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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