Training & Racing

Importance of High Intensity Exercise – Part 1

Importance of High Intensity Exercise – Part 1

High-intensity exercises sequentially recruits all the different types of muscle fibers in your body, starting with the smaller motor units made up of slow-twitch fibers, also known as Type I.  These are the muscles involved in aerobic metabolism (i.e. steady state running).  They recover quickly and have a lot of endurance; perfect for long distance runners.  Next in line are the intermediate fast twitch fibers or Type IIA, followed by the stronger, fast-twitch fibers known as Type IIB.  The main difference between a slow twitch motor unit and a fast twitch motor unit is the fast twitch motor unit controls more muscle fibers or cells and these cells are bigger; thus producing more force.

Your fast-twitch fibers are largely glycolytic and store a lot of glucose. When these muscles are recruited (through high intensity exercises), it creates the stimulus needed to grow muscle. At the same time, it enlarges the glucose storage reservoir in the muscle, which in turn enhances your insulin sensitivity.  Normalizing your insulin is one of the primary health benefits of exercise, and this is particularly true in the case of high-intensity exercise according to Dr. Doug McGuff.

Long, slow distance running does not do this as efficiently, which is due to the gradual recruitment of slow-twitch motor units. Since they recover quickly, you’re merely recruiting that one group over and over again while exercising, instead of tapping into the fast twitch fibers as you would with high intensity exercise. As a result, your Type II fibers will not be as well conditioned or ready when called upon to run faster during a race.  Another downside to long distance running is the highly catabolic effect, or breakdown of your muscles.  High-intensity exercise, on the other hand, promotes muscle building and that stimulus is what triggers the body to make an adaptive response to hold on to lean muscle when trained regularly and given ample rest.

So what constitutes high intensity exercise? It’s training that focuses on performing exercises to the point of momentary muscular failure.  The fundamental principles of high intensity training are that exercises should be brief, infrequent, and intense. When exercises are performed with a high level of effort, it is thought that it will stimulate the body to produce an increase in muscular strength and size.


Boone A. Ebel
Endurance Athlete
Certified Fitness Trainer
Specialist in Performance Nutrition


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This post was written by:

- who has written 8 posts on Trail Running Club.

After graduating from Western Illinois University in 2004, Boone furthered his career in the health & fitness industry and owns a training and coaching business in Arizona. A lifetime runner and outdoor enthusiast, Boone enjoys competing in endurance events on both road and trails. When he is not training clients, odds are you can find him on the trail somewhere around the Southwest.

Since 2009, Boone has 33 Top Ten finishes including: -1st Overall 2014 San Pedro Park 10K -3rd Overall 2014 San Luis Obispo Marathon -4th Overall 2014 Oakland Marathon -2nd Overall 2013 Tucson Half Marathon -1st Overall 2013 Pass Mountain 12K (CR) -3rd Overall 2013 Canyon de Chelly 55K -4th Overall 2013 Santa Rosa Marathon -7th Overall 2013 Phoenix Marathon -2nd Overall 2013 Desert Classic Marathon -1st Overall 2013 Rose Bowl 10K -2nd Overall 2012 Mesquite Canyon Trail 50K -4th Overall 2012 Lost Dutchman Marathon -2nd Overall 2012 XTERRA McDowell Mountain 15 mile run -6th Overall 2011 Zane Grey Highline Trail 50 Mile Endurance Run -4th Overall 2011 Old Pueblo 50 Mile Endurance Run -1st Overall 2010 Sedona 50K -1st Overall 2010 Flagstaff 50K

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