Fast vs Slow Twitch Muscle Fibres & Training

Many of you may know from previous newsletters, I’ve taken up cycling a bit more seriously over the last few months. As I’ve become more serious it has lead me to training with more advanced cyclists with the advantage that I am now getting coaching on cycling techniques.

One of the most effective tips I was given was to increase my cadence (the number of times the pedal does a full rotation in a minute). Coming from a rowing background where the stroke rate very rarely goes above a 42, I had brought the habit of slower/stronger muscle contractions with me into cycling. The term used to describe this type of cycling is called “grinding”. I’ve known from very early on that I was a grinder but as I initially started cycling as a form of cross training for rowing, I thought very little of it.

Now cycling is my focus and I’m committed to getting stronger/faster, I noticed I had seemed to plateau at a certain point about 2 months ago. Noticing my frustration a fellow cyclist suggested I increase my cadence. After committing to increasing my cadence for two weeks I began to notice a marked improvement in both my speed and stamina as well as noticing my body begin to change (becoming much leaner very quickly). In explaining how muscles work more efficiently at a higher cadence he brought up the difference between fast and slow twitch muscle fibres.

As I had studied muscle function at Sutherland Chan his explanation tweaked something in the back of my mind. He had mentioned the higher cadence would rely more heavily on the fast twitch muscle fibres (Type II) vs the slow twitch muscle fibres (Type I), so back to my textbooks I went to learn more about how muscles work. Hopefully this article will help you understand how your muscles work a little better and thereby allow you to design a workout which will support your long term goals.

The first thing to understand is muscles are made up of a combination of both fast (Type II) and slow (Type I) twitch muscle fibres. In general we are born with about 50% of each muscle type. Although this is variable and may help explain why certain athletes excel at certain sports (i.e. an athlete with 80% Type II fibres will have an advantage in sprinting where as an athlete with 80% Type I fibres will have an advantage in marathons).

Type I (Slow Twitch) Muscle Fibres

Type I fibres create ATP (the source of energy muscles use to create contractions) aerobically. This means they use oxygen in a chemical reaction with glycogen to create ATP. This is a more complex process taking longer to complete but it is also more efficient (creating no waste and more ATP-six ATP molecules/glycogen molecule) in the long run. Type I fibres produce a weaker contraction but fatigue at a much slower rate than Type II fibres. They are also able create glycogen from fat stored in the cell as well as the surrounding adipose tissue.

Type II (Fast Twitch) Muscle Fibres

Type II fibres create ATP anaerobically. This means they break down glycogen directly into ATP. This process is not very efficient in the long term as only 2 ATP molecules are produced per glycogen molecule as well as two fatty acid molecules. This process generates a great deal of waste which must be expelled from the muscle fibre. Type II fibres produce a stronger contraction than Type I fibres but fatigue very quickly.

What tweaked in the back of my mind in my fellow cyclists explanation of how muscles work more efficiently was his suggesting a faster cadence would call upon more Type II muscles fibres during the ride. This is a common misconception I’ve heard when people speak about how their muscles function and therefore it affects the way clients design their exercise routines.

Although it is true Type II fibres contract faster with more force, they also create much more waste (lactic acid) which in the end will limit the length of time the muscle fibres can work. So even though Type I fibres contract more slowly with less force they create no waste thus allowing them to continue to fire for much longer periods of time.

In general if you are looking to build muscle bulk/strength you will want to focus on calling on Type II fibres in your exercise routine i.e. short, intense workouts focused on going to the point of exhaustion. If this is your goal, you should be working at 85% or higher of your maximum heart rate. Doing strengthening exercises with heavy weights and working to exhaustion or sprint training will help develop these fibres.

If you would like to increase your endurance you will want to focus on strengthening Type I fibres. These would be longer routines at a more moderate pace focusing on building your aerobic capacity. By increasing the efficiency with which the cardiovascular and circulatory system can deliver oxygen to your muscles you will increase the ability of Type I fibres to function over long periods of time. Exercising at 75-85% of your maximum heart rate for extended periods of time (60 minutes to a few hours).

If you have any questions about this article or anything else, please feel free to contact me at your convenience.