Muscle Fibers: What’s the difference?

Type I (Slow-Twitch)
ROLE: Endurance-focused, slower to fatigue. Has a lot of mitochondria (the energy units of the cell); not much power.
FIND IT: Muscles that work hard over long periods of time, such as postural muscles, the calves, and the hip flexors.
USE IT: For endurance activities such as distance running or cycling.

Type IIX (Fast-Twitch)
ROLE: Power-focused. Capable of creating explosive strength but fatigue quickly with a well-developed glycolytic system for fast energy.
USE IT: For power moves like sprints and vertical jumps.

Type IIA (Fast-Twitch Combo)
ROLE: A mix of type I and type IIX, so able to use both aerobic and anaerobic energy systems.
USE IT: For longer-lasting power over time, such as running up flights of stairs.

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Which work first?

Although they may have a majority of one fiber type or another, most muscle groups are a combo of both type I and IIs. Slow-twitch (type I) are the first to step into action when your muscle contracts; if they can’t generate enough necessary force, the type IIs come into play.

Muscle Fiber Facts

Average percentage of type I and type II (both IIX and IIA) in men.

Color of type I fibers (which contain more blood-carrying myoglobin molecules).

The age at which most of us start to see a drop-off in the number of type IIX and type IIA muscle fibers. Type I decrease a little more slowly.

Color of type II fibers, which are noticeably lighter in color than type I.

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Train Your Type

Although genetics play a strong role in your natural fiber makeup, your training will have some effect on which type of muscle fibers you develop, says Mark Blegan, Ph.D., chair of the Department of Nutrition and Exercise Science at St. Catherine University in St. Paul, MN. Here’s how to target each:

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Type I
STRENGTH TRAIN: Lighter resistance, higher reps; isometric moves like planks or single-leg balances, circuit training, or body-weight exercises with short rest intervals.
CARDIO: Longer duration, lower intensity (like running a 10K or a marathon).

Type IIX
STRENGTH TRAIN: Heavy resistance, lower reps.
CARDIO: Short, high-intensity intervals, such as Tabata (20 seconds on, 10 seconds for recovery); explosive, power-based moves such as kettlebell swings or cleans.

Type IIA
STRENGTH TRAIN: Moderately heavy resistance, moderate reps. Rest 45 to 60 seconds between sets.
CARDIO: Longer intervals (e.g., one to three minutes on, one to three minutes for recovery).