"HIIT-ing it After Arm Workouts Will Ruin Your Gains", Study Says and Confuses Statistical and Practical Significance

Does this look as if sprinting would impair muscular development of arms or any other muscle? I mean, come on - look at the average sprinter: Many gymrats dream of the arms and overall muscular physique they have; no wonder that the data from the full-text shows a different picture than the abstract would suggest.

I have repeatedly written about combining strength and classic endurance training. With endurance first, endurance last and even endurance in-between the studies yielded often very different results in terms of what would be the optimal way to combine both. With a few exceptions in which resistance training was combined with crazy endurance training sessions, however, I've yet never written about nor seen compelling evidence for the often-heard claim that "cardio ruins your gains".

For HIIT, i.e. high-intensity interval training, a recent study from the Nippon Sport Science University does now claim, though, that my that combining HIIT and weight training may be a very bad idea, ... an idea that may in fact, just as the broscientific nightmare suggests "ruin your gains, bro!" Upon closer scrutiny, however, things appear less unambiguous than the abstract would have it...
The purpose of the study was to examine whether or not lower limb sprint interval training following arm resistance training influences training response of arm muscle strength and hypertrophy. Or in short: Will doing HIIT sprints immediately after an arm workout ruin the strength and strength gains you've primed before? 

Figure 1: According to the study, you better don't do HIIT sprint training after an intense arm workout if you don't want to ruin the strength and size gains you "primed" with curls and co (photo from Kikuchi. 2015)

The subjects, twenty previously only lightly trained men, were divided into resistance a training group (RT, n=6) and concurrent training group (CT, n=6).

• The RT program was designed to induce muscular hypertrophy (3 sets x 10 repetitions (reps) at 80% 1 repetition maximum [1RM] of arm curl exercise), and was performed in an 8-week training schedule carried out 3 times per week on nonconsecutive days. 
• Subjects assigned to the CT group performed identical protocols as strength training (ST) and modified sprint interval training (4 sets of 30-s maximal effort, separated in 4m 30-s rest intervals) on the same day. 

The relevant study outcomes the researchers evaluated were the changes in maximal oxygen consumption (VO2max), muscle cross-sectional area (CSA), and 1RM that were measured before and after the 6-week study.

Figure 1: Relative changes in VO2max (conditioning), muscle size (CSA) and strength (1-RM) over 6 wks (Kikuchi. 2015).

As the data in Figure 1 reveals, significant increases in VO2max from pre- to post-test were observed only in the CT group (p=0.010, ES=1.84), but not in the RT group (p= 0.559, ES= 0.35). The rest of the results in Figure 1, however should be kind of surprising to anyone who has read the researchers conclusion that "our data indicate that concurrent lower limb sprint interval training interfere with arm muscle hypertrophy and strength" (Kikuchi. 2015).

No, you are not mistaken. The average muscle size and strength gain in the combined training group was larger. The reason the scientist still claim that their study would show that HIIT impedes strength adaptation is a statistical one. While the changes in the CT group had p-values p > 0.05 and were thus not statistically significant. The (albeit smaller) mean increase in the RT group was significant. Accordingly, the corresponding "effect size" in the RT group is larger than the one in the CT group and thus HIIT training must be bad, right? Well,... I don't think so.
References:

• Kikuchi et al. "The effect of high-intensity interval cycling sprints subsequent to arm-carl exercise on muscle strength and hypertrophy in untrained men: A pilot study." Journal of Strength and Conditioning Research Publish (2015): Ahead of Print | DOI: 10.1519/JSC.0000000000001315