person at rest is 5 liters/minute. Resting cardiac output is the same for trained and untrained individuals--the difference is that trained individuals have a larger stroke volume, therefore resting heart rate is less.

In the capillaries, oxygen is extracted from arterial blood and used to produce energy. The amount of oxygen extracted is referred to as the a-v O2 Difference. As the cells become metabolically more active during aerobic exercise, oxygen consumption increases dramatically.

Oxygen consumption is the way that we measure the intensity of aerobic exercise. The term for oxygen consumption is VO2. Because of differences in body size, aerobic intensity is measured in milliliters of oxygen per kilogram of body weight per minute, or ml/kg/min.

VO2 is determined by two factors: a-v O2 difference and cardiac output. Since a-v O2 difference becomes maxed out at submaximal effort, aerobic capacity is ultimately determined by cardiac output.

From a resting value of 5 liters/min, cardiac output increases during maximum exercise to 20-22 l/min for untrained individuals to 40 l/min for elite athletes. Given that we only have 5 liters of blood supply, that means that during exercise we must circulate our entire blood supply 4-10 times per minute. The demands of exercise not only dramatically increase the need for cardiac output, but also the need to deliver blood to the working muscles. We have a tremendous vascular capacity, much greater than our blood supply. Therefore, the ability to divert blood flow from less-essential areas like the liver and kidneys to working muscles is an important training adaptation. At rest, blood flow to the kidneys and liver represents almost 50% of cardiac output (2.45 l/min), with 20% going to the muscles. During moderate exercise, flow to the liver/kidneys decreases both in relative and actual amounts (3% of total, 1.2 l/min), while blood flow to muscles increases to 71% of total output or 12.5 l/min.

Here is a list of cardiovascular adaptations to exercise:

Central/Short term
Increased stroke volume
Increased plasma volume
Increased maximum ventilation
Increased amount of O2 extracted from inspired air
Improved blood shunting
Improved recruitment of muscle fibers (mechanical efficiency)

Peripheral/Long Term
Increase in a-v O2 difference
Increase in enzymes and cellular structures (e.g. mitochondria) needed for ATP production
Increased capillary density in working muscles
Increased ability to clear lactic acid
Increased stroke volume

Stroke volume is listed twice because there is an initial increase due to the increase in plasma volume and there can also be long-term moderate thickening of the left ventricular wall in response to high-volume aerobic training.

Improvements in lung function with exercise are modest, primarily because a healthy person has a substantial lung capacity to begin with. In the absence of pulmonary disease, lung function is not a limiting factor in exercise performance. Even at maximal effort, we do not utilize our entire lung capacity. Maximum aerobic effort is determined by maximum cardiac output.

The central nervous system (CNS) plays in important role in exercise training. As mentioned earlier, the CNS determines the shunting of blood flow to the working muscles. The CNS is also involved in mechanical efficiency, i.e. the recruitment of muscles to perform a given activity like walking or running.

Some central adaptations/improvements can start to take place in 2-4 workout sessions, but are generally considered to begin to occur in 2-6 weeks. Peripheral adaptations can take 6-12 months.

Musculoskeletal system
In terms of fitness training, the musculoskeletal system is designed to generate force and apply that force through the levers of the limbs. This involves the muscles themselves, connective tissues such as tendons and ligaments, multiple hormones, enzymes and proteins, and, again, the central nervous system.

Muscle cells (often called muscle fibers) are long, cylindrical cells about the diameter of a human hair, and often running the length of the muscle itself. Muscle fibers are grouped into larger bundles surrounded by connective tissue. The connective tissue ultimately forms a muscle sheath which is continuous with the tendons that attach the muscle to the bone.

Motor neurons (nerve cells) innervate muscle cells. A muscle fiber is attached to only one motor neuron, but one motor neuron controls many muscle fibers.

The interior of the muscle fiber consists of hundreds of thousands of myofibrils. Myofibrils contain the apparatus that contracts the muscle cell, a series of filaments that slide inward when activated, causing the muscle to contract.

The maximum force capacity of a muscle is believed to be related to the cross sectional area of the muscle--the larger the cross-sectional area, the more potential for applying force. The amount of "strength" generated by a muscle group is also affected by the intensity of stimulation, number of muscle fibers recruited, and the frequency and coordination of muscle fiber stimulations.

Skeletal muscle fibers are commonly grouped into 2 classes based on morphological and specific physiological characteristics--slow twitch (Type I) and fast-twitch (Type II). Fast-twitch fibers are subdivided into Type IIA and Type IIB. Type I fibers develop force slowly and have a longer twitch time; they also have more oxidative capacity. Type II fibers develop force more rapidly with a faster twitch time. The distinction of IIA and IIB is based on a relative difference in oxidative capacity--IIA can increase oxidative capability in response to training (although not to the extent of a Type I fiber) whereas IIB fibers are more "anaerobic" and do not change with endurance training.

The number and type of muscle fibers is genetically deterimined. Strength training does not increase the number of muscle fibers and endurance training results in only modest adaptations in Type IIB fibers. Elite sprinters have a genetically high percentage of Type IIB fibers; elite marathoners a high percentage of Type I. The rest of us are decidedly average ;-)

Here are the musculoskeletal adpatations to exercise:

Central/short term
Increased recruitment of muscle fibers
Inreased mechanical efficiency
Amount of sythesis and storage of hormones

Peripheral/long term
Increased muscle size (hypertrophy)
Increased volume of proteins and cellular structures involved in muscle contraction
Increased strength of connective tissue
Cellular adaptations to more effectively utilize hormones

When starting a strength-training program, virtually all of the initial increases in strength are due to neuromuscular facilitation. The body learns to recruit muscle fibers more efficiently, in greater numbers, and with improved coordination. The average person has a large unused muscle capacity, so there is a tremendous ability to "use what you have" before you ever need to "make more". Often, muscle hypertrophy does not occur until strength levels have increased at least 100% (not that difficult for an untrained person to achieve).

The amount of hypertrophy one can achieve is dependent on genetic factors (muscle fiber amount and type) and volume of training. Just lifting heavier weights will not make you look like Ahnauld in his younger days--you need to do a substantial volume of work as well (and, like most elite athletes, champion body builders are born with particular body types, so chances are, even if you train like Arnold, you still won't look like Arnold.)

Hypertrophy is also dependent on hormones. Women do not have the same levels of testosterone as men, so they will not get as large. Women should not shy away from heavier weights out of a fear of "looking like a bodybuilder".

Again, short-term adaptations can take place in 2-6 weeks. Long-term depends more on type and volume of training, but, while you can see improved tone and definition in 6-weeks, substantial increases in size will probably take 3-9 months.

Practical ConsiderationsFirst of all, this is more proof that exercise benefits can be realized in a relatively short period of time. Someone who is not currently exercising should not think it takes weeks and months or a superhuman effort to see improvement. It doesn't.

It also means that those who want to train seriously should realize that, after the initial improvement, further gains will come more slowly--but they will continue for an extended time.

For those with busy schedules, it is important to understand that improvements are lost in roughly the same amount of time they were gained. And, it is easier to maintain than it is to gain. So, if you are faced with a busy stretch of work or family issues, 1 or 2 quality workout sessions per week can be enough to help you maintain, and with a short layoff, you can quickly get back to form.

This holds true for illness and injury, but with one difference. After a longer layoff, you can get back into things relatively quickly, but you probably will have lost some long-term improvements.

The other caveat about returning from extended injury or illness is that your "central" abilities will improve faster than your musculoskeletal system can handle. In other words, your perceived fitness level will return faster than your tendons' and ligaments' ability to handle the physical strain of movements. So don't ramp up the volume too quickly.

Core Training
In the strength training part of the diary, I dealt with the adaptations that occur with weight lifting. I did not address core training. Core training involves primarily muscle fiber recruitment and coordination, as opposed to muscle hypertrophy. I will address core training more in the diary on "Specificity of Training".

Other Issues
There are two other issues I want to address that don't fit the topic as closely, but I couldn't figure out where else to put them, and they are topics that are frequently misunderstood.

Heart Rate Training
Monitoring heart rate can be an important tool for making sure you are exercising efficiently and a motivator as well. However, heart rate response to exercise tends to be oversimplified and it is important to know some things to get the most out of this method.

First of all, heart rate is only an indirect measuring tool. As stated earlier, the intensity of aerobic exercise is based on cardiac output and oxygen consumption. It is not practical to measure oxygen consumption. However, in most cases there is a linear relationship between heart rate and VO2, and measuring heart rate is VERY practical and convenient, so we can use heart rate to gauge aerobic intensity. However, I must repeat that increased heart rate per se does not always mean aerobic activity is taking place.

During aerobic exercise, heart rate increases as a part of increased cardiac output. During strength training, heart rate can increase, but it is due to pressure changes in the intrathoracic cavity, NOT because of increased cardiac output. Therefore, just because heart rate increases during strength training, that does not mean you can perform strength training and aerobic training simultaneously.

The biggest challenge in using heart rate monitoring to measure aerobic intensity is the variation of maximum heart rates and heart rate response to exercise in the general population. All heart rate guidelines use a percentage of maximal heart rate (HRmax) as the measurement of intensity. Measuring HRmax directly requires you to take a maximal exercise test--a REAL maximal exercise test, not a doctor's stress test. That can be risky, expensive, and is definitely not a pleasurable experience. Most people use a formula to estimate HRmax. The problem is that, within the normal population distribution, the SEE for all HRmax calculations is 10-12 beats per minute. That means that fully 1/3 or the population has a true HRmax that is 10-30 beats/min above or below the calculated HRmax. That's a significant variation. (And that does not include medication effects).

In my experience I have had numerous clients become confused and distressed when they first put on their heart rate monitors and saw a response substantially different that what the "guidelines" predicted. In extreme cases, I was contacted by younger runners who had actually stopped running because they were afraid they were damaging their hearts.

Another factor affects HR response to exercise and that is cardiovascular drift. That simply refers to the fact that heart rate tends to drift upward during the length of an exercise session, even when intensity is kept constant. The likely reasons are increased body core temperature and loss of plasma volume through perspiration. I have noticed a 20 beat/min increase over the course of a 30-40 min run on a treadmill, with no change in speed. If you are doing a programmed workout based on a target heart rate goal, the exercise machine will actually start to decrease your intensity when this occurs, thus lowering the quality of the workout.

The thing to do with a new heart rate monitor is to put it on and be a passive observer for the first few sessions. Do your usual routine and see what happens. Compare your heart rate response to your rate of perceived exertion--if it feels easy, it probably is, regardless of  the heart rate. Once you have determined your individual response, you can then use the HR monitor to guide your workouts. But keep it simple. I have seen programs that get very detailed about different heart rate "zones"--I think those plans are devised mainly to make it seem more complicated so that you will "need" the paid expertise of the author/presenter.There are three basic "training zones": easy, medium, and hard. It doesn't have to be any more complicated than that. 

Also keep in mind that exercise heart rate can be affected by illness or stress. Always compare your heart rate to you overall feelings of exertion and guide your efforts accordingly.

Lactic Acid
This is just a personal thing with me. Lactic acid is often described as a "waste product" that "builds up in the muscles" and is responsible for muscle soreness. None of these are true.

Lactic acid is formed during exercise metabolism and increased levels of lactic acid contribute to fatigue. But lactic acid is not a "waste product"--it is a dynamic substrate that can traverse a number of different metabolic pathways. It can be used directly as fuel by cardiac muscle tissue, it can be used as an oxidizeable substrate for aerobic metabolism or it can be incorporated into amino acids and proteins.

Actually both trained athletes and untrained individuals produce the same amounts of lactic acid at any given relative intensity. The difference is that trained athletes can clear lactic acid more efficiently.

And lactic acid has nothing to do with delayed onset muscle soreness (DOMS). DOMS is most likely caused by microtrauma-related edema and is especially related to eccentric muscle contractions.


Fitness Science Vol 1 - The Stress Response

Volume 1 starts with the basic principle that underlies all exercise training: The Stress-Response-Adaptation Principle, sometimes known at General Adaptation Syndrome (GAS). Quite simply, this means that physiological systems respond to appropriate stimuli. Repeated stimuli, or stresses, frequently lead to adaptations.

Not all stimuli have the desired response. Too little stress, and no adaptation occurs. Too much stress, and negative adaptation occurs, i.e. overtraining or injury. So the purpose of any exercise training is to apply the appropriate amount of stress to force the body to adapt in ways that are positive and increase one's functional capacity (or fitness). In other words, your body will do what you ask it to do.

The effectiveness of a stressor, or training stimulus, in creating a postive adaptive response is specific to the individual and is relative to any given point in time or level of conditioning. Obviously, a workout that might result in improvement for a sedentary 60-year old adult would be useless for an Olympic marathon runner. But it also means that the workout that was effective for that 60-year old in week 2 may no longer be as effective in week 4 or 5 and certainly will not be effective in week 11 or 12.

A workout plan--or training stimulus, or exercise precription--is usually described according to the following: Type, Intensity, Frequency, and Duration.

Type refers to the type of activity chosen: running, walking, cyling, lifting.

Intensity refers to the effort level required to mobilize the systems and processes within the organism and move it from the current "status quo" or homeostasis.

Frequency and Duration are the degree and frequency that the simulus is applied, enough to result in the adaptations necessary so that the body can perform at a higher level.

Since this diary is about stress-response, I will focus on intensity. As I mentioned earlier, the training stimulus must be sufficient to initiate adaptaton, but not so intense that it leads to breakdown. I often use a scene from the old I Love Lucy show as an example. Lucy and Ethel take jobs at a candy manufacturer. They are responsible for taking the individual candies that are coming down a conveyor belt and packing them into candy boxes. At first the belt is moving slowly and there are few candies, so they are able to easily keep up. The belt then starts to move more quickly and more candies appear. Lucy and Ethel become increasingly frantic and disorganized, and finally the entire operation falls apart.

The opening scene on the belt is your body at rest in its current state of fitness. It can easily handle what you ask it to do. The first modest increase represents a training stimulus. The women had to work harder and move faster to keep up, but they were able to do so, even though they weren't as coordinated or efficient as they could have been. As the belt speed and candy volume increased even more, they could keep up for short stretches, but grew increasingly frantic in their movements and fell further and further behind. At max speed and volume, everything fell apart.

This is what happens to your body during exercise. The first modest increase in intensity causes you to work harder and be more focused. Repeating that stimulus will result in the body adapting to that workload and becoming more coordinated and more efficient. Going to the second higher level, the body can keep up for short periods, but not for very long. To perform at this intensity, the time interval needs to be short, followed by recovery. This is what is known as interval training, and can be a useful strategy. At the highest, all-out level, a beginner will not be able to keep up--internal systems will go haywire. This is when injuries and other adverse medical events can occur.

How do you measure intensity? Two common methods are heart rate and perceived exertion.

Many exercisers are familiar with the concept of heart rate monitoring. In short, while performing cardiovascular exercise, heart rate is a good gauge of intensity--i.e., the harder you work, the higher the heart rate. There are a variety of formulae for determining ideal heart rate training ranges. The most common is to subtract your age from 220 (220-age) and use that number for your Maximum Heart Rate (HRmax). Exercise heart rate is then calculated as a percentage of HRmax.

A more accurate method is the Heart Rate Reserve (HRR). You still estimate HRmax the same as before, but now you also include Resting Heart Rate (HRrest). HRrest is best measured first thing in the morning while at "rest". To determine your HRR training rate, do the following:

  1. Calculate estimated HRmax (220-age)
  1. Measure HRrest.
  1. Calculate HRreserve: (HRmax-HRrest)=HRreserve
  1. Calculate desired intensity percentage of HRreserve: For 60% effort: (HRreserve x .60)
  1. Add that result back to HRrest to determine 60% training heart rate.

Example: 50 year old with a resting heart rate of 60 who wants to work out at a 60% intensity. HRmax=160 (220-60). HRreserve=100 (160-resting HR of 60). 60% of HRreserve=60. Add that back to HRrest and you get a 60% training heart rate of 120 beats per minute.

I like to do heart rate monitoring, and I monitor mine during my workouts, but it takes some experience and insight to really make it effective. I will go into more detail in the next diary, but suffice to say, there are many variables involved with heart rate and you really need to learn about YOUR specific response to exercise before you can use it most effectively.

Rate of Perceived Exertion (RPE) is an effective tool. As the name implies, you pay attention to your general sense of exertion and use an RPE scale to assess your effort level. The most common RPE scale is the Borg Perceived Exertion scale. It looks like this:

6 No exertion at all

7 Extremely light


9 Very light - (easy walking slowly at a comfortable pace)


11 Light


13 Somewhat hard (It is quite an effort; you feel tired but can continue)


15 Hard (heavy)


17 Very hard (very strenuous, and you are very fatigued)


19 Extremely hard (You can not continue for long at this pace)

20 Maximal exertion

Sometimes you will see a modified version of the Borg scale, listed as a 1 to 10 scale, but I prefer the original.

The scale also corresponds to what is often called the "talk test". The idea is that you should exercise at a level where you can comfortably carry on a conversation (although the listener should be able to tell you are exerting yourself) but not sing a song. If you can only talk with extra effort, that might be an intensity level too high--or certainly higher than you need to be for general health and fitness.

The "conversational level" of exertion corresponds to 12-14 on the Borg scale, the "extra effort level" with 15-17. A beginner can see results at 9-11.

For strength training, the most common measure of intensity is a percentage of the maximum weight you can lift one time--referred to as a 1 Repetition Maximum or 1RM. Since it is not advisable for beginners to perform a 1RM lift, we estimate it by saying your lifting weight should be the amount of weight you can lift 12-15 times while maintaining good form. The 15th rep should be the last one you could do. This is for beginners. More experienced recreational lifters should work at a higher intensity--i.e. at weight that you can lift at least 8 times, but no more than 12. (Again, there are a lot of variations to this that will be addressed in the next volume).

So what does this mean in practical terms?

First of all, if you are beginning an exercise program--or returning from a layoff due to injury, illness, or sloth--you don't have to work that hard to see improvement. A 40% intensity level for 15-20 minutes is enough. At this level, you can work out every day or even twice a day. If 15-20 of sustained activity is too much, then break it into several intervals with 1-2 minutes of recovery. For a beginner, working harder does not get you into shape any faster--it just hurts more.

After about 12 weeks, if not sooner, you should increase the minimum intensity to 55%-65%.

A regular exerciser must be aware that, as the body adapts to a certain level of effort, the intensity, frequency, or duration must be changed/increased in order to see continued improvement (if that is your goal). Exercisers should include variety in their workout routines--do some mild interval training, some harder, shorter workouts, some cross training--to vary the training stimulus so that the body can continue to adapt. A common mistake for experienced exercisers is to get stuck in the same routine. For those who are exercise-averse, these variations do not have to be severe or uncomfortable--interval training doesn't mean running wind sprints--just different enough to provide an alternate stimulus.

The biggest mistake that beginning strength trainers make is to stay at the same weight level and just do more reps. I have seen this with many people, especially adults over 40. They start a routine, and then after 3-4 months they are still lifing the exact same weight--just lifting it 20 times instead of 8-12 or 12-15. Whatever your intensity level, once you can comfortably lift the weight the max number of times with good form, you must increase the weight to continue to progress.

It is also helpful to vary your strength training routine on a regular basis, to go through "cycles" of intensity or modality. Example: rotate through cycles of selectorized machines, free weights, or cable machines. You'll get better overall results and be less prone to boredom or injuries.

In summary:

  1. The body repsponds to external stimuli--it will do what you ask it to do.
  1. "Fitness" is a dynamic condition, so you want to have the right routine for your goals, medical history, and level of conditioning.
  1. You don't have to kill yourself to see results, but you must maintain a focus and purpose to your workouts.

HRMs cannot count calories during strength training

Since this keeps coming up, I decided I am going to write this once and then just repost it.

I know that we are all concerned with counting calories, and we all want to know our exercise calories. Unfortunately, there is no consistent way to calculate actual calories burned during strength training. I also understand that, human nature being what it is, we tend to place great faith in fixed numbers--there is something very authoritative and definitive about that number on our wrists that makes it hard to resist--even in the face of all scientific facts to the contrary. (Remember--for a long time people had trouble accepting that the earth was round).

It's just something you are going to have to live with. Heart rate monitors CANNOT count calories burned during strength training.

The most commonly accepted method for measuring the calories burned for a particular activity is to measure oxygen uptake (VO2).

During *steady-state*, *aerobic* exercise, the TCA cycle is the primary means of producing energy, and oxygen is utilized at a relatively consistent rate depending on the intensity of the exercise. There is an observable and reproducible relationship between heart rate and oxygen uptake. If we have some individual data--resting heart rate, maximum heart rate, VO2 max, weight--it is possible to make reasonably accurate estimates of caloric expenditure based on the percentage of HRmax or percentage of HRreserve at which someone is working.

From the other perspective, basic exercise activities that have a common movement--walking, running, cycling, stairclimbing--have been extensively studied and equations to predict energy cost have been developed that are applicable to most of the general population. Cross trainers/ellipticals are the exception since they do not have a common movement design.

It is under these conditions and with these types of activities that calorie estimating equations and heart rate monitor estimations are the most accurate--exercises and exercise movements that are aerobic in nature and that are performed at intensities between 40% of VO2 max and the lactate threshold.

If an activity does not meet these criteria, then prediction equations and heart rate monitors become less accurate.

When it comes to strength training, they are not accurate at all.

There is a mistaken belief among many people--repeated even by many "experts" on bodybuilding websites--that ANY increase in heart rate reflects aerobic conditioning and an increase in caloric expenditure. This is not true. The primary reason is that the increase in heart rate that occurs with strength training results from a different physiologic mechanism than it does during aerobic exercise.

The increased heart rate that occurs with aerobic exercise is the result of the need for increased cardiac output--the heart must pump more blood to meet the energy demand of the activity. Heart rate increases because of a VOLUME load.

The increased heart rate that occurs with strength training is the result of changes in intrathoracic pressure and an increase in afterload stress. There is no corresponding increase in cardiac output, and thus only a modest increase in oxygen uptake. Heart rate increases because of a PRESSURE load.

So, unlike aerobic exercise, the increased heart rate during strength training DOES NOT reflect either an increase in oxygen uptake or a significant increase in caloric expenditure. Moving quickly from machine to machine to keep the heart rate elevated does not change this fact. It is still a pressure load, not a volume load.

Does this mean that strength training is a less useful activity for weight loss, or that it does not contribute to maintaining a calorie deficit? Of course not. Strength training is a critical part of a weight loss program. Strength training may only have a modest observable calorie burn--actually it's more like a simmer--but it can contribute to an overall calorie deficit in other ways--a modest "afterburn", conservation of lean muscle mass, the metabolic effects of more rapid protein turnover, for example. But the effects of strength training are not general in nature--they are very specific to the individual, and they are affected by so many different variables, it is impossible to formulate an equation or prediction table that is applicable to the general population. Since the focus of this article is strength training and heart rate monitors, I will not go into detail about the many benefits of strength training and weight loss.

So far I have been discussing "traditional" strength training programs--i.e. structured routines consisting of "sets" and "reps" at relatively heavy intensities--up to 10RM-12RM.

What about "circuit training"? There are two basic types of circuit training--One features alternating cardio and strength stations; the exerciser performs one set at a strength station, followed by a 1-3 min cardio interval, and alternates. The second type features a circuit of strength machines only. Traditionally, circuit training routines feature higher-reps, higher speed of movement and lower resistance levels--often 40% of 1 RM. Because of the lower resistance (or in the case of the first type, the inclusion of cardio intervals), these types of circuit training will involve more of a dynamic volume load, and thus a higher caloric burn. Using HRMs is still problematic, however, because the inclusion of upper-body lifting movements and the higher resistance (compared to aerobic exercise) means that HRMs will most likely OVERESTIMATE caloric expenditure--by as much as 30%-35%. For example, a heart rate of 85% of max that would normally reflect a VO2 of 70% of max might reflect a VO2 of only 51% of VO2 max during circuit training.

Again, in this article, I am not evaluating different training methods, just discussing the accuracy of HRM calorie estimates from these types of activities. 

Does this mean that heart rate monitors are not useful? Not at all. For a number of aerobic activities--most ellipticals, spin classes, running outdoors, other aerobic-style classes--they are still the best option for estimating calories. And they can be used for circuit training and some mixed classes or cross fit workouts--both as a more vague estimate of calories burned, but also for workout-to-workout comparisons. And for many people, by the time you get to the point where you can and need to start doing more intense lifting and circuit workouts (e.g. tabata, crossfit), calories burned during a workout is less relevant anyhow.

My muscles are burning--this must be a good exercise, right?

A key technique used in infomercials for a new "fitness" device is to take the device into a crowd and invite random people to try it out. The viewer is then shown a series of testimonials saying "I could really feel this working" or "my (legs, abs, butt, etc) muscles were burning so I know it really works." Even without an infomercial, some people come to the same conclusion when they try out a new device, a new exercise, or a new class. If you try something and it makes your muscles fatigued, or burn, or sore the next day, it must be effective, right?

Not necessarily. Exercise training effects are very specific. An exercise movement will recruit the muscle fibers necessary to meet the demands of that movement. If one does the same movement over and over, some muscle fibers may became highly trained, while others nearby are trained only a little or not at all. If you engage in a movement that has different demands, then different muscles are recruited and others are activated in different patterns. If the movement is different enough, the muscles may become quickly fatigued, or if it is different and more intense, one might feel a burning--and then fatigue. However, this is an ACUTE response to the immediate demands of the movement--it only means that, at that moment, you were asking these muscles to do more than what they were used to doing. If they weren't doing much of anything before, then it doesn't take much to overload the muscle and cause fatigue.

Fitness (strength and cardio) improvement occurs as a result of the SYSTEMATIC APPLICATION OF A PROGRESSIVE TRAINING LOAD. The body always tries to maintain "homeostasis"--an internal, stable equilibrium. So it will adapt to a training load--but ONLY to the specific demands of that load. Once it adapts, it reestablishes equilibrium and no further adaptation takes place.

So when you are evaluating any fitness product--any device, any shoes, any class, any movement--you have to ask: what is the resistance method? is the resistance method designed so that it is applied effectively? And, most importantly--CAN THE RESISTANCE BE PROGRESSIVELY INCREASED? If the answer to the last question is "NO", then this device, product, etc is probably going to be ineffective and not worth the money.

Even though you may "feel" different muscles working when you do the movement or use the device, if there is no progressive overload, your body will quickly adapt to the "load" and you will no longer see improvement. Of course by then, the "return window" has expired or you just don't bother to send it back, or the power of auto-suggestion makes you think it's YOUR fault that you aren't doing better.

This is especially true of machines that use only body weight as resistance--and use only a small part of the body--and that use a dynamic movement such as gliding, rocking, or a spring mechanism to assist the movement. It is also true of products that contain an inherent “instability” that causes muscles to initially work “differently”, but provide no way to change the instability pattern or increase resistance.

Often the manufacturers will show you "EMG studies" that show the muscles being activated. That's a time-honored smokescreen. All that shows you is the immediate response--it doesn't show the level of resistance and it can't show whether or not there is a training effect.

Examples of these types of products include: Leg Magic, most infomercial ab machines, most "thigh" machines, and “tone up” shoes. (Although some people just find the “tone up” shoes really comfortable—that’s a different issue).

This does not mean that ALL new movements or exercises are ineffective--far from it. Doing a variety of exercises that activate different muscle groups or works them in new ways is important--as long as you can progressively increase the resistance. In that case, if you do a new lifting movement that results in fatigue/burning/soreness, that is often a good thing. That might mean it's an area you may have been neglecting.
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