CSA itself can vary greatly due to glycogen uptake from metabolic stress (which would contribute to both the strength-endurance and sarcomere hypertrophy), and in certain subgroup of individuals lack of a sufficient erk1/2 signal may not lead to "ideal" protein synthesis levels. Thus the rationale for adding a burn set or drop set during 5s. For similar reasons, I request a mate of mine, who does Max-OT, to add a burn set at the end of his routines. He also noted it made a palpable difference in "fullness" and endurance-performance.
One could have generated even greater metabolic stress than the single burn set by performing the 5x5 HS protocol using a density-training variation (i.e. extremely short periods.) Or, one could have performed a static hold with the leg extension for up to 60 seconds.
Finally, it's unclear to what extent and frequency both parties were in their load progression during the HS period. If loads stagnated in both groups after awhile, then the differentiator would likely be glycogen storage and uptake, which again would be determined by which group had experienced more metabolic stress. Or to put it another way, it's unclear whether the overall mechanical strain is relatively the same between both groups. Then, in that case, then again the only true differentiator is the metabolic stress.
The differentiation in stress is essentially underlined by the first portion of the study, detailing GH secretions. GH levels from exercise responds to metabolic stress (specifically hypoxia.) The "medium" intensity set was essentially a variation of a drop set, and naturally would generate the most hypoxia. Thus, the higher acute GH levels. In turn, the HC protocol would generate less hypoxia than the medium intensity set, but higher than the HS set. Thus, it's mid-level rank in GH.
Again, it's really not a question of intensity or varying volume. It's relevant from a performance standpoint, but it's (and most mainstream periodization theory) still too high level to fully assess what is really going on in terms of hypertrophy stimulus.
cheers,
Jules
