Historically, cardio has received a bad reputation in bodybuilding circles. This reputation appears to have stemmed from two ideologies: 1) when you do cardio, you burn muscle to fuel your workout; and 2) there is some type of interference effect between lifting weights and doing cardio. Both beliefs hold some scientific merit; however, the overall body of literature seems to suggest that regularly performing aerobic exercise may actually be good for bodybuilders. To begin, let’s briefly compare some of the physiological and psychological benefits of weightlifting and cardio. If you feel you already have a decent understanding of the health benefits of exercise, please feel free to skip to the “Cardio Burns Muscle” section.
Benefits of Exercise
The benefits of both resistance exercise and cardiovascular exercise have been well established (Haff & Triplett, 2016). To highlight all the benefits of exercise is outside the scope of this article, but the take home message is quite simple: “Exercise is good for you.” Time and time again, exercise has been shown to reduce the risk of many common diseases such as cardiovascular disease, diabetes, stroke, and even some cancers (Warburton, Nicol, & Bredin, 2006; Westcott, 2012). With the benefits of exercise in mind, many individuals question if there is a modality of exercise (e.g. aerobics, weightlifting, calisthenics) that offers superior health benefits.
Currently, the number one cause of death is heart disease (Center for Disease Control and Prevention, 2015), and for this reason, we will focus on the cardiovascular benefits of weightlifting and aerobic exercise. Two of the main ways you can evaluate the cardiovascular benefits of exercise is by looking at changes in cardiovascular fitness and changes in blood lipid profiles. VO2max is the gold standard for measuring cardiovascular fitness and simply measures how much oxygen the body can consume. Since a higher VO2max is linked to lower cardiovascular disease risk factors, VO2max is one way to measure cardiovascular health (McMurray, Ainsworth, Harrell, Griggs, & Williams, 1998). When it comes to seeing which mode of exercise results in a greater increase in VO2max, the answer is quite clear: aerobic exercise causes far greater increases in VO2max than weightlifting (Haff & Triplett, 2016). When it comes to measuring whether cardio or weightlifting results in greater improvements in blood lipid profiles, the answer becomes less clear.
To investigate this question, I dug up 11 experiments that compared the changes in blood lipid profiles between cardio and weightlifting. Blood lipid profiles are simply a way to measure the concentrations of specific fats in the body. This typically includes: total cholesterol, high-density lipoproteins (‘good’ fats), low-density lipoproteins (‘bad’ fats), and triglycerides. When looking at the data, it appears that aerobic exercise may offer greater improvements in cardiovascular health than resistance exercise. Of the experiments investigated, 4/11 (Banz et al., 2003; Chaudhary, Kang, & Sandhu, 2010; Fenkci, Sarsan, Rota, & Ardic, 2006; LeMura et al., 2000) showed greater improvements in the cardio group than the weightlifting group, and 7/11 (Behall, Howe, Martel, Scott, & Dooly, 2003; Blumenthal et al., 1991; Boardley, Fahlman, Topp, Morgan, & McNevin, 2007; Hersey et al., 1994; Schjerve et al., 2008; Smutok et al., 1993) showed similar improvements.
If the majority of the experiments showed similar improvements in blood lipid profiles, you may be asking why I am biased to thinking that cardio may offer greater improvements in cardiovascular health. Simply put, if both modes of exercise did offer similar improvements, 36% of the experiments investigated shouldn’t have shown statistically greater improvements in blood lipids for the aerobic exercise group. Furthermore, I noticed a trend within the aerobic exercise group to see greater improvements in blood lipid profiles than the resistance training group. For example, Behall et al. (2003) showed a 20.3% reduction in cholesterol for postmenopausal women performing aerobic exercise compared to only a 13.3% reduction in postmenopausal women performing resistance exercise. There was also a 25.6% reduction in low-density lipoproteins in the aerobic group compared to a 19.9% reduction in the resistance group.
While aerobic exercise may offer greater cardiovascular health benefits, there are some benefits resistance exercise offers that aerobic exercise does not. For example, a resistance training program results in greater increases in strength, hypertrophy, and bone mineral density  than an aerobics training program (Haff & Triplett, 2016). Sarcopenia is the loss of muscle mass with age and effects 45% of the elderly population (Janssen, Shepard, Katzmarzyk, & Roubenoff, 2004) and osteoporosis effects 39.7% of Caucasian women (Melton, Chrischilles, Cooper, Lane, & Riggs, 2005). Resistance training may be able to counteract both sarcopenia and osteoporosis which are the cause of a large degree of falling related injuries in the elderly. Falling can be fatal but even minor injuries reduce the quality of life by decreasing functional independence, mobility, and increasing fear of falling, isolation, and depression (Marques et al., 2011; Stevens, Mack, Paulozzi, & Ballesteros, 2008). Falling is the leading cause of injury among the elderly (Stevens et al., 2008) and results in a $31 billion expense for Medicare alone (Burns, Stevens, & Lee, 2016).
Psychological benefits of exercise
Less often discussed are the psychological benefits of exercise. Exercise has been shown to improve depression, anxiety, stress, self-esteem, body-image, emotional well-being, cognitive functioning, and overall quality of life (Lox, Petruzzello, & Martin Ginnis, 2017). While most of this research has been centered around aerobic exercise, it does appear that resistance exercise offers similar psychological benefits (O’Connor, Herring, & Caravalho, 2010). Unfortunately, the mechanisms behind these benefits remain unclear, making it difficult to conclude if one mode of exercise may offer an advantage over the other.
Since cardio and weightlifting are two different modes of exercise, it is my belief that they offer slightly different psychological benefits making it best to perform both modes to optimize mental health. This belief is largely drawn from experiments that have shown more positive results when combining both modes of exercise (i.e., Colcombe & Kramer (2003) found greater improvements in cognitive functioning from concurrent training) and experiments which have only found benefits from one mode of exercise (i.e., Dinoff et al. (2016) found only aerobic exercise to increase brain-derived neurotrophic factor (BDNF)). For any bodybuilder reading this who still does not think cardio is necessary for improving overall health, I would like to point out that resistance training is a relatively new form of exercise that is somewhat specific to humans. Conversely, aerobic exercise has been performed for centuries and is seen across most animal species. This offers an evolutionary perspective for why aerobic exercise may be needed in addition to resistance exercise. History is also littered with quotes from famous philosophers about the importance of aerobic exercise:
“Lack of activity destroys the good condition of every human being, while movement and methodical physical exercise save it and preserve it" -Plato
“Exercise till the mind feels delight in reposing from the fatigue” -Socrates
“The moment my legs begin to move my thoughts begin to flow” -Thoreau
To summarize, aerobic exercise may offer greater improvements in cardiovascular health while resistance exercise offers greater increases in strength and muscle size. Both modes of exercise offer psychological benefits but combining both modes may have the best effect on improving overall mental health. So long story short, “Exercise is good for you.” Now let’s address the two reasons raised earlier as to why cardio is bad for bodybuilders.
Cardio Burns Muscle
A normal healthy body’s preferred source of fuel is carbohydrates. This is largely because the muscles have a form of the enzyme hexokinase which has a high affinity for glucose to initiate glycolysis (metabolic pathway that breaks down glucose into energy; Tymoczko, Berg, & Stryer, 2015). The next preferred source of fuel is fat as fats can be used to produce a large amount of energy. Amino acids are the least preferred source of fuel but will still play a small role in providing the body with energy; typically, within proportion to the workload (Rennie & Tipton, 2000). The amino acids that are primarily used for fuel are glutamate and the branched chain amino acids (leucine, isoleucine, and valine). Without going into too much detail, it is important to realize that proteins are constantly being broken down and synthesized in the body; regardless of if a person is exercising or not. Most of the proteins that are broken down are either damaged or have a short lifespan to begin with. Other proteins, like the proteins that make up our muscles or eyelids are more stable and will not be readily oxidized to provide energy (Tymoczko et al., 2015).
The majority of the amino acids that we will use to fuel exercise comes from the free amino acid pool, which is basically the amino acids that are floating around in the blood. During short-term exercise, glutamate concentrations decrease and alanine concentrations increase (most likely due to an increase in pyruvate from glycolysis which can be used to synthesize alanine). Overall, about 3-6% of the energy from short term exercise will come from this amino acid pool. During prolonged exercise, additional amino acids will play a role in metabolism and their contribution to energy may be up to 15% (Eberle, 2014). Nevertheless, unless a person is under extreme conditions, these amino acids are most likely not coming from muscle tissue. If for some bizarre reason muscle tissue is broken down to provide some energy, this will easily be offset by the increases in muscle protein synthesis (process of the body making proteins) after the cessation of exercise (Haff & Triplett, 2016).
To summarize, our bodies do not like to use proteins as a source of fuel. We will derive the majority of our energy from carbohydrates and fats, and the miniscule amount of energy that we do get from proteins will most likely not come from muscle tissue. If a person is still worried about losing muscle mass from exercise, they can simply consume a high carbohydrate, moderate protein meal about two hours before they exercise (Kerksick et al., 2017).
A popular notion among many bodybuilding circles is that cardio interferes with the gains in strength, power, and hypertrophy from resistance exercise. Some argue that adding cardio to a resistance training program will put further stress on the body which will lead to unintentional overtraining. Others argue that cardio sends catabolic signals which will have a negative impact on the adaptations from weightlifting.
Overtraining occurs when there is an imbalance between training and recovery causing long-term decrements in performance (Meeusen et al., 2013). Overtraining is a taboo subject in research as to my knowledge, it has never been directly studied in a longitudinal experiment (and probably never will be). However, since many athletes can attest to the symptoms of overtraining, it is accepted as something which can happen. The symptoms of overtraining have a large degree of variability and can be quite serious based on published case studies (Meehan, Bull, Wood, & James, 2004). While overtraining is something that athletes should be conscious of, it is highly unlikely that adding a little bit of cardio into your exercise regimen will cause you to become overtrained.
The argument that aerobic exercise induces catabolic signaling which interferes with adaptations from resistance training stems largely from the 5’ adenosine monophosphate-activated protein kinase (AMPK) / protein kinase B (Akt) switch hypothesis. This hypothesis stems from an experiment which showed shocking rodents with low frequency stimulation (representing aerobic exercise) activated AMPK which turns on a catabolic signaling cascade to provide energy to the cells; whereas shocking the rodents with high frequency stimulation (representing resistance exercise) activated Akt-tuberin mammalian target of rapamycin (Akt-TSC2-mTOR) which causes an anabolic signaling cascade. The authors concluded that aerobic and resistance exercise promote opposing signals which interfere with each other (Atherton et al., 2005).
There are several problems that I have with the AMPK/Akt switch hypothesis; the most obvious being that no human study has been able to support this hypothesis (Fyfe, Bishop, & Stepto, 2014). In fact, human studies have repeatedly shown aerobic exercise to activate anabolic pathways such as mTOR (Mascher, Ekblom, Rooyackers, & Blomstrand, 2011) and resistance exercise to activate catabolic pathways such as AMPK (Dreyer et al., 2006). Furthermore, AMPK is not necessarily ‘bad’ and ‘catabolic.’ Yes, AMPK is activated when energy stores are depleted to help provide energy to the cells but, as stated earlier, our bodies won’t start breaking down muscle tissue to provide this energy. And yes, AMPK may suppress muscle protein synthesis by activating tuberous sclerosis complex which inhibits Ras homolog enriched brain from activating mTOR (leading to less anabolism) . However, the real-world application of this knowledge may be limited as human studies have shown that mTOR signaling seems to not be impaired even when cycling for 30-minutes at 70% VO2max after performing resistance exercise (Apro, Wang, Ponten, Blomstrand, & Sahlin, 2013). Lastly, AMPK signals other things to happen such as creating more mitochondria to help the body produce more energy (Reznick & Shulman, 2006) .
As you can see, much remains to be determined about the molecular mechanisms of this interference effect. Data from in vitro (test tube) and rodent experiments seems to suggest one thing, but human data seems to suggest another. Aerobic exercise probably does cause some catabolic signaling to take place, but this is easily offset by the anabolic signaling from exercise (both aerobic and anaerobic). Now that we understand some of the potential mechanisms behind the interference effect, let’s look at what the data from longitudinal studies says.
To begin, I would like to direct your attention to a 2012 meta-analysis which compared the effects of weightlifting alone, cardio alone, or performing both modes concurrently on hypertrophy, strength, and power (Wilson et al., 2012). In this analysis, only power reached a level of statistical significance showing that weightlifting alone was better than concurrent training. While this finding has important implications for power-athletes (e.g. discus, shot-put, boxing), the implications for bodybuilders are limited. While power was the only variable that reached a level of statistical significance, the effect-sizes (measure of the differences between two conditions) seemed to favor resistance training alone over concurrent training. The effect sizes for hypertrophy were 1.23 and 0.85 and the effect sizes for strength were 1.76 and 1.44, for resistance training and concurrent training, respectively. Even though these levels did not reach statistical significance, many readers of this paper concluded that concurrent training will compromise gains in strength and hypertrophy.
One major problem that I have with this paper is the methods which were used to gather the data. No sensitivity analysis was used to see if there were any influential studies which had a large impact on the outcome of the meta-analysis. Without going into too much detail, sometimes experiments can find odd results that are different from the norm. If these odd results are combined with multiple other experiments which found ‘normal’ results, the entire meta-analysis can now misrepresent the body of literature. For this reason, I decided to look up the studies used in this meta-analysis and any other relevant experiments that I could find. The criteria for my search was that the study lasted at least 5-weeks and measured strength in terms of repetition-maximum and/or had some measure of muscle hypertrophy.
This was a lose literature search where I took the relevant studies used in the Wilson meta-analysis and then searched PubMed/Medline for “concurrent training” experiments published after February 2012. I was left with a total of 302 studies that I scanned the titles/abstracts for inclusion. This narrowed my list down to 22 experiments. While there are most likely studies that I overlooked, I think 22 experiments will give us a good number to work with.
In table 1 below, I have provided the data that I extracted from the experiments. I was mainly concerned with changes in strength and hypertrophy and decided to exclude power from my analysis. Strength is mainly represented by percent change in one repetition-maximum between the beginning of the study and the end of the study. If two lifts were tested (e.g. squat and bench press) I decided to provide the percent change for each lift separately (e.g. CT1, CT2). For simplicity, I decided to exclude the changes in maximal voluntary contraction and/or peak torque from this analysis.
Table 1. This table summarizes the findings from the 22 experiments reviewed in this article.
It was a little more complicated to indicate changes in hypertrophy. I indicated most of the studies by the percent change in lean body mass (Cantrell, Schilling, Paquette, & Murlasits, 2014; Chtara et al., 2008; Davitt, Pellegrino, Schanzer, Tjionas, & Arent, 2014; Fyfe et al., 2018; Fyfe, Bartlett, Hanson, Stepto, & Bishop, 2016; Glowacki et al., 2004; Hennessy & Watson, 1994; Hickson, 1980; McCarthy, Agre, Graf, Pozniak, & Vailas, 1995) or percent change in muscle cross sectional area (Bell, Petersen, Wessel, Bagnall, & Quinney, 1991; de Souza et al., 2013; Lundberg, Fernandez-Gonzalo, Gustafsson, & Tesch, 2013; Lundberg, Fernandez-Gonzalo, & Tesch, 2014; McCarthy, Pozniak, & Agre, 2002; Mikkola, Rusko, Izquierdo, Gorostiaga, & Hakkinen, 2012; Tsitkanou et al., 2017). I provided the mean effect for changes in limb girth for Jones, Howatson, Russell, & French (2013) and changes in vastus lateralis thickness for Ahtiainen et al. (2009). For Bell, Syrotuik, Martin, Burnham, & Quinney (2000) and Kraemer et al. (1995), I combined the increases in type I and II fibers but for Putman, Xu, Gillies, MacLean, & Bell (2004) I provided the changes in fibers separately for reasons discussed later in this article. Lastly, I provided both the percent change in lean body mass and combined the percent change in cross sectional area of type I and II fibers for Hakkinen et al. (2003).
Of the studies investigated, 11/17 showed that resistance training alone results in greater improvements in strength (Bell, Syrotuik, Socha, Maclean, & Quinney, 1997; Cantrell et al., 2014; Chtara et al., 2008; de Souza et al., 2013; Fyfe et al., 2016; Glowacki et al., 2004; Hennessy & Watson, 1994; Hickson, 1980; Kraemer et al., 1995; McCarthy et al., 1995; Tsitkanou et al., 2017). While some of these effects were minor (de Souza et al., 2013; McCarthy et al., 1995), some were pretty substantial (Hickson, 1980) suggesting that there is an interference effect for strength athletes. As discussed later, these detriments may be avoided by separating aerobic and anaerobic training sessions, consuming proper nutrition, or altering the parameters of aerobic exercise performed.
If you are a bodybuilder, these changes in strength probably don’t discourage you too much as you’re mainly concerned with the changes in hypertrophy. Of the studies investigated, 10/21 showed that concurrent training results in greater improvements in hypertrophy (Ahtiainen et al., 2009; Cantrell et al., 2014; Chtara et al., 2008; de Souza et al., 2013; Fyfe et al., 2016; Lundberg et al., 2013, 2014, McCarthy et al., 1995, 2002; Mikkola et al., 2012). This suggests that there is mixed evidence on the effects of concurrent training on hypertrophy and an individual can choose to believe if concurrent training increases, decreases, or doesn’t change hypertrophy. At this time, there appears to be no right answer to this question.
One of the potential reasons why some experiments may have found greater improvements in hypertrophy with resistance training alone could be due to caloric intake. As concurrent training will expend more energy, an athlete will need to consume more food to maintain their weight. Indeed, Hennssey & Watson (1994) and Häkkinen et al. (2003) found a greater decrease in percent body fat in the concurrent training group suggesting that they were underfed throughout the experiment. If caloric intake was better controlled, these studies may have found equivalent improvements in hypertrophy for the concurrent training group. Furthermore, the additional food intake may have helped better fuel the participants workouts and perhaps allow them to lift more weight for a one repetition-maximum.
Another potential reason some experiments found greater improvements in strength and hypertrophy with resistance training alone could be because the training program was simply too exhausting. Hickson (1980) had his participants perform 5 weightlifting workouts for 30-40 minutes and 6 aerobic workouts for 40 minutes each week. This equates to about 7-hours of exercise a week, which is a lot considering the participants were not engaged in a regular exercise program for at least 3-months prior to beginning the study. Also, the cardio sessions themselves were pretty intense with the participants “running as fast as possible” for 40-minutes, 3 times a week (Hickson, 1980, p. 256). While the participants were not overtrained, it is likely that they were experiencing significantly greater fatigue from the workout regimen than the resistance training group. This could have led to inferior improvements in strength and hypertrophy.
Now you may be wondering why 10/21 of the experiments showed concurrent training to result in greater hypertrophy than weightlifting alone. There isn’t a whole lot of evidence to support a proposed mechanism, but I can hopefully provide some general knowledge about exercise that may help. A common notion among bodybuilders is that you damage your muscles in the gym while lifting weights so that they grow back stronger once you leave the gym. From a biological perspective, we can reword this statement to say that in the gym we activate AMPK to help fuel our workouts so that muscle protein synthesis can be elevated for 48-72 hours once we leave the gym (Kumar, Atherton, Smith, & Rennie, 2009). As you can see, the catabolic pathways that we activate during exercise are of little importance since the anabolic pathways are activated for a much longer time after the cessation of exercise. Interestingly, one study found greater AMPK activation following resistance training alone when compared to concurrent training (Fyfe et al., 2018). This same study also found evidence that ribosomal RNA, which helps make up the ribosomes where the translation of messenger RNA to proteins occurs, increases to a greater extent following concurrent training. Additionally, I would hypothesize that the increase in calcium and free radicals from aerobic exercise would lead to an enhanced activation of calcineurin and reactive oxygen species which may lead to superior hypertrophy. Also, the increase in cell swelling, hypoxia (lack of oxygen), metabolic stress, growth hormones, and testosterone from cardio may further help with hypertrophy (Schoenfeld, 2010).
A Couple Studies of Note
As a general rule of thumb, aerobic training will transition the characteristics of fast-twitch muscle fibers to slow-twitch, and resistance training will lead to the transition of the characteristics of slow-twitch fibers to fast-twitch. Bodybuilders prefer the later as fast-twitch fibers appear to have a 50% greater growth capacity (Adams & Bamman, 2012). Interestingly, the studies that did muscle biopsies before and after the training intervention found some surprising results. Fyfe et al. (2018) showed resistance training alone to result in a greater increase in type I fibers than concurrent training. Kraemer et al. (1995) showed a significant decrease in type IIb  and increase in type IIa in both groups and Putman et al. (2004) showed a significant decrease in type IIa/IId(x) in both groups. Putman et al. (2004) also showed a greater increase in muscle fibers for the resistance training group but there is a large discrepancy between the hypertrophy of type I and II fibers. As shown in table 1, the concurrent training group showed a greater increase in type II fibers while the resistance training group showed a greater increase in type I fibers. This evidence questions some of the popular ideas about fiber type transitioning but ultimately shows that there are little differences in fiber type transitioning between concurrent training and resistance training alone. If anything, concurrent training may result in greater hypertrophy of type II fibers.
Lastly, Davitt et al. (2014) compared the effects of performing endurance exercise first followed by resistance exercise to performing resistance exercise first followed by endurance exercise. While there were no significant differences for any measure between the groups, there was a trend toward favoring performing cardio before weightlifting. This is interesting as I would guess that it would be better to do weightlifting first as you would be fresher when performing your heavy resistance exercise. This is supported by a recent meta-analysis which concluded that it is best to perform weightlifting before cardio (Murlasits, Kneffel, & Thalib, 2018). At this time, the data seems to suggest that performing cardio and weightlifting separately may be best (Murach & Bagley, 2016; Sale, Jacobs, MacDougall, & Garner, 1990).
As you can see, there is seems to be little evidence to suggest that bodybuilders should avoid cardio. If you are a strength or power athlete, cardio may be something to be weary of. Performing a minimal amount of cardio (150-minutes of moderate intensity each week) and consuming adequate nutrition to maintain your current body composition may help prevent the decrements in strength and power. Ultimately, it is something that an athlete should experiment with individually. To finish this article, I would like to provide a summary of some of the main points made and some of the potential ways to perform concurrent training to maximize your gains.
Summary Points and Practical Recommendations
1. Aerobic exercise results in superior cardiovascular benefits compared to weightlifting
2. Weightlifting results in superior improvements in strength and hypertrophy (and to some extent, bone mineral density) which may help counteract rising rates of sarcopenia, osteoporosis, and their respective falling-related injuries.
3. Cardio does not burn muscle. While proteins do provide some of the energy needed for exercise, the proteins will most likely not come from muscle tissue.
4. Power athletes are better performing strength training alone.
5. At this time, it is more or less unknown what effects concurrent training has on hypertrophy. Due to the enormous amount of health benefits observed from aerobic exercise, I would hypothesize that concurrent training is probably a good idea.
6. High-intensity interval training may be better than moderate-intensity continuous exercise for trying to increase muscle mass (Wilson et al., 2012).
7. There is evidence to suggest that it may be better to perform cardio and weightlifting in separate sessions or separated by at least 6 hours (Murach & Bagley, 2016). If you have to perform weightlifting and cardio in the same session, it is probably best to do weightlifting first (Murlasits et al., 2018).
8. The mode (e.g. elliptical, stair master, treadmill) of aerobic exercise may be important. Wilson et al. (2012) showed that the interference effect may be body part specific (i.e., you would see smaller increases in strength and hypertrophy if you ran on leg day compared to running on chest day).
9. Wilson et al. (2012) found running but not cycling resulted in decreases in strength and hypertrophy. This could be due to the high impact nature of running which may lead to more muscle damage and fatigue. Additionally, Murach and Bagley (2016) recommend performing cycling over running.
10. Consume 0.14 gram of protein per pound of bodyweight (~20-30-grams) and 0.5 grams of carbohydrate per pound of bodyweight (~70-100-grams) after exercise to help replenish glycogen stores and stimulate muscle protein synthesis (Perez-Schindler, Hamilton, Moore, Baar, & Philp, 2015).
 Any weight-bearing activity results in increases in bone mineral density. So, while weightlifting will result in greater increases in bone mineral density than cycling or swimming, activities such as running may result in equivalent increases bone mineral density. More evidence is needed to determine if weightlifting or running is better for increasing bone mineral density. At this time, it appears that concurrent training may provide the most optimal increases in bone mineral density (Zhao, Zhao, & Xu, 2015).
 There are many other catabolic pathways that AMPK may play a role in but for the purpose of simplicity, I choose not to discuss them. For the curious, I direct you to Fyfe, Bishop, and Sept, 2014 for a review.
 I have only glanced the surface of the acute molecular interference between aerobic and anaerobic exercise. For the curious reader, I direct you to a Current Opinion paper that summarizes the evidence that contradicts the interference effect (Murach & Bagley, 2016). This paper discusses some studies that have shown an increase in anabolic signaling from concurrent training.
 Type IIb is only found in animals but this was a 1995 study.
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