No Reduction in Protein Synthesis W/ Concomitant Training Involving HIIT Training for Non-Strength Trained Muscles

The results of the study and hand strongly suggest, but don't finally prove that a brief leg-focused HIIT like spinning will not impair your biceps gains even if it's done immediately before weight training.

You will probably have read about the problem that doing endurance before or immediately after resistance training will blunt the protein synthetic response to exercise. In fact, I have written about this topic repeatedly. The latest article was published only two weeks ago ("How Much Cardio Messes W/ Your Weight Training Results Depends on Recovery Times: Cardio + Weights in One Session vs. AM + PM Training vs. Doing Each on One Day" | read more).

Now, a recent study from the Swedish School of Sport and Health Sciences shows that the question whether "cardio" in the broadest sense will interfere with strength training gains cannot be answered with a simple yes or no.
In their study, Moberg, et al. (2014)set out to conduct a series of studies to examine the influence of endurance exercise on the acute stimulatory effect of resistance exercise on anabolic processes. To this ends, the effect of endurance exercise on a previously inactive muscle was investigated. The aim was to examine the influence of resistance exercise on mTORC1-signaling and rate of protein synthesis in the triceps brachii muscle with or without preceding intervals of high-intensity cycling.

Eight trained males performed, in a randomized fashion, two sessions of heavy resistance exercise (RE) with the triceps muscles, where one session was preceded by intervals of high-intensity cycling (E+RE), 5 x 4 min at 85% of VO2 peak. Mixed muscle protein fractional synthetic rate (FSR) was measured at rest, prior to exercise, and during a 3 hour recovery period following exercise by continuous infusion of L-[ring-13C6] phenylalanine. Muscle biopsies from the triceps brachii was collected twice at rest separated by three hours, directly after resistance exercise and following 90 and 180 min of recovery. Signalling in the mTORC1-and AMPK-pathway was assessed using western blot technique.
It is important to point out that the same amount of work with regard to load, total number of repetitions and total time under tension was performed in the two trials.

Figure 1: Fractional muscle protein synthesis response in the biceps brachii and corresponding changes in signalling proteins in the HIIT + resistance training (E+RE) and the resistance training only trial (Moberg. 2014).

Unlike in previous studies, where the "cardio" exercise was usually a steady state exercise and the muscle protein synthesis was measured in muscles that were trained during the "cardio" and resistance training session, mostly specifically the leg muscles, the muscle protein fractional synthesis rates (FSR) in the study at hand increased from 0.050 ± 0.006 %/h at rest to 0.078 ± 0.008 and 0.082 ± 0.0016 %/h following E+RE and RE, respectively, with no difference between trials.

The same goes for the signalling protein AMPK and eEF2,  as well as for the alleged protein synthesis gauge mTOR which was increased by 76% and 108% above rest directly after the E+RE and RE, respectively, and remained elevated in both trials during the entire recovery period.
References:

• Lundberg, Tommy R., et al. "Aerobic exercise does not compromise muscle hypertrophy response to short-term resistance training." Journal of Applied Physiology 114.1 (2013): 81-89.
• McCarthy, J., et al. "Combined strength and endurance training: Functional and morphological adaptations to ten weeks of training." No. NHRC-92-26. Naval Health Research Center San Diego CA, 1992.
• Moberg, Marcus, et al. "High-intensity cycling performed prior to resistance exercise does not influence mTORC1-signaling and the rate of muscle protein synthesis in the triceps brachii." 19th annual Congress of the European College of Sport Science, 2-5 July 2014, Amsterdam The Netherlands. 2014.