So where does this leave us? We can use weights as heavy as our 6RM or as light as our 15RM and trigger growth at least for a short time; both extremes creating a deficiency in metabolic and stress respectively. If our goal then is to maximize gains we will need to spend most of our time in the middle, or the 9- to 11RM (~75%1RM) rep range. In this range we have the most effective blend of load stress and metabolic stress.
In reality, the intensity sweet spot is a moving target. It would not be right if I were to stop at this point, leaving out a major real-world problem that all long-term lifters experience at some point. Most of the research available showing minimum and/ or maximum effective loads used relatively untrained subjects. One of the reasons untrained subjects are used in the majority of hypertrophy research is that the aim of the researcher is to successfully achieve measurable hypertrophy and untrained muscles are very sensitive to just about any loading stimulus. Hypertrophy is difficult to achieve in highly trained lifters, especially in the short period of time used in most studies. As a result, estimates about effective training intensities are generally below what would be most effective for veteran lifters.
There is a hypertrophy-specific principle which states, “the effectiveness of any given load is determined by the condition of the tissue at the time the load is applied.” We see this principle at work when the weight loads you began making progress with no longer produce results. What happens is that as the tissue adapts to the weight loads you use, the threshold for creating an effective stimulus goes up.
The reduced effectiveness of a given weight load over time is what researchers call the “repeated bout effect” (RBE). The RBE is caused by a number of adaptive mechanisms. For example, the growth signal that is initiated by load stress becomes weaker and shorter. As a result the anabolic effect of each training session is significantly reduced. Whereas an untrained lifter will induce an anabolic state within his muscles that will last well over 24 hours, a veteran lifter will only stay anabolic for at best about 12 hours. This assumes proper diet and supplementation before and/or after his workouts.
So what’s a lifter to do to overcome the RBE? Well, you have to progressively increase the load and metabolic stress over time. And when you can no longer increase the load and metabolic stress, you need to decondition the muscle so that the minimum effective loads become effective as the starting point of your next hypertrophy-specific training period. Keep it in the intensity sweet spot. - FLEX