Arnold Schwarzenegger is taking a closer look at how we build slow and fast-twitch muscle fibers. In his recent Arnold’s Pump Club Newsletter (November 10, 2026), he explained that lifting heavier weights may provide an advantage for fast-twitch muscle fibers, making it an important aspect of training as we age.
“Light Weights Build “Slow-Twitch” Muscle – Any time a new study pokes a hole in an old gym belief, it’s worth a look. For decades, it’s been suggested that lighter weights train your slow-twitch fibers, and heavy weights train your fast-twitch fibers.”
Muscle growth was Arnold’s top priority for decades. It catapulted him to the top of the IFBB Pro League, helping him secure seven Mr. Olympia titles in his prime. At 79, Schwarzenegger’s goals have shifted to longevity and functional training, which can help keep him mobile and capable of leading such a hectic schedule.
“Slow-twitch fibers are designed for endurance and low-to-moderate intensity exercise. The fast-twitch fibers are for speed, strength, and explosive power. Both are important. But a new study suggests that slow-twitch fibers grow the same from both light and heavy loads.”
Arnold Schwarzenegger Says Light and Heavy Weights Are Needed for Muscle Growth
In the newsletter, Schwarzenegger pointed out research suggesting that as long as you take sets close to failure, light or heavy weights can build real muscle.
“A recent meta-regression analyzed people who had thigh-muscle biopsies before and after about 10 weeks of heavy or light training. The fibers grew about the same regardless of load.”
Several studies have found that both heavy and light weights build muscle when you take sets close to failure. So it’s not that one is better at building muscle than the other; it’s just hard for people to tell when they are close to failure, meaning they don’t hit the reps that trigger the most growth.”
After looking at fiber type, they found that fast-twitch fibers led to more growth as weight ramped up. However, the same couldn’t be said for lighter weights with slow-twitch muscle fibers.
“The biopsies let scientists take a closer look at fiber type, and the idea that “low weights favor slow-twitch muscle fibers” didn’t hold up. Fast-twitch fibers trended toward more growth as weight increased. At the very light end (20 to 30% of your max), slow-twitch only looked better because fast-twitch grew less.”
“So the idea isn’t quite right. Light weights grow slow-twitch just fine, but heavy lifting does too. And that means light weights may fall short for fast-twitch.”
While he’s not completely dismissing light weights for fast-twitch growth, he does believe heavier weights will get you to your goals faster:
“The likely reason is how your body recruits muscle. It calls on slow-twitch fibers first and brings in fast-twitch as effort climbs. Light sets taken to failure eventually get there. Heavy sets may get there sooner and keep those fibers working longer. In fairness, the fiber-type differences were small. The trials were small, and the researchers call the results exploratory.”
“But we’re sharing this because the research adds another piece to the puzzle about why everyone needs to push heavier weight, relative to their own strength, at least once or twice per week. Fast-twitch fibers tend to shrink first as we get older, which means if you don’t go heavier, they don’t grow the same, leaving your body vulnerable.”
Schwarzenegger believes light and heavy weights have room in anyone’s program, including his. He continues to offer the bodybuilding community wisdom and practical tips to keep the progress coming.
