Darren Burns

There are many variables to think about when putting together a training routine, including which exercises, how many reps, how many sets, how much weight, how often to train each muscle group, not to mention diet and supplements. One of the most important of these variables is weight load.

There are two factors critical to inducing muscle hypertrophy: load stress and metabolic stress. Load stress is applied by lifting and lowering a weight, as well as forcing a passive muscle to bear a load (e.g., loaded stretch). Metabolic stress is created by actively contracting a muscle against resistance, which leads to the accumulation of metabolic byproducts such as lactic acid and free radicals.

Both load stress and metabolic stress activate anabolic signaling pathways within the muscle cell. Loading acts through a mechanism called mechanotransduction, whereby membrane-bound mechanosensors, which are activated when the muscle cell membrane is strained and stretched, turn this mechanical signal into chemical signals that tell the cell to build more structural and contractile proteins. The buildup of metabolic stressors such as lactic acid and free radicals activates anabolic signaling pathways, although the exact mechanism is still being ironed out. An additional effect of metabolic stress that is no less important than activating anabolic signaling pathways is the induction and activation of muscle satellite cells. These satellite cells add nuclei to the muscle cells, allowing them to grow.

We control load stress by adjusting the weight loads that we use. In general, the heavier the weight, the greater the load stress. But can a weight be too heavy to build muscle? A recent review of research looking at this question concludes that ~6RM (~85% 1RM) is probably the heaviest weight load that is still effective for growth in most people. Studies involving weight loads heavier than this fail to produce growth comparable to more moderate weight loads. But why would this be, if mechanical load is fundamental to triggering growth? As weight loads go up, time under tension tends to go down, which reduces metabolic stress. From all the available research, it is clear that some level of metabolic stress is necessary to maximize growth. This is why routines advocating low reps only have limited utility for bodybuilders. The principle to be taken away is, as weight loads go up, load stress goes up but metabolic stress goes down.

Just as there exists a maximum effective load, research also points to a minimum effective load. A 15RM (65% 1RM) load appears to be the minimum amount of weight that leads to significant growth in most people. Lighter weight loads emphasize metabolic stress if sets are taken to failure. At the same time, load stress is minimized, which reduces the utility of routines advocating high reps only. The principle to be taken away is, as weight loads go down, metabolic stress goes up but load stress goes down.

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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

Darren Burns