The scientific literature clearly delineates that "intensity" is the prime stimulus for our bodies to change and improve as a result of the engagement in a resistance exercise program. Intensity is the most important controllable factor (uncontrollable factors include percentage of fast twitch or slow twitch muscle fibers, etc) in an individuals response to an exercise program. It appears that the other variables of an exercise program, although important, are simply not as meaningful as the intensity with which we exercise.
This leads us to an important question: What is intensity? Intensity can be defined as a percentage of our momentary ability to perform an exercise. Stated otherwise, it has nothing to do with the amount of weight we lift, it has everything to do with our effort. Lifting a relatively heavy weight for 6 reps or a relatively light weight for 20 reps are both deemed intense so long as it is utterly impossible to lift a 7th rep or a 21st rep. When looking to produce better results, faster results, or break through a plateau, the first place to turn your attention to should be your training intensity. Most well-intended trainees err in almost the opposite direction. They add more exercises, add more sets and/or increase the number of weekly workouts. All of theses are steps in the wrong direction. Increase your intensity...and less workouts and less exercises will become MORE.