Not a whole lot. First of all, both groups consisted of experienced exercisers. This requires more discipline than I can muster and likewise for most of my patients. Thirdly, the fact that both groups improved is interesting because the only change in the control group was to exercise to muscle failure. I believe that the ideal number of sets for most people who work out is two sets as a minimum and three if time allows.
Most individuals are a bit inflexible and prone to injury.
The first set can be a kind of warm-up, getting your muscles and ligaments ready for a harder set to follow. The second set can be done to muscle failure, or at least to fatigue, and that is usually sufficient. Some people prefer a third set, especially those who do higher reps, want to burn calories, or are looking for tone rather than strength and bulk. I believe that individuals who are not prone to injury and have limited time could carefully use this approach. Finally, we would all probably improve our strength, endurance and body composition if we were enrolled in a thirteen week study and knew we were going to be examined before and after.