Muscle Fibers - There’s A Difference
Jul 29th, 2007 by Kaiser
So I was in boxing class this morning for an hour and half - my first time there in a week. Of course I was feeling winded, especially since Saturdays are primarily cardio days. It could have been worse, but Saturdays are usually soft so I wasn’t challenged to push that hard - a big difference from the MMA (mixed martial arts) classes on the weekdays.
When my arms were feeling like jello and my swings had the power and speed of a feather, I got the idea to post a little bit on the different classes of muscle fibers we all possess. I weight train hard with weights at least 5 days a week - why would my muscles be giving out from the boxing?
It’s because not all types of training work the same types of muscle. We actually possess different muscles for different classes of activity.
Fast, explosive movements require Type II Muscle Fibers - they’re used for short, explosive movements such as sprinting and weight-lifting. The Type II class is further sub-divided into two other categories, IIa and IIb. Type IIb are the fastest firing, but also tire the fastest. They are the ones worked most during high-intensity weight training; they also possess the greatest potential for growth.
So with me, even though my Type II fibers have been trained hard with weights, they only have a limited capacity for sustained activity. My Type I fibers had become de-trained, and that was leading to my fatigue.
The types and number of muscle fibers we have are for the most part genetic. Everyone has a combination of Type I, IIa, and IIb fibers, but in different proportions. In that respect, many of us are born “pre-disposed” to certain sports. For example a successful marathon runner has an overwhelming number of Type I fibers. Although they are highly trained, they are known for being very thin due to the smaller size of these fibers. They would have a hard time succeeding at sprinting for that reason.
Compare that to a an NFL running back, someone who needs to run for short powerful bursts of only a few seconds at at time. There bodies are packed densely with muscle, due to the impact that intense training has had on the size of their IIb fibers; the same can be said of bodybuilders.
Most of us are somewhere in between - depending on the type of our training, we can push the potential of one type of fiber over another. But for optimal fitness and performance, we need to train both types of fibers. That’s why I always try to provide multiple types of training to my clients during their sessions - so they can avoid episodes like mine this morning ! 
