I came to the conclusion sometime ago that drop-set's are pretty much the most intense way to do resistence exercise. Done properly you can fatigue pretty much every type II fibre in the muscle. Recovery time after 1 drop-set is noticably longer than for example just working with a weight at your 10RM.
I began experimenting with drop-sets on my biceps to test thier effectiveness as predicted by the size principle and I have some observations to report.
I have only been resistence training seriously now for about 6 weeks, the first time I did a drop-set on my biceps (went close to failure at each weight), it took about 5 days to recover from 1 set! During the recovery period there was extreme pain and discomfort, I would have to keep the bicep muscle in a maximal extended position in order to relive some of the DOMS. I guess this was just becuase I was not used to working out.
Its now been about 2.5 weeks since then and now my recovery time for for 1 drop-set on biceps is about 36 hours. Muscle mass has gone up significantly, strength a bit.
More interesting though, I had assumed that drop-sets would be superior to just doing high reps with a low load to fatigue all fibres, mainly becuase overall it should of meant doing less reps in total.
However it seems that isnt true, instead I found myself doing the same total number of reps in a drop set compared to just doing straight reps to failure with the lowest weight I would use in the drop-set.
Recall to neurons are exactly like logic gates, either they are firing or they arent, there is no "half" firing. With the above anecdote in mind, this suggests that fibres have a fixed and finite "firing" time before they become totally fatigued. This is also independent of the weight you are lifting.
Still, I think there is a bone density advantage to using heavy weights atleast 50% of the time, as stress on bones is higher with higher weight. And we know from Wolff's law that bone stress determines bone density.