How to prioritize your training hours to best meet your fitness goals.
Most people know there’s a difference between fast-twitch and slow-twitch muscle, but not many people know what the difference actually is — and if they do, they probably apply it to running, not weightlifting. The truth is, fast-twitch and slow-twitch muscle fibers have a big impact on resistance training, high-intensity interval training, powerlifting, Olympic lifting, and pretty much every other type of training.
Your muscle fiber composition (specifically the ratio of fast-twitch to slow-twitch muscle fibers you have) profoundly impacts your ability to sprint, run, jump
I’m willing to bet that if you put a cross-country runner and a 100-meter dash sprinter in a weight room, this would happen:
This is a classic example of fast-twitch versus slow-twitch muscles at work. In this post, you’ll learn all about the two main types of muscle fibers as they relate to resistance training, plus whether it’s possible to get better at one type of lifting when you have a genetic predisposition to the other type.
What are muscle fibers, anyway?
If you’re at all attuned to the fitness industry, you’ve probably seen the term “muscle fibers” on innumerable Instagram posts and Facebook ads. While the term itself has become pretty ambiguous and is often misused, the definition is quite simple: A muscle fiber is a single muscle cell.
Muscle fibers help control the physical forces within the body, and when they are grouped together (in lumps called fascicles, singular fasciculus) they move your body. There are two main types of muscle fibers in skeletal muscles: fast-twitch and slow-twitch.
Before we dive into the differences between the two types and their implications, let’s preface with the fact that separating muscle fibers into two groups is a vast oversimplification of the way the body works. There are actually three broad classifications of muscle fibers — slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG) — and with all things physiology, caveats abound.
But for simplicity’s sake, this article covers a high-level overview of fast-twitch and slow-twitch so you can use the information in your training without becoming overwhelmed by scientific babble.
What are fast-twitch muscle fibers?
Fast-twitch muscle fibers are also known as type II muscle fibers. As the colloquial name implies, these muscle fibers contract quickly and produce powerful get-up-and-go forces. However, they tire out very quickly, too. Fast-twitch muscle fibers do not use oxygen to produce energy; instead, they rely on anaerobic metabolism (again, simplifying, as FO muscle fibers do use oxygen), which is why they fire so fast but fatigue in a short time.
What are slow-twitch muscle fibers?
Also known as type I muscle fibers, slow-twitch muscle fibers are fatigue-resistant and primarily facilitate smaller ranges of motion (as compared to fast-twitch). Slow-twitch muscle fibers contain a lot of blood-carrying myoglobin, so they are also known as red fibers, and because of their aerobic metabolism, they’re the first to activate when a muscle contracts.
Fast-twitch vs. slow-twitch: The main difference
Fast-twitch and slow-twitch muscle fibers differ primarily in the fact that they contain different levels of mitochondria, which is what affects the way they fire. Slow-twitch muscle fibers have more mitochondria than fast-twitch muscle fibers; because fast-twitch muscle fibers have fewer mitochondria, they fatigue quicker.
This brings us to the practical difference: the activities they are used for. Fast-twitch muscle fibers give short but powerful bursts of energy, which makes them essential for activities like sprinting and jumping. Slow-twitch muscle fibers are used for endurance activities, such as long-distance running or cycling. It’s actually been proven that athletes who are successful in endurance have more slow-twitch fibers than fast-twitch fibers.
Fast-twitch vs. slow-twitch: Which is better to have?
Which type of muscle fiber is “better” is entirely subjective and depends upon your goals. If you’re a sprinter, you’ll want more fast-twitch muscle fibers because the quick firing mechanism is an asset to someone who needs to run fast. If you’re a marathoner, however, more slow-twitch muscle fibers will benefit your efforts the most.
Similarly, Olympic weightlifters who regularly test one-rep maxes at explosive lifts will need to train and gain type II (fast-twitch) muscle fibers. Someone who competes in obstacle course races that require muscular endurance will need to train type I (slow-twitch) muscle fibers.
CrossFit athletes, on the other hand, desire a healthy balance of type I and type II muscle fibers, because the sport requires them to perform well at many athletic activities — but not exceptionally for any single activity.
How to train fast-twitch muscle fibers
Training fast-twitch muscle fibers involves lots of explosiveness and speed. High-effort sprints, for instance, can help strengthen your existing fast-twitch muscle fibers as well as increase your total number of such muscle fibers.
Low-rep, high-load strength training, especially compound and explosive lifts (e.g., power cleans, back squats, deadlifts, snatches), help tremendously. When strength training for fast-twitch muscle fibers, stick to the one-to-three rep range.
Plenty of bodyweight exercises can help, too: Box jumps, jump squats, and other plyometric exercises enhance your fast-twitch abilities.
How to train slow-twitch muscle fibers
Slow-twitch muscle fibers are made for endurance and are activated during aerobic activities, so train them that way. Extended aerobic workouts, such as power walking, jogging, cycling, rowing, and hiking will train your slow-twitch muscle fibers.
But don’t think you’re stuck to cardio workouts only. Weight training can improve your muscular endurance significantly, as well as your cardiovascular endurance. To strengthen slow-twitch muscle fibers with resistance training, perform sets of 10 to 15 reps or more. Keep your loads low to moderate to ensure you meet the ideal rep count. Tempo training also helps.
Smarter Sweat takeaways
You can’t change anything about the genes you were handed (unless, perhaps, you’re an expert biohacker). But, you can change the way you train if you feel compelled to enhance the abilities of one muscle fiber type over another.
As with everything else in fitness, if you want to get good at something, there is nothing to it but to do it. You want to get faster? Add sprints to your routine. You want to become more powerful? Start practicing Olympic-style lifts and prioritizing low-rep, high-load training. You want to tackle your first marathon? Better start going for distance on the roads and increasing your weight training sets to the 15-plus rep range.
Those suggestions are, of course, a simplification of what one might really need to do to enhance their slow-twitch or fast-twitch abilities. For the best results, follow an online training plan specific to your goals (like our upcoming Smarter Sweat Endurance program), or work one-on-one with a trainer who can support you.
By Megan Kittrell and Amanda Capritto
Megan is a trusted assistant to Smarter Sweat. She helps with everything from blogging to administration to research. Amanda is the co-founder of Smarter Sweat. She's a certified personal trainer, health coach, and functional fitness coach. She also owns another business, where she writes about all things fitness, nutrition, travel and lifestyle.
About Smarter Sweat
Smarter Sweat is a fitness company built from the ground up by fitness experts Amanda Capritto and Ashley Phantz. Amanda and Ashley are both dead-set on cutting through the clutter of the fitness industry and providing raw, real, utterly honest information about fitness and wellness.