Researchers have been studying the effects of combining aerobic and resistance training on muscle growth (hypertrophy). Some previous research suggested that aerobic training might interfere with the muscle growth that comes from resistance training. . However, this new research shows that aerobic training either does not inhibit the muscle growth stimulus from resistance training or may even enhance it. In addition, the new research found that the cross-section and volume of the muscle increased. These results were found by comparing combination training with resistance training. The research also looked at specific biological factors related to muscle growth and found that some factors that help with protein synthesis increased, some decreased, and others stayed the same. There were also changes or no change in factors related to breaking down proteins. Overall, these new findings challenge earlier conclusions about the effects of combining aerobic and resistance training on muscle growth.

Here is a review article of recent scientific literature

Diametrically opposed evidence, compared to previous research, regarding the interference effect of physiological adaptations on muscle hypertrophy during combined resistance training, aerobic endurance training

TZANETAKIS IOANNIS MSc Exercise Physiology

1.Introduction

The “principle of specificity” is one of the key pillars of maximizing desired physiological adaptations. However, the requirement to develop many different abilities in the body in order to optimize athletic performance , as well as recommendations for general health and fitness from organizations such as the American College of Sports Medicine (2013), have led to the implementation of training programs with simultaneous training of abilities, which appear to have interfered with each other’s adaptations. This was demonstrated by Hickson RC (1980) in a study in which the sample was exercised with 6 aerobic training sessions and 5 resistance training sessions (weights etc.) per week. Both types of exercise were done at high intensity. Results showed that while resistance training did not interfere with endurance adaptations, resistance training negatively interfered with resistance training adaptations. Since then and for many years, the results of this research were considered by the scientific community almost as dogma and the findings were found to be verified, as in the meta-analysis by Wilson JM et al. (2012). However, a more recent review by Murach K,Bagley (2016) showed the existence of contradictory evidence that endurance training enhanced the hypertrophic stimulus.
The incorporation of both of these types of exercise into periodization training programs is called concurrent training.

Apró W. et al. (2015) applied 2 exercise protocols, one with combined interval aerobic interval training followed by resistance training, and 2 weeks later, the same sample underwent the same resistance training. The results of the study showed that the increase in AMPK activity due to the prior aerobic effort did not negatively affect mTORC1 activity during the combination training and, at the same time, that there was no increase in AMPK at the end of the individual resistance training. The second finding contradicts some previous research, but this is probably due to the fact that the trainees in the present study were more trained and familiar compared to the sample in the earlier study which showed the opposite. The increased activity of mTORC1 is also explained by the increase in ribosomal kinase S6K1 and factor 4E-BP1
Pugh JK, et al. (2015) applied 2 exercise protocols, one with combined resistance training followed by interval aerobic exercise training, and 7- 25 days later, the same sample underwent the same resistance training. A greater increase was observed in the PGC-1a protein that acts as a transcriptional co-regulator with respect to genes that regulate energy metabolism and is the main regulator of mitochondrial biogenesis in the case of HIIT followed resistance training. In addition, similar inhibition was observed in the proteolysis markers FOXO3A and atrogin-1 in both conditions. A greater increase in PGC-1a-ex1b factor was also observed after the combination training test than after resistance training alone, whereas the increase in this factor is typical after resistance training. However, it has not been adequately documented that this factor is associated with muscle hypertrophy as there is conflicting evidence from previous studies. Although in a study by Ruas JL, et al. (2012) we can speculate that this increase is associated with muscle hypertrophy.
Fernandez – Gonzalo R. Et al. (2013) implemented a 5-week exercise protocol in which one leg was exercised with a combination of aerobic exercise training of gradually increasing intensity until failure , followed 6 hours later by resistance training on a flywheel until failure. They found that after concurrent training the exercised leg showed 5% greater increase in peak power than leg exercised with resistance training alone, 6% greater increase in Vastus Lateralis volume in the same leg after MRI measurement,, 8% greater increase in transverse cross-sectional area, same increase in peak torque in both legs. Two times greater increase in PGC-1a, two times greater decrease in MuRF-1 and one and a half times decrease in atrogin1 after concurrent training .Both of the previous two factors are markers of proteolysis. No differences in mTOR phosphorylation were detected, a small difference of 0.3 in the increase in phosphorylation of p70S6K during concurrent training. Although the authors note that the markers indicating adaptations for increased anabolism are only short-lived, and cannot be generalized for long-term conclusions , I note that in the present study, hypertrophy of the external platys , both from MRI and from the increase in muscle cross-sectional area was observed.
Apró W, et al. ( 2013) in this research implemented a protocol in which they had an inverted exercise order in their combination training compared to the 2015 research. That is, resistance training preceded aerobic training and this was continuous moderate intensity training, as opposed to the HIT they included in the 2015 research. The results showed that aerobic exercise did not interfere with the hypertrophic stimulus, at least to the extent revealed by markers of the mTORC1 metabolic pathway, namely S6K1 and eEF2. In addition, all transcription factors showed similar responses between concurrent training and resistance training alone, with the exception of PGC-1, which showed greater gene expression in concurrent training
In the study by Lundberg TR, et al (2013) which lasted 5 weeks, the 2 training protocols compared were: the first, unilateral aerobic exercise on an ergobicycle , followed by resistance training on flywheel knee extensions, to the concentric and eccentric failure. And second, the training of the other leg with the same resistance training. There was a striking increase in muscle volume of the outside flat, measured by both MRI , and a considerably larger increase in cross-sectional area, measured by biopsy after the training intervention with concurrent training, compared to the considerably smaller increase resulting from resistance training. These increases are speculated by the researchers to be attributable to both an increase in sarcoplasmic proteins and myofibrillar constituent proteins. A greater increase in type I muscle fibers was also observed during concurrent training.
In the study by Lundberg TR, et al. (2012) which preceded the above research by the same researchers, using a similar training approach, greater activation in mTOR and p70S6K signaling was detected after concurrent training, compared to resistance training alone. This fact demonstrates to us that at least the expression of these factors is not compromised when aerobic training precedes resistance training. And myostatin remained suppressed for a longer time after combination training than after resistance training, and hence it does not appear that protein synthesis is inhibited by this training approach. Rather, we would say that it favors and enhances the anabolic environment.

Resistance Training (RT) , Endurance Training (ET), Concurrent Training (CT), High Intensity Interval Training (HIIT), Cross Sectional Area (CSA),Muscle Fiber (MF)

2.Conclusions
New research on the issue of muscle hypertrophy during combined aerobic and resistance training, challenges the established view that aerobic training causes physiological interference, which inhibits the hypertrophic stimulus, mainly coming from resistance training. Some of the studies, using both MRI and biopsies, showed no inhibition of the hypertrophic stimulus, while others showed that concurrent training enhances it. There was also an increase, per se, in the transverse cross-section of the muscle, as well as in its volume. All results came from a comparison of concurrent training with resistance training. Rather enhancement of mTOR signaling is shown, and gene expression, either enhanced, or negatively unaffected by aerobic training, is shown. Indeed, protocols in which aerobic training precedes resistance training, and protocols following it, have been tested. In the above studies, by examining physiological – gene expression factors, we see that either those related to protein synthesis show an increase, or those related to protein synthesis show a decrease, or the inhibitors of protein synthesis are stable. There is also a decrease, or stability, in factors related to proteolysis. If anything, the latest research and reviews directly challenge the conclusions previously drawn.

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