Aerobic Exercise & Strength Training- Does It Help Or Hurt?
One of the greatest obstacles to the realization of our fullest potential in any field is the idea of convention. Convention can heartily sustain the life force of myths and concepts poorly understood by the public at large, even when confronted by volumes of well researched science that contradicts them. Nowhere is the hold of convention more pronounced than in the related fields of diet and exercise, where training protocols are often prescribed or implemented based on what is popular or what everyone has traditionally done. Not much thought is given to whether the programs are necessarily the best practices for the goals sought. The universal recommendation that aerobic exercise needs to be a part of everyone’s strength training regime is a case in point, as volumes of research highlight the negative impact of aerobic exercise on strength training, power development and muscle development. Not that there is anything wrong with aerobic and distance training, as it does indeed serve several purposes. Its health benefits are many and well documented. However, most employ aerobic exercise as a way of reducing body fat, building endurance or improving recovery time. All these benefits can be better achieved through other more anaerobic based and time efficient forms of conditioning such as high intensity training [1,2,3,4] and attention to dietary intake. More importantly numerous studies have shown that concurrent aerobic exercise can in many cases negatively impact strength and power gains as well as increase the likelihood of overtraining and bring about negative hormonal responses to exercise even when used in relatively small amounts. [1, 5,6,7,8] Such findings disqualify aerobics as a universal requirement for everyone, especially those interested in optimizing strength, power and or muscle mass to their fullest potential. A group that includes not only athletes whose disciplines require maximum strength and power output but also those seeking the toned, tight and sculpted body and those wishing to increase strength and skeletal muscle as a means of decreasing the motor related decline that comes with aging. 
The Impact of Aerobic Exercise on Adaptations For Power & Strength
The human body is a very specific machine, one that is individually adapted to the very stresses that it encounters throughout the course of everyday life. The process of increasing strength and muscle mass is in fact a stress response adaptation to a very specific form of overload. When lifting weights, for example, at a level of intensity or with a load sufficient enough to trigger a need for our body to adapt, a number of hormonal, neuromuscular and chemical events occur. Events that lead to an adaptive anabolic environment that can promote increases in muscle size and or increased ability to generate force. (See my Article on How Muscles Get Bigger & Stronger) By engaging in sustained steady state aerobic exercise such as distance running, our body is exposed to a very different form of stress-and the adaptations for this form of activity are far different than those required for increased power, strength and muscle mass. The physique of a sprinter is far different from that of a marathon runner as the two activities create different physical adaptations thus it should come as no surprise that many studies confirm that continued aerobic exercise can bring about decreases in muscle power. This may be due to neuromuscular and hormonal adaptations favorable for reducing the amount of energy expended while exercising- factors which would make an endurance athlete more efficient as opposed to maximizing mechanical power. The sheer volume requirement of endurance type training as well may be a consideration as well as it appears to interfere with the recovery required for maximal results from strength and power training.
The Impact Of Aerobic Training On Power and Rate Of Force Development
Repetitive activity associated with aerobic exercise has been shown to reduce the ability to generate force in the high velocity, low frequency region of the force velocity curve- in other words it makes our muscles less powerful.[1,5] Power and strength are closely related but not exactly the same thing. Strength is defined as the capacity for gross muscular effort. Power on the other hand refers to the speed at which effort can be performed. Its development is paramount for athletic performance since most movements in any sporting discipline are executed as forcefully and as quickly as possible. Even in sports that one might consider primarily aerobic in nature such as basketball or soccer due their extended duration of play actually consist of repeated bouts of explosive movements that make up a very small proportion of playing time. [9,10] A good example of differences between power and strength would be a comparison between a powerlifter bench pressing 440lbs and an Olympic level 100m sprinter exploding out of the blocks. Both movements require the ability to generate larger forces at high speeds, but the sprinter’s acceleration would be almost twice as powerful as the powerlifter. Contrast that with the amount of high speed force required by a baseball player to swing a bat from maximal backswing to contact with the ball where the baseball player generates just under ten times more force than the powerlifter bench pressing 440lbs. The baseball player would not likely be able to lift as much as the powerlifter, but he or she might be more powerful. That being said, you can be strong and not powerful but you can’t be powerful without a certain base of strength as the two are very directly related. (The most powerful athletes, Olympic lifters are indeed the strongest.) Now we have an understanding of the significance of strength and power, we can look at how it can be affected by distance type training.
A study published in the European Journal of Applied Physiology took an in-depth look at how power is affected when individuals do strength training concurrently with endurance training as opposed to strength training on its own. During the three week study, two groups of male participants were made to train twice a week doing the same program of resistance exercise with one group doing an additional two days of continuous aerobic exercise on a rowing machine with sessions lasting anywhere from 30 to 60 minutes in duration. At the end of the study there were similar increases in maximum one repetition lifts and isometric strength tests, but only the strength training group saw an increase in rate of force development (ROFD) and the associated rapid neural activation. No changes whatsoever were observed in the group doing both strength training and aerobic exercise. Rate of force development refers to the speed at which force can be produced and a faster ROFD means you are able to do quicker and more explosive movements- qualities essential for strength increases as well as athletic performance. The study concluded that even small amounts of aerobic exercise can lead to interference in explosive strength development.
How Aerobic Exercise Affects Muscle Development for Maximum Power & Muscle Mass
Other studies have demonstrate that sustained aerobic type exercise not only affects rate of force development, but also decreases peak power development through changes in the way muscles are recruited. There are basically two types of skeletal muscle fibers in our bodies Type I and Type II. (See the chart above for a detailed breakdown of the differences.) Type I fibers are what you would recruit primarily if you were running or doing any form of sustained aerobic activity for a considerable amount of time (usually longer than 30 minutes) and someone like a successful marathon runner would tend to have a fairly high distribution of them as an adaption to the sustained endurance training they undergo. Such fibers are highly resistant to fatigue, have a dense network amount of capillaries transporting oxygen rich blood to them and use triglycerides (fats) as their primary fuel source. (Thus the origin of the idea that aerobic activity is best for burning body fat- which it is not- See my article on Rethinking the Need for Aerobics) Not to get too technical but Type I fibers gain their resistance to fatigue from their ability to generate ATP (an important muscle fuel source) through the use of oxygen (aerobic metabolism) which is provided by the many blood vessels found in such muscle fibers.
Type II fibers are more efficient for short bursts of speed and power. These muscle fibers are not recruited to a significant extent during low intensity exercises such as endurance training.(13) Type II fibers have various sub divisions and use glycogen (and creatine phosphate) as their main fuel source and can use either anaerobic or aerobic oxidative metabolism to generate ATP. Type IIx and type IIb fibers are used primarily for explosive movement or any short term anaerobic activity. Generally, these muscle fibers are found in high distributions on more muscular athletes like bodybuilders, powerlifters and sprinters as an adaptation to the short high intensity training protocols that they regularly engage in. These fibers also produce more power than all the other fibers and rely on an anaerobic (without oxygen) metabolism to create ATP. Their development is an important part of any program where strength and power production is a primary goal, and is an important part of any regiment geared towards building lean muscle mass as other muscle fibers do not develop to the extent of these fibers. Type IIa fibers are a bit of a cross between Type II and Type I fibers as they have a fairly high resistance to fatigue and use both anaerobic and aerobic metabolism to operate. They act as a sort of a bridge between long term and short term activity and allow us to perform movements of moderate intensity for periods up to about 30 minutes.
How are muscle fibers affected by aerobic exercise? A decade long Canadian study found that subjects who regularly engaged in high intensity aerobic training verified the idea that our bodies do indeed adapt to the specific stresses it has to regularly undergo. The percentage of Type I fibers in those who regularly participated in distance training was 70.9% as opposed to 37.7 percent in the control group who did not exercise. Endurance training appeared to promote a transition from Type II to Type I fibers at the expense of the more powerful Type II fibers. Interestingly enough, Type IIa fibers in the endurance trained group members actually had a reduced aerobic capacity as a result of the years of training. This decrease in percentage of fast twitch fibers significantly compromises strength and speed capabilities as high intensity conditioning requires an increase in the functional properties of fast twitch Type II fibers relative to slow twitch Type I fibers. Although in a large part individual proportions of muscle fiber types are genetically predetermined, what we do can make a large difference in how our body adapts. The more aerobic exercise you do, the greater promotion of Type I fiber -while the more explosive type anaerobic training you do can increase the proportions of Type II fibers.[15,16,17]
Effects Of Concurrent Aerobic Exercise & Strength Training On Cortisol & Testosterone Levels
Another area for concern is the impact of concurrent aerobic training with strength training on your hormone levels. Some studies have found that aerobics done in certain quantities can produce a net catabolic (breaking down) effect on muscle tissue. This catabolic effect can be traced to either a decreased release of testosterone or an increase in the stress hormone cortisol from combining the two forms of training. A study of US Army soldiers published in the Journal of Applied Physiology took 35 men and split them into four groups. The first group performed whole body high intensity strength training for four days a week focused on increasing muscle size and strength. The second group did upper body strength training only and the third group performed aerobic type endurance training only. A program consisting of two days of continuous aerobic exercise at 80-85% VO2max for 40 minutes and two days of interval training at 95-100% VO2max. The fourth group did a combination of both strength training and aerobic training protocols. Researchers found a significant increase in exercise induced and total cortisol response after in members of the strength & aerobics training group. Whereas those in the strength training only group saw a decrease in cortisol levels and an increase in testosterone levels. Changes that promote an anabolic environment favorable to increased muscle growth and strength increase. The study concluded that the catabolic effects noticed in the strength and aerobic trained group came as result of extreme stress placed on the adrenal glands which systematically lead to a form of overtraining. Other studies of concurrent strength and aerobic type endurance training found similar increases in cortisol (a catabolic stress hormone) among those training in both modalities.[5,18]
The Role of Aerobics In Promoting Overtraining When Combined With Strength Training
Our muscles get stronger, bigger and more powerful as a direct response to the stress of exercise. However, it must be noted that these changes can only occur if the volume and or intensity of training is not so much that our bodies are unable to adapt to it. When we are unable to recover from exercise induced stress, we are said to be overtrained- and overtraining can bring about a long term decrease in performance and muscle related improvements. Recovery from overtraining can take several days to several weeks  and we know now that sustained aerobic exercise combined with strength training may result in less than optimal hormonal profiles and other factors associated with overtraining. (5,7,19)
The human body has a limited set of resources available to it to help recover from the stress of exercise. An insufficient recovery is the central causative factor of overtraining. That being said, given the large volume of exercise that accompanies conventional endurance based aerobic training, the amount of repetitive stress placed on joints during such forms of continuous exercise and the large amount of energy substrates consumed, it is not surprising that the stage is set for overtraining when it is combined with the rigors of high intensity anaerobic training. The increase in oxidative stress during continued aerobic type training may also have a negative impact on net protein turnover. This impact can result in muscle fiber atrophy . All of the aforementioned factors create an environment that prevents optimal adaptation to strength and power training.(1)
Inadequate recovery can also lead to a lasting fatigue that diminishes the ability to develop tension during power and strength movements. Reduction in power generation from overly high training volume then reduces your ability to learn and master power related skills [21,22,23] while also increasing your risk for injury. The common scenario for people starting an exercise program that includes both weight training and aerobics is that they tend to quit several weeks into the program after an enthusiastic start. Considering the amount of stress inflicted on the body by combining both modes of training it is no surprise that an otherwise untrained individual would experience fatigue and a psychological aversion to training over time. It wouldn’t be because of a lack of willpower or consistency but simply because they are overtrained.
Alternatives to Aerobic Exercise for Endurance Development
Aerobic fitness is indeed an important component for both health and athletic performance. However, prolonged aerobic exercise isn’t the only way to increase endurance and cardiovascular capacity nor is it the most efficient. High intensity training and high intensity interval training has been shown to produce metabolic endurance adaptations similar to and in some cases superior to aerobic exercise.[24, 25, 26] (Read my article here on high intensity training and endurance) Other studies found that similar increases in maximal aerobic capacity (VO2max) and improvements in 1 mile run times can occur not just as a result of high volume continuous endurance training but also from high intensity interval training as well. Interval training however takes less time to complete. It has the added benefits of improving anaerobic capacity and power output whereas conventional endurance training only positively affects the aerobic energy system. 
The Main Source Of Contention- Inclusion Of Aerobics For Body Fat Reduction
Perhaps the most common reason for employing aerobic exercise with strength training is for purposes of reducing body fat. Conventional thinking has long been that aerobics burn significant amounts of calories- enough to create a negative energy deficit that would bring about a reduction in body fat. For this reason, most employ aerobics into their training regime as a way to keep their body weight under control. Considering that aerobic exercise does indeed use fats as an energy source it is easy to see some logic to that approach. However, when compared to high intensity anaerobic training modalities, the idea of aerobics as the best way to lose body fat comes into question. Studies confirm that high intensity training methods create greater post exercise energy expenditure and fat utilization [27,28,29,30] and favor negative energy and lipid balance to a greater extent than low to moderate intensity aerobic type exercise. That being said you don’t need aerobics to burn fat if the intensity of your anaerobic training is sufficient and if you maintain a sensible diet. (See my article on High Intensity Training & Weight Loss Here)
Are there benefits to aerobics? Absolutely. By no means should this article be construed as a negative hit against such forms of exercise. For many aerobics exercise offers not only numerous health and performance benefits but psychological ones as well. It can be an excellent tool for relieving symptoms of depression and elevating mood. Many people run or do aerobics not so much for a physical effect but for the high that comes with exercise. High intensity training has been shown to elicit similar psychological advantages as well in elevating mood and combating depression  but it would be unrealistic to expect everyone to gravitate towards that form of training. Thus, if your goals revolve around maximizing strength and power for athletic performance, the science of the matter would suggest that adding aerobics to a program of weight training could be counterproductive. Those seeking the toned and taut look that comes from building quality lean muscle mass while minimizing body fat levels should also note that aerobics isn’t the only way to shed body fat and that they might be better served avoiding it altogether and instead focus on high intensity training protocols and proper diet to reduce body fat and increase aerobic fitness levels. On the other hand, runners and endurance athletes as a whole would benefit greatly by adding strength training to their exercise programs as it may help maintain normal levels of testosterone. 
While some studies have found seemingly contradictory findings regarding concurrent strength and aerobics training, these differences appear mainly to be due to differences in protocols, exercises used, length of the studies and the age and fitness levels of the participants selected. Studies using high intensity strength training as a control however seem to confirm the effects of overtraining as a result of performing both training modalities concurrently as well as a decrease in power generation and strength friendly muscle fiber development. Also of importance is the increased cortisol production observed in those studies of concurrent aerobic and strength training protocols. In two decades of experience as a trainer, I have consistently seen significant increases in muscle mass, strength and power generation in endurance athletes I have worked with when they cease endurance training altogether in the offseason or as a result of injury. However, these increases occurred without any subsequent loss of aerobic fitness capacity when they eventually resumed distance based training. In the end, it is up to you to determine what your goals are and ensure that the program you use helps you attain those goals as opposed to being locked into the idea that aerobic exercise is a universal requirement for everyone.
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