Western blotting results revealed that no changes were seen in the controls, but the phosphorylation (p) of p70S6K, p85S6K, rpS6, and MAPKs p38Alpha and p38Gamma were increased after both REs. However, the increase in p-p70S6K, p-rpS6 and p-p38Gamma were larger after HRE. p-Erk1 and p-Erk2 MAPKs increased only after HRE.While scientists are still figuring out the exact mechanism that triggers / drives muscle growth it is contemporarily accepted that mTOR/MAPK-signaling are the major players in an adaptation process at the end of which hypertrophy is the well deserved reward for all the strenuous in the gym.
On a side-note: In a recent meta-study (Krieger. 2010), i.e. a review of all available literature on the topic the author concludes:
multiple sets are associated with 40% greater hypertrophy-related effect sizes than 1 set, in both trained and untrained subjects.The findings of Hulmi, on the other hand, give us an idea of the underlying biological reasons for the superiority of multi-set training.