Training for Size & Strength - Does the Rest Matter? Study Finds 7-9% Greater Increase in Muscle Size With Decreasing Rest Periods.

Image 1: If you want to build Arnold-esque arms you better not sit around too long in-between your sets.
"Short rest periods to burn fat, medium rest periods to build muscle and long rest periods to build strength" - it's actually pretty likely that one of your trainers, gym buddies or fatherly mentors told you something along those lines in the past. In view of the results of a soon to be published international study by Brazilian researchers from the State University of Campinas and the Federal University of Rio de Janeiro and their American colleagues from the Eastern Illinois University, the University of Memphis and the Colorado College (Souza-Junior. 2011), this is probably the next item on list of widely accepted bodybuilding myths that have a spark of truth to them... at least for recreational strength trainees who use some creatine monohydrate to promote their strength and mass gains.

For their study, the results of which are going to be published in the next issue of the Journal of the International Society of Sports Nutrition, Tacito P. Souza-Junior and his colleagues recruited 22 "recreationally trained" men with a minimum of one year resistance training experience at a frequency of 4 sessions a week, who were randomly assigned to one out of two exercise protocols, which differed only in the time the subjects were allowed to rest in-between sets (cf. figure 1).
Figure 1: Identical training protocol for all subjects participating in the study (compiled based on information from Souza-Junior. 2011)
The only difference between the groups was that half of the subjects trained with a constant rest time of 2 minutes between sets over the whole 8 weeks (CI group), while the remaining subjects had to decrease their rest times from week to week (DI group) according to the scheme illustrated in figure 2. The training sessions were supervised and the subjects were " verbally encouraged to perform all sets to voluntary exhaustion". Considering the overall workload and the training frequency, this were probably pretty hard weeks for the 22 trainees.
Figure 2: The rest times decreased according to a standardized protocol by 15 sec each week.
In addition all subjects, who btw. did not follow a standardized diet, consumed the proven creatine + maltodextrin mix (7 day loading phase with 20g/day creatine + 20g maltodextrin followed by a maintenance dose of 5g creatine + 5g maltodextrin taken immediately post workout) that has been used in numerous studies before.

Figure 3: 1RM performance (in kg) for bench press and barbell squat before and after the 8-week training period in subjects with constant and decreasing rest periods (data adapted from Souza-Junior. 2011).
Now, if the initially stated "wisdom" held true, then the 11 subjects with constant rest periods should either have gained more muscle (if you consider 2 minutes a "medium" rest period) or built more strength (if you would say that 2 minutes belong to the realm of "long" rest periods) - yet figures 3 and 4 seem to indicate that neither of that was the case.
Figure 4: Muscle CSA (in cm²) of arm and tigh muscles before and after the 8-week training period (data adapted from Souza-Junior. 2011).
If we have do yet a closer look at the effect sizes, there is yet a notable advantage of the DI protocol in terms of the measured increases in muscle CSA with +14% and +19% in arm and tigh CSA in the constant rest interval group (CI) and +21% and +28% in the decreasing rest interval (DI) group.

There is a spark of truth to every myth

To give you an idea of how significant - and I am talking about "posing significance" not statistical significance here - this is, I have calculated the respective increases in arm- and tigh-circumference, which would differ by 0.4cm and 0.7cm, respectively. Not really outstanding, but nevertheless an important finding of which the researchers say that it lends support to the notion that
decreasing [rest] interval[s] seems to be more efficient than constant interval to produces [sic!] hypertrophic responses.
It has yet to be stated that the 11 subjects in the decreasing rest interval group paid dearly for this increase in muscular hypertrophy, as their "exercise performance" as measured by the total workload per session decreased profoundly from week 1 to week 8: -35% total volume for barbell squats and -30% for bench presses. The subjects who used constant rest periods, on the other hand, increased their total volume by +20% for squats and by +30% for bench presses. That being said, all powerlifters out there better stick to their constantly (long) rest periods if they do not want to compromise their game.