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Know Your Fibers: Use Five Exercise Variables to Target All Muscle Fiber Types

By Billy Polson

Most of us pay attention to reps and weight when we're putting together a workout plan, but how about changes in sets, tempo, time under tension, rest/recovery? Each of these additional factors can make a big difference in the muscle fibers you are recruiting as well as the results you are achieving in your workouts. Most muscles contain a combination of fiber types, with different muscles having different ratios depending on their roles within our movement. In building your workouts, you want to think the composition of your muscles, and how variations can help all of the different fiber types to develop.

There are three basic types of muscle fibers:

Type 1 Muscle Fibers: Type 1 muscle fibers are slow twitch, tonic muscle fibers that are used for stability, endurance, and posture. These muscles are recruited if your time under tension is greater than 70 seconds. (Time under tension would be the entire time it takes to complete each set or activity.) For example, if you have poor posture with rounded shoulders, you should work on strengthening the Type 1 fibers of your lower middle trapezius in order to hold your shoulder blades down for extended periods of time. A good exercise would be cable pull-downs, keeping your shoulders retracted for the entire set and performing the set for at least 70 seconds. Also, if you are new to weight training and still trying to gain better shoulder and hip stability, you should focus your workouts on training the Type 1 fibers in your body before trying to lift weights that are too heavy for your abilities.  

Type 2A Muscle Fibers: Type 2A muscle fibers are fast twitch, phasic muscles that are used for brute strength work. These muscles are recruited when time under tension is between 20 and 70 seconds. For people looking for strength and muscle mass work, these are the muscle fibers you need to be focusing on. As an example, if you are looking to increase the muscle mass of your chest, you should be focusing your work on strengthening the Type 2A fibers of your chest. Therefore, a good exercise would be cable presses and performing the set with enough weight so that you were maxed out on reps after 20 to 70 seconds.

Type 2B Muscle Fibers: Type 2B Muscle Fibers are fast twitch muscle fibers that are used for sprinting and power movements. These muscles are recruited when time under tension is less than 20 seconds. Sprinters and people involved in power sports and activities should have a large focus on these fibers in their training. For example, if you are tennis player, then for your cross court sprints or power shots you are doing some major recruitment of 2B fibers. Therefore, in your workouts you should be focusing on power trunk rotations for power shots and full-speed, two to three-second side shuffles for runs.

Now that the different muscle fibers are clear, let's break down the best way to hit each of them. Essentially, you need to think about how each of them is impacted by the variables I mentioned above—sets, tempo, time under tension, rest/recovery. By varying these elements in a workout, you can target specific fibers in line with your goals.  

Muscle Fiber Type Ideal Time Under Tension Tempo Reps Sets Rest/Recovery
Type 1 Fibers (Stability, Posture, Endurance) 70 - 120 seconds  (therefore a set focused on these fibers should last between 70 and 120 seconds each time) Ideally these are slow movements (i.e. on a push-up do a 3 count down a 2 count hold at bottom and a 3 count up 3-2-3. Or a 6 count pull-up, 6 up, no hold at top and a 6 count down 6-0-6) Always perform a minimum of 13 reps here Range should be between one and 5 depending on time available 60 - 120 seconds
Type 2A Fibers (Strength, Muscle Mass)For muscle mass growth: 40 - 70 seconds.   For relative strength increases (increases in strength without increases in weight/size): 20 - 40 seconds  Ideally these are moderate speeds (i.e. on a squat do a 2 count down a 2 count hold at bottom and a 2 count up 2-2-2. Or a 4 count military press, 1 up, 2 hold at top and a 1 down 1-2-1) Always perform 6 - 12 reps. Range should be between five and 10 depending on time available 90 - 120 seconds
Type 2B Fibers (Power and Sprinting)Less than 20 seconds  Ideally these are fast movements (i.e. on a step-up do a 1 count up, no hold and a 1 count up 1-0-1. Or a power medicine ball chest pass as fast as possible  X-X-X) Perform either fewer than five reps or less than 20 seconds Range should be between eight and 15 3 - 5 minutes (sometimes up to 10 minutes for power athletes)
Remember—think about the kind of fibers you need to train for your particular goals; then put together the variations on each exercise that will put an emphasis on those fibers. And remember, have fun and kick ass!

About Billy Polson: Billy Polson is co-founder of the award-winning Diakadi Body personal training gym and creators of RealJock's 12-week Workout Programs. Billy is a certified Exercise Coach through the Paul Chek Institute as well as a Certified Personal Trainer through The National Academy of Sports Medicine. Have burning questions about your fitness that you want Billy and Diakadi co-founder Mike Clausen to answer? Send an email to