If you do weight training first and foremost to build muscle mass rather than strength, then you might be interested in the study that sports scientists from the Federal University of Parana published in the Journal of the International Society of Sports Nutrition.
The article describes an experiment involving 22 experienced amateur bodybuilders, all of whom were creatine users . The first week they took 20 g creatine and 20 g maltodextrin daily; after that the dosage went down to 5 g creatine and 5 g maltodextrin a day.
All subjects trained six times a week and worked in cycles of three workouts in which all of the main muscle groups were covered; they all did the same exercises. They trained with weights at which they could manage 8-10 reps and did four sets of each exercise.
Half of the subjects trained for the eight weeks that the experiment lasted with two-minute rests between sets [CI]. The other half [DI] took increasingly shorter rests. During the first two weeks the DI group took two-minute rests between sets, but from week three they decreased the rests by 15 seconds each week. So in week three the subjects in DI took 105-second rests, in week four 90-second rests, in week six 60-second rests, in week seven 45-second rests and in week eight 30-second rests.
During the experiment the training volume (the number of kilograms the subjects lifted times the number of reps) increased for the group with a constant rest period. In the other group the training volume declined. The graph below shows the volume development for the bench presses.
Both groups gained strength, but the increase in maximal strength increased more in the group with constant rest periods than in the other group.
When it came to muscle mass, however, it was the training with increasingly shorter rests that led to better results. The researchers measured the subjects' muscle dimensions with MRI scans and saw that the muscle mass in the arms [CSAA] and in the thighs [CSAT] increased more in the DI group.
"Decreasing interval seems to be more efficient than constant interval to produce hypertrophic responses", the researchers conclude. They say they are now doing a bigger study, and that they will also look at subjects who don’t use creatine.
J Int Soc Sports Nutr. 2011 Oct 27;8(1):17.