If you were curling the maximum weight that you could lift for one rep, then just about every single muscle fiber from slow to fast would be recruited to lift that weight. However, when you pick up a weight that you can curl for 20 reps, you may recruit only the slow-twitch muscle fibers because the weight is so light that the fast-twitch muscle fibers don’t need to be bothered. Since the fast-twitch muscle fibers grow the biggest and strongest, and the slow-twitch muscle fibers need to stay small for maximal endurance, it has been assumed that to really build maximal muscle size the weight must be heavy enough to recruit the fast-twitch muscle fibers.
Now back up to that weight that you could complete 20 reps of curls with. Yes, when you do the first few reps you are recruiting only the slow-twitch muscle fibers. But as the reps continue and those slow-twitch muscle fibers fatigue, the brain starts calling on more and more of the bigger fast-twitch muscle fibers with each and every rep completed. And if you take that 20-rep set to absolute muscle failure (the point where you can not complete another rep) then you recruited the same amount of fast-twitch muscle fibers as you did when you used a weight that limited you to one rep. This is the real key to how higher-rep sets performed with lighter weight may be better for muscle growth than doing fewer reps with heavier weight—training to muscle failure.
While recruiting the maximum number of fast-twitch muscle fibers may be crucial for muscle growth, there appear to be other factors involved. After all, if muscle growth were only about recruiting fast-twitch muscle fibers, then the best way to grow would be to always lift a weight that limits you to one rep. Yet, as we all know, that is not the best way to grow muscle. In fact, that’s not even the best way to increase muscle strength. This is due to the fact that metabolic stress is also important for both muscle growth and muscle strength.
Metabolic stress is created within the muscle when by-products accumulate from the biochemical pathways that are used in the muscle cells to produce the energy required to allow the muscles to continue contracting to lift the weight. These by-products signal other chemicals in the muscle, such as insulin-like growth factor-1, to come in and induce muscle hypertrophy. The by-products even signal other chemicals to be released from other areas of the body. For example, growth hormone levels rise as the levels of the metabolic by-product lactic acid rise. These may all be important for increasing the process of muscle protein synthesis, which is ultimately how muscles grow.
To best way to maximize metabolic stress is to do more reps. Higher reps increase the amount of biochemical by-products that are produced. So as long as you take that high-rep set to muscle failure, you recruit the same amount of fast-twitch muscle fibers as a heavier set for fewer reps, but you also induce greater metabolic stress. And while all of this sounds good in theory, there is also research that supports this contention. Now does training with lighter weights sound a little more compelling?
The majority of the research on lighter-weight/higher-rep training comes out of the Exercise Metabolism Research Group at McMaster University in Ontario. Their first research to grab the attention of strength scientists, as well as well-read bodybuilders everywhere, was a 2010 study they published in the online journal PLoS (Public Library of Science) ONE. They had trained men perform four sets of leg extensions using either a weight that limited them to 4–5 reps per set or a weight that allowed them to complete about 25 reps per set. They took all sets to muscle failure, except for one additional group who used the 25-rep weight but did not go to full muscle failure. When the men did four sets with light weight for higher reps to muscle failure, muscle protein synthesis of the quadriceps was elevated by as much as 60% more than when they did the 4-rep sets to failure. However, both the 4- to 5-rep sets and the 25-rep sets when taken to failure increased muscle protein synthesis far greater than the increase seen with the 25-rep sets that subjects were not taking to muscle failure.
A 2011 study by the same researchers looked at what happens to muscle protein synthesis following heavy-weight or light-weight training when subjects consumed a protein shake 24 hours after the workout. They performed four sets of leg extensions using either a weight that they could lift for about 4–5 reps or a weight they could lift for about 25 reps. Those doing 4–5 reps took each set to muscle failure, while half of those doing the 25-rep sets went to muscle failure on all sets and half did not go to muscle failure. They reported in the Journal of Nutrition that having a protein shake 24 hours after training to failure using either the heavy weight (4–5 reps per set) or the light weight (about 25 reps per set) significantly boosted muscle protein synthesis higher than when they drank the same protein shake at rest or after weight training but not going to muscle failure.
One trend that you should be noticing right away from these two studies is that the most critical factor in increasing muscle protein synthesis, both immediately after training and for the 24 hours that follow, is to take all sets to muscle failure.
That is the only way to recruit all of the fast-twitch muscle fibers and instigate enough metabolic stress. These data finally shut up all those strength-training experts who will try to tell that you shouldn’t train to muscle failure at least not on most sets. They claim that it leads to over-training and it can actually interfere with strength and muscle growth. Of course, the first thing that you’ll notice about most of these so-called experts is the fact that their upper arms barely measure 15 inches. And if their lack of muscle mass doesn’t tip you off, then that kind of nonsensical reasoning about muscle failure tells you right away that this “expert” does not understand the basics of muscle physiology and more specifically, never learned about the size principle. It also tells you that they haven’t been paying attention to the newer research that has been conducted, such as these two studies.
As far as overtraining goes, you have no worries about that as long as you follow a solid nutrition and supplement plan like the ones you find in FLEX. Our ancestors used to be on their feet all day hunting and/or plowing the fields. They did intense labor for the majority of the day and rested only at night when the day was over. Do you honestly think that, after sitting on your ass all day at your desk job or in school, training intensely for 1–11⁄2 hours is going to cause you to overtrain?! Get real! This is just an excuse for those who don’t want to put in the hard work it takes to build real muscle. They’ll blame their genetics when really they should be blaming their poor work ethic and shoddy training techniques. Bottom line—if you take anything home from this article it’s that you should be taking most if not all of your working sets to muscle failure or close to it for maximum muscle growth. The debate is over.
Another finding from these two studies is that training with lighter weight done for higher reps appears to be better at promoting muscle growth than heavier weight done for fewer reps. But hold up, there. Two problems exist with these two studies. The first is the fact that these studies measured only muscle protein synthesis. Yes, that is an indication that the muscle is in a heightened state of muscle growth. But they did not measure actual muscle growth over time training these ways. They just looked at a snapshot from one workout. It is a pretty good snapshot, but it still does not prove that this will lead to greater muscle growth. The second problem is that the heavy-weight sets were done with a weight that limited the subjects to just 4–5 reps per set. Even the most egomaniacal bodybuilder who is more worried about showing off for his fellow gym members rather than getting actual results, typically does not train in this low-rep range. So it really isn’t a fair comparison of heavy weight versus light weight as far as what those terms mean for the average bodybuilder.
The McMaster University researchers finally did a follow-up study to see just how well these light-weight, high-rep sets hold up for instigating real muscle growth compared with more realistic rep ranges. They had subjects perform one of the three leg extension training programs that were performed three times a week for 10 weeks: 1) one set to failure using a weight that limited them to 8–12 reps; 2) three sets to failure using a weight that limited them to 8–12 reps per set; or 3) three sets to failure using a weight that allowed them to complete 20–30 reps. Immediately after each workout the subjects ate a protein bar that provided 30g of protein, 33g of carbs, 11g of fat, and 4g of the branched-chain amino acid leucine.
They reported in a 2012 issue of the Journal of Applied Physiology that both the subjects training with three sets of 8–12 reps to failure and those training with three sets of 20–30 reps to muscle failure increased leg muscle size by about 7%, which was more than double the increase of those training with just one set of 8–12 reps to failure. This increase in muscle size was visible in both the slow-twitch and the fast-twitch muscle fibers. The take-home point here is that to maximize muscle growth you definitely need to perform more than one set per exercise, but whether you train in the 8- to 12-rep range or the 20- to 30-rep range doesn’t really matter, as long as you take each set to muscle failure.
But before you leave thinking that it doesn’t matter what rep range you choose, the researchers also measured muscle strength and muscle endurance before and after the 10-week program. They found that the group training with either one set or three sets of 8–12 reps per set increased their one-rep max strength significantly more than the group doing three sets of 20–30 reps. All groups had a similar increase in the number of reps they could complete with 80% of their one-rep max (1RM, using a weight with which they could complete about 10 reps at the start of the program). But muscle endurance, which the researchers measured by the number of reps the subjects completed using 30% of their 1RM, only increased in the group using three sets of 20–30 reps.
So, although training with both heavy weight and light weight can elicit similar increases in muscle growth, they offer different benefits to the muscle’s capacity. The heavier weight leads to better increases in muscle strength, while the lighter-weight training induces increases in muscle endurance that the heavy-weight training does not.
Change Is Good
The real take-home message from these studies is that you should never stick with one rep range for too long. Regardless of how heavy or how light you go, you will be able to increase muscle growth as long as you train to muscle failure. Changing up your weight from heavy weight for fewer reps to lighter weight for higher reps also allows you to increase muscle strength and muscle endurance. Having greater strength and muscle endurance allows you to train with heavier weights and to complete more reps with a given weight, which can help to better influence muscle growth. And regardless of what each rep range offers, the simple fact that you are changing it, keeps the muscle “guessing,” which prevents it from stagnating, and allows for better continued gains in muscle size, strength, and endurance. This is known as the Weider Muscle Confusion Principle, and it works well... Now we know why!
And if these research studies aren’t enough to persuade you to lighten up from time to time and do some high-rep training, then consider the fact that Tom Platz, who arguably built some of the biggest legs in body-building, was known to frequently do squats with sets as high as 30, 50, and even 100 reps. And even Zack Kahn, who is one of the biggest IFBB pros ever, is known to frequently utilize 50-rep sets.
A great way to change up your rep range is to use linear periodized microcycles, where you start out with lighter weight for higher reps and gradually increase the weight each week until you are down to very heavy weight for few reps. Try my Up and Down 11-week plan that increases the weight each week starting with reps in the 21–30 range and brings you down to just 3–5 reps per set. Then you reverse the order to reduce the weight each week until you are back to doing 21–30 reps per set. You can repeat this indefinitely as long as you change up other variables such as the exercises used and the rest periods allowed between sets. Just be sure to take each working set to muscle failure or close to it, and I can guarantee you will realize new gains in muscle growth.