Do you wonder why some people can naturally run faster and others can lift more weights? It is all about the differences between fast-twitch vs slow-twitch muscle fibers.
Your skeletal muscles are made up of bundles of individual muscle fibers called myocytes. Each myocyte contains many myofibrils, which are strands of proteins (actin and myosin) that can grab onto each other and pull. This shortens your muscles and causes them to contract.
Types of Muscle Fibers
You have two main types of muscle fibers: slow-twitch (Type I) muscle fibers and fast-twitch (Type II) muscle fibers. Fast-twitch fibers can be further categorized into Type IIa and Type IIb fibers. Your muscles contain a genetically determined mixture of both slow- and fast-twitch muscle fibers. On average, you have about 50 percent of each type of fiber in most of the muscles you use for movement.
You may be wondering why all of this matters. Well, these different types of muscles respond differently to training and physical activity. Each fiber type contracts in a different way. So, by changing your workout routine, you can change your muscle composition.
Slow-Twitch Fibers
Slow-twitch (ST or Type I) fibers contract slowly and are highly resistant to fatigue. In terms of structure, they have a small motor neuron and fiber diameter, a high mitochondrial and capillary density, and a high myoglobin content. In terms of energy, they have a low supply of creatine phosphate, which is a high-energy substrate that you use for quick, explosive movements. Your slow-twitch fibers also have low glycogen content and an abundant amount of triglycerides (the stored form of fat). They don’t contain many of the enzymes involved in glycolysis but do contain many of the enzymes involved in the oxidative pathways (Krebs cycle, electron transport chain). You use slow-twitch fibers for aerobic activities that require low-level force production, such as walking and maintaining your posture. Most of your everyday activities use slow-twitch fibers.
Fast-Twitch Fibers
Fast-twitch (FT or Type II) fibers, on the other hand, contract quickly and are not very resistant to fatigue. These fibers can contract more quickly than slow-twitch fibers because of the activity of the enzyme (myosin-ATPase) that breaks down ATP inside the myosin head of the contractile proteins of these fibers. Fast-twitch fibers are high in creatine phosphate and glycogen and contain moderate triglyceride stores. They have both a high glycolytic and oxidative enzyme activity. You use fast-twitch fibers during prolonged anaerobic activities where you’re exerting a lot of force.
Thomas DeLauer Breaks Down Muscle Fiber Types
How You Can Test Your Fast-twitch vs Slow-twitch Muscle Fibers Composition
There’s a couple of methods you can use to figure out your proportion of fast-twitch vs slow-twitch muscle fibers.
You can easily determine the fiber composition of your muscles with a simple weightlifting workout. First, pick the muscle group that you want to test and an exercise that will use those muscles. Then, perform as many repetitions at 80% of your max weight as you can. If you do fewer than 7 repetitions, then your muscle group is likely composed of more than 50% fast-twitch fibers. If you can perform 12 or more repetitions, then that muscle group has more than 50% slow-twitch fibers. Or, if you can do between 7 and 12 repetitions, then your muscle group probably has an equal proportion of muscle fibers.
Because lifting weights uses many muscles at once, you can’t use this test on individual muscles. You can only use it to determine the composition of your muscle groups. If you want to determine the fiber-type composition of an individual muscle, you’ll need to get a needle biopsy of the muscle.
How You Can Optimize Your Workout for Fast-Twitch vs Slow-Twitch Muscle Fibers
Now that you know the composition of your muscle groups, you can optimize your workouts to meet your fitness goals. Your fiber type proportion plays a major role in a few specific areas, including the:
- amount of weight you can lift
- the number of repetitions that you can complete in a set or interval workout
- amount of muscle mass that you will be able to develop
When you’re working out at a low intensity, your slow-twitch muscle fibers are recruited first. They have the lowest firing threshold. As you increase the intensity of your workout, you’ll recruit larger motor units. The largest motor units contain the fast-twitch fibers. They have the highest threshold and are recruited last. So, if you only perform low-intensity workouts, then you’re probably only using slow-twitch motor units. If you’re working out at a high intensity, such as lifting weights or performing drop-down sets, then you first recruit your slow-twitch motor units, then the fast-twitch A and finally fast-twitch B motor units.
You can’t change one type of muscle fiber to the other type. You can, however, change the area that each fiber type takes up in a muscle. The key is to focus on the intensity of your workout. You can change your muscle composition by increasing the amount of weight you lift or your number of reps.
Target Slow-Twitch Fibers to Increase Your Endurance
If you want to build your muscular endurance, you’ll need to train your slow-twitch muscle fibers. You can do this by increasing your number of reps. When you do this, your fast-twitch fibers will atrophy while your slow-twitch fibers hypertrophy. Your muscles’ endurance capabilities will then increase proportionately, while their strength and mass will decrease. This is why you get weak and lose muscle mass quickly when you stop training.
Target Fast-Twitch Fibers to Build Your Muscle Mass
Although targeting your slow-twitch fibers will increase your endurance, you won’t be able to lift as much weight as you can with a higher proportion of fast-twitch fibers. Slow-twitch fibers have lower in mass than fast-twitch fibers. So, if you want to be stronger and increase your muscle mass, you’ll need to target your fast-twitch muscle fibers. What sets world-class weightlifters apart is they build there type IIa fast-twitch muscle fibers (1).
To build your fast-twitch fibers, you’ll need to increase the intensity of your workouts. You can increase your intensity by using heavier weights and fewer repetitions. Heavyweights recruit your fast-twitch fibers, which can produce a much greater force than slow-twitch fibers. Hypertrophy is the muscle breakdown you need for your muscles to grow, and it will only occur in muscle fibers that you overload. So, to increase your maximum strength, your goal should be to do 5 to 8 reps at 80% of your max weight. Depending on the specific intensity of your workouts, you could change your muscles to a proportion of 75% fast-twitch fibers and 25% slow-twitch. You should also focus on some basic compound exercises such as bench press, squats, and deadlifts.
