Fitness, Science

What You Need To Know About Slow-Twitch Fibers

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Have you ever felt the need to run for a while, really “Forest Gump” it in a serious cardio session? Or maybe you’ve played a sport that required a lot of movement and running for extended periods of time. During these activities, slow-twitch muscle fibers are your best friend. Your slow twitch muscles are great for endurance activities like long distance running, cycling, and sports such as soccer. These muscles can perform work for a substantial amount of time without fatigue.

Remember from my fast-twitch blog, fast-twitch muscles are good for rapid, explosive types of movements like jumping or sprinting. These muscle fibers contract quickly, much quicker than slow-twitch, but get tired super fast, plus they consume an enormous amount of energy.

Your muscles, for the most part, are made up of a mixture of both slow and fast twitch muscle fibers.

The amount and type of muscle fibers differ by individual of course. Your soleus muscle, located in your lower leg hanging out with the good ole gastrocnemius, and your back muscles that are involved in maintaining posture, contain mainly slow twitch muscle fibers. DO you know why? You guessed it, endurance! Muscles dense with slow twitch fibers are considered red, because they contain numerous blood vessels. Slow twitch muscle fibers require a robust, oxygenated blood supply as they use oxygen to produce the energy needed to contract. Here’s a great explaination on muscle contraction.


  • Slow-twitch fibers contain mitochondria, which are organelles that use oxygen to create adenosine triphosphate (ATP), which is the chemical energy that fuels muscle contractions and most cell activities. I’ll put it this way, ATP is microscopic and you produce your body weight in it each day! This is the reason slow twitch fibers are considered aerobic (use oxygen). They need that oxygen to fuel the mitochondria to produce the ATP needed to function. Whew! 
  • Slow-twitch fibers are also called red fibers because they contain more blood-carrying myoglobin, which creates a darker appearance.
  • The slow twitch muscle fibers produce their own source of energy, and can maintain an exerted force for an extended period of time, however they are not able to generate a significant amount of force.
  • Slow-twitch fibers are the first recruited when a muscle contracts. Pending their ability to generate the amount of force necessary, the fast-twitch muscle fibers are engaged.
  • The muscles responsible for maintaining posture have a large amount of slow-twitch fibers.
  • You can increase mitochondrial density in these muscles via steady-state endurance training. This type of training will improve your body’s ability to use oxygen to produce ATP.


Isometric contractions

  • Little-to-no joint movement, will cause constant contraction in your slow-twitch muscle fibers and can help improve their ability to utilize oxygen to produce energy. Example exercises: single-leg balance, front plank, side plank.

Circuit training

  • Alternating from one exercise to the next with little-to-no rest (using light weight), will challenge slow-twitch fibers.

High-repetition sets

  • Use lighter weights with slower movement tempos and high repetition (15+) to engage the slow-twitch fibers. This keeps your body in a state of aerobic metabolism while fueling the activity.
  • Body-weight exercises are an effective way to improve aerobic metabolism, in turn improving the efficiency of slow-twitch fibers.

Short rest intervals

  • Approximately 30 seconds between sets will challenge the slow-twitch fibers.

To summarize, slow-twitch muscle fibers have unique functional characteristics, and they can be trained to be more aerobically efficient with the proper exercise program, of course.

I want to share ways you can get in-depth training, advice, and begin making your health a priority. 

Again, I want to sincerely thank you for taking the time to read my post. Don’t forget to tell friends and family about me! Share if you’d like and comment below to get a conversation started.  I wish you the best, and I hope to hear from you soon!

Coach Slice


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