Does light weight build muscle
- New research on the growth of type I and II fibers suggest we may have been selling our slow-twitch fibers short by only training heavy.
- Type I fibers are maximally stimulated with longer duration sets requiring lower loads. Type II fibers respond best with short sets with heavy weights.
- There are a lot of ways to vary intensity in your program, like periodizing training rep ranges over time, as well as using heavy weights with multi-joint exercises and using lighter weights with single-joint movements.
"Lift heavy to grow" is a favorite mantra of coaches and gym rats alike. Heavy weights maximally recruit the large motor units associated with type II fibers, and since the type IIs are strength-related fibers with the greatest growth potential, focusing on their maximal recruitment is the key to getting swole, right?
Well, not so fast.
Don't Short-Change Your Slow-Twitch
Type I fibers are like the Rodney Dangerfield of the bodybuilding world – they get no respect. Slower, weaker, and often smaller than their fast-twitch counterparts, the type I's only claim to fame is an ability to contract repeatedly, albeit without much force.
Relegated to a life of 5K's, marathons, and unsettlingly tight running shorts, at first glance this fiber type's ability to resist fatigue seems to be more of a bodybuilding curse than a blessing. As such, training philosophies typically revolve around stimulating and annihilating the type II fiber, with little consideration for the lowly slow-twitch fibers.
However, new research on the effects of different training intensities and the growth of type I and II fibers suggest we may have been selling our slow-twitch fibers short and, in the process, missing out on pounds of potential muscle (1).
It's time to re-think our training philosophies in the context of fiber-type specific hypertrophy.
Big Weights and Type II Fibers
Certainly, a wealth of studies suggest type II fibers do, in fact, grow more with high intensity strength training (2). The caveat here is "high intensity." It's not necessarily that type II fibers have an innate ability to outgrow their slow-twitch relatives, but that they show superior growth when trained at higher intensities (>50% 1RM).
Our current understanding of the hypertrophy of each fiber type may be more a consequence of the way we've studied them (high intensity) than what actually happens in the gym (2, 3). The best summary of this relationship is a 2004 paper from Dr. Andrew Fry, who compiled the data from various studies on the growth rate of fiber types and found that, at most training intensities, the type II fiber reigns supreme.
As training intensity decreased below 50% 1RM, the type I fibers eventually outgrew the IIs, but the rate of growth in this range was still nowhere near what was achieved at higher intensities, regardless of fiber type. After reading a study like this, not much would change in our training recommendations, but there are limits to the type of analysis (regression) performed by Fry (2).
The biggest limitation is that there just weren't that many low-intensity training studies out there to compare (2, 3), and a paucity of any that directly compared high-intensity against low intensity training while accounting for growth of the different fiber types.
Add to that recent evidence on the growth rates of muscle fibers in response to differing training intensities (1) and you'll soon see that our type I fibers are capable of more than we give them credit for.
The Case for Type I
While they may be rare, there are enough studies for us to infer we've probably underestimated the hypertrophic capacity of our type I fibers. Recently, Mitchell et al. (1) performed a now infamous training study demonstrating that, when taken to failure, training with low loads (three sets at 30% 1RM) can produce comparable hypertrophy to training at higher intensities (three sets at 80% 1RM).
Looking at the individual fiber types, while the data may not be statistically significant, we see that the type I fibers responded slightly more to low intensity training (19% change versus 14%) and type II fibers more to the high intensity training (15% versus 12%).
Ultimately this suggests there's more to the equation than the number of plates you have on the bar and tentatively supports what may be intuitively obvious: Type I fibers are maximally stimulated with longer duration sets requiring lower loads, while Type II fibers respond best to short sets with heavy weights.
A common complaint with most training studies is that the researchers use untrained college students. What happens in these guys' underdeveloped physiques may not represent what would happen in highly trained muscle. Fortunately, when we look at the muscle fibers of various trained athletes, we see support for our fiber type hypertrophy theories.
Bodybuilders typically emphasize volume, fatigue, and use moderate repetition ranges (4), while their powerlifting (5) and Olympic weightlifting counterparts emphasize load and/or movement speed. Not surprisingly, bodybuilders display significantly greater hypertrophy of type I fibers compared with the strength-oriented athletes (2).
Taking all evidence into account, it seems reasonable to conclude that differing training intensities can produce comparable whole muscle hypertrophy (1, 6-8), but the fiber-types affected may differ.
As with most things in the scientific world, it's not a cut and dried issue. Two additional studies have investigated this topic – albeit with slightly different designs – and both found high-intensity training to be superior for growth irrespective of fiber type (9, 10).
Here's where it gets interesting. While there are exceptions, studies that equate work between high and low intensity conditions tend to favor high-intensity training for both fiber-type specific and overall muscle growth (10, 11). Those that don't match the work performed between conditions find equivalent results across training intensities.