I actually meant to say repeating - not zig-zagging...
Actually...let me be more specific on my WO.
I initially was incrementing "properly" during 15s (1-2-3-4-5-6) and continued the trend into the 10s. However, I had to adjust for the 5s because I didnt want to zig-zag for those. So I ended up repeating during the 5s so I wouldnt have to reduce weight on the first WO of 5s below the the final WO of 10s (ie the 10rm).
So in the 5s I'm doing 7-7-8-8-9-9 carrying on from 1-2-3-4-5-6 in the 10s.
Gains started coming during 2nd half of 10s and are pretty noticeable now into 2nd half of 5s. I don't like the idea of zig-zagging personally - but thats only a psychological thing
Back in 2003 (ive had a 2 year WO gap), i DID do quite a bit of zig-zagging. However, my gains were still quite substantial. 1-1.5 inches gained for each body part.
I'd take it then, atleast from my experience, that zig-zagging and repeating have negligible differences.
For the ERK1/2...metabolic demands drop as volume is reduced (ie going from 10s to 5s). This then causes reduced activity of ERK1/2 which is involved in hypertrophy. So I take it that the dropsets makes up for the decreased volume and increases metabolic demand...hence increasing activity of ERK1/2 and hence hypertrophy.
And on 5s again... if muscles can adapt in as little as 48hrs, repeating 5rm for 2 weeks (eg 150lb) would be pointless? Well maybe not pointless but not a efficient as further loading?