Kettlebells and your fast twitch/slow twitch muscles

Posted: April 28, 2011 in Uncategorized

My first weight lifting class was in high school, back in about 1983.  I remember signing up for it because the football coach was teaching it… and most likely football players would be taking it.  (…my mind as a 16 year-old!)  By the time I went to college I was hooked on strength training and took the 1-credit course nearly every semester.  In these classes we did more than just lift weights though… we learned about the muscular body and studied the proper names and locations of all the muscles, and we learned about fast-twitch and slow-twitch muscles.  It’s been awhile since college… and the other day the topic came up when I was running stairs, and I was trying to recall the differences… so I thought it was time for a refresh, and maybe a good blog post!

Fast-twitch muscles fibers contract quickly, providing an athlete with powerful, short bursts of energy.

Slow-twitch muscle fibers contract more slowly and will end up providing greater endurance over a longer period of time before they fatigue.

So I learned that most people have a 50-50 balance of fast to slow-twitch muscles, but occasionally an athlete can have a greater percentage of one or another.  Take an Olympic sprinter, for example.   He probably has 80% fast-twitch muscles.  He is, in a sense, born with a genetic predisposition to have quick bursts of energy.  If his parents put him in boxing school as a kid, he probably would have done well there too… short, quick punches rely on fast-twitch muscle fibers for the explosive bursts of energy.  (Similarly, so do kettlebells!)  But, put the dude in a marathon, and he may not do so well with only a 20% slow-twitch composition.

So perhaps this means that we should pick our sport wisely.  However… I began to wonder… could an athlete change their composition with the right training?

Old-school thinking was that the balance of fast and slow twitch muscles you were born with remained the same throughout your life.  For the most part, this is still true, however it has recently been discovered that an athlete could change the way the muscle fiber responds or behaves.  So, in the example of the sprinter turned marathon runner… he could train his fast-twitch “short-bursts-of-energy” muscles to take on the qualities of slow-twitch “high-endurance” muscles, but he would still have the same percentage of fast to slow… the muscle fibers will always be fast-twitch, but they might act like slow-twitch.

During kettlebell training, we need both aerobic endurance as well as explosive power and strength.  So we are tapping both our slow and fast-twitch muscles, especially if we only take a 15-second rest between sets.

By the way, I highly recommended writing down your kettlebell workout before you begin, and then turning on some great music and knocking it out.  A planned workout eliminates standing around thinking about what exercise you should do next, which gives your body more of a rest and less of an aerobic workout.  In my opinion, if you’re not dripping in sweat within the first 5 minutes of your kettlebell workout, you’re taking too many breaks!

I am currently designing a new workout that combines yoga and kettlebells… it should be a killer… look for it soon!


  1. You said that most people have 50:50 balance between fast twitch and slow twitch, and have the example that sprinters have 82:20 split. Is that really the case? because if it were so then this would be a HUGE difference, I would have thought that such vast differences would exist only between different species.

    • Thanks for your question… it would seem like we’d be super human or something with that much difference! Here’s a bit from an article I read recently about the 80-20 ratio:
      *Fiber Type and Performance*
      “Our muscle fiber type may influence what sports we are naturally good at or whether we are fast or strong. Olympic athletes tend to fall into sports that match their genetic makeup. Olympic sprinters have been shown to possess about 80 percent fast twitch fibers, while those who excel in marathons tend to have 80 percent slow twitch fibers.”

      Here’s the link to the whole article:

  2. […] Kettlebells and your fast twitch/slow twitch muscles ( […]

  3. kettlebell workout says:

    Thanks for posting that up, it’s very interesting how much different there is, if I didn’t know and were to take a guess, I would have said that there would be around 5% difference max between humans in the slow:fast twitch ratios.

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