Concurrent training (CT) seems to impair training-induced muscle hypertrophy. This study compared the effects of CT, strength (ST) and interval training (IT) on the muscle fiber cross-sectional area (CSA) response, and on the expression of selected genes involved in the MSTN signaling mRNA levels. Thirty-seven physically active males were randomly divided into four groups: CT (n=11), ST (n=11), IT (n=8) and control group (C) (n=7) and underwent an eight-week training period. Vastus lateralis biopsy muscle samples were obtained at baseline and 48 hours after the last training session. Muscle fiber CSA, selected genes expression and maximum dynamic strength (1RM) were evaluated before and after training. Type IIa and type I muscle fiber CSA increased from pre- to post-test only in the ST group (17.08% and 17.9%, respectively). The SMAD-7 gene expression significantly increased at the post-test in the ST (53.9%) and CT groups (39.3%). The MSTN and its regulatory genes ActIIb, FLST-3, FOXO-3a and GASP1 mRNA levels remained unchanged across time and groups. 1RM increased from pre- to post-test in both the ST and CT groups (ST=18.5%; CT= 17.6%). Our findings are suggestive that MSTN and their regulatory genes at transcript level cannot differentiate muscle fiber CSA responses between CT and ST regimens in humans.
Alex’s notes: Cardio? Weights? Both? Which first? These questions are all valid, and how you design your fitness routine will depend on your goals. Performing both cardio and resistance training is commonly recommended to the masses for health promotion, but recently it has been hypothesized that doing both results in diminished returns of both. With regard to strength and hypertrophy, concurrent training may enhance the mitochondrial biogenesis pathways while inhibiting protein synthesis. Specifically, it has been shown that although concurrent training results in similar strength and hypertrophy gains as pure strength training, type-I muscle fiber hypertrophy may be diminished.
The study at hand aimed to verify this through gene expression. 37 individuals with no previous weight training experience underwent twice weekly training sessions for eight weeks of strength training (ST), interval training (IT), or concurrent training (CC). Importantly, the CC group alternated their exercise order (resistant training or interval training first) every training session. The strength training was aimed at hypertrophy, using a 6-12RM for 3-5 sets of leg press, leg extension, and leg curls. The intervals were performed at 80-100% of VO2 max. The diet was not controlled and the participants were simply instructed to maintain their current diet and not take any supplements.
Muscle hypertrophy was only significant in the ST group, but maximal strength increased similarly in both the ST and CC groups. IT and CC also increased aerobic fitness while ST did not. These results make sense when thinking about the principle of specificity, but the lack of hypertrophy in the CC group is interesting, especially since the myostatin pathways response was similar in both groups. CT training literally blunted hypertrophy across the entire spectrum of muscle fibers, and in beginners nonetheless, which is more concerning given that beginners do not need as great a stimulus to induce adaptation. These results are in accordance with previous studies looking at concurrent strength and endurance training.
I suppose the take-away is that if your goal is strength and aerobic fitness, CC is good. However, if maximal hypertrophy is your goal, then you best avoid cardio training.