What’s the Diff?
Your muscles are made up of three main types of fibers:
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: During endurance activities such as distance running or cycling
TYPE IIX (Fast-twitch)
ROLE: Power-focused. Capable of creating explosive strength but fatigue quickly with 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: Longer-lasting power over time, such as running up a few flights of stairs
WHICH WORKS 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.
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:
STRENGTH TRAIN: Lighter resistance, higher reps; isometric moves like planks or single-leg balances, circuit training, body-weight exercises with short rest intervals
CARDIO: Longer duration, lower intensity (like running a 10k)
STRENGTH TRAIN: Heavy resistance, lower reps
CARDIO: Short highintensity intervals, such as Tabata (20 seconds on/10 seconds for recovery); explosive, power-based moves such as kettlebell swings or cleans
STRENGTH TRAIN: Moderately heavy resistance, moderate reps. Rest 45 to 60 seconds between sets.
CARDIO: Longer intervals (i.e., 1 to 3 minutes on, 1 to 3 minutes for recovery)