30 Min "Moderate Intensity Cycling" Enough to Cut Biceps Muscle Gains by 59%, Follow Up on 2015 HIIT Study Says

"Cardio Can BOOST Your Gains?! Do it Before Weights and be Rewarded With 28% Increased Fiber Size & VO2 Gains" - This previously discussed study seems to refute the results of the study at hand; but there are a few important differences: muscle groups involved, type of exercise, timing.

Putting things into perspective: As usual, there are a few issues you should keep in mind when interpreting the results - with the most important issue being the lack of dietary control. With neither control nor suggestions for energy and protein intake, it's not clear if and to which extent the reduction in biceps gains may be mediated by a decreased ratio of protein intake/energy requirement and/or a reduced ratio of total energy intake/energy requirement.

In addition, the absolute differences don't sound anywhere as huge as the 59% relative difference in CSA gains. While the SEP group upped their sleeve size from 8.8 to 9.9 cm² (+1.1 cm²), the CT group made it from 9.7 to 10.2 cm² (+0.5 cm²) - that's 0.6 cm² more for the "biceps only" group.

It's also odd that the size gains were impaired, while the strength gains were not. Previous studies usually found detrimental effects on both (Hickson 1980, Hennessy 1994, Leveritt 1999, Häkkinen 2003) - albeit in most cases in studies where cardio and strength training trained the same muscle group (legs). The mechanistic explanation for these observations is that the muscle receives disparate signals (Docherty 2000), with one saying build more mitochondria and the other signaling the muscle to increase protein synthesis, is thus not really applicable the study at hand. Plus: The results are inconsistent (Alabinis 2003) and depend on the "modality, frequency, and duration of the endurance training selected" (Wilson 2012).

A working model of the intracellular signaling networks mediating exercise-induced skeletal muscle adaptations. While RT works the mTOR signaling cascade, "cardio" ramps up AMPK signaling, which may (a) inhibit mTOR signaling via TSC and (b) suppress resistance exercise-induced muscle-protein synthesis (Nader 2006).

With respect to the observations in the study at hand, the authors speculate that the reduced gains are a potential consequence of reduced "phosphocreatine (PCr) recovery in strength trained muscle [...] due to blood redistribution for subsequent leg exercise" (Tomiya 2017). The authors elaborate: "Increased blood flow in arm muscles is important not only for early recovery of PCr but also for muscle development after strength training. Blood flow might be decreased in the arm during lower extremity exercise due to redistribution". It would be nice if we had fractional muscle synthesis or other data that would give us more than speculative insights into the underlying mechanism of the effect... also, if the scientists' hypothesis is correct, any type of exercise that doesn't involve the previously trained muscle group would compromise your gains. If that's indeed the case, this would mean that you'd better switch to a 6-way split if you want to maximize your gains. Studies that this way of training is superior, on the other hand, doesn't exist.

Furthermore, the previously discussed study by Kazior suggests, albeit for cardio before weights, increased gains in fiber sizes, when the same muscle (here legs) are trained during both, the cardio and resistance training regimen. Would be interesting to see what pre-lifting arm cranking would have done to the subjects' gains... but hey: that's a chance for another follow-up | Comment!