by CycleOps Power Master Trainer, Charles "Joey" Adams

Are you a slow twitcher, a fast twitcher or do you just twitch when you get on your bike? It comes down to the specificity of how you train and knowing your power zones. Cycling is primarily an aerobic sport. The longer you use and develop your aerobic engine the stronger you will ride, unless you are trying to become a track sprinter. As you take a close look at the physiology of muscle fiber types you discover the keys to zone training and your power zones. In a very simplistic sense the Fast Twitch IIb fibers are your sprint fibers (the Phosphogen or PCr system below), the Fast Twitch IIa fibers are your Anaerobic Glycolysis fibers, and the Slow Twitch fibers are the Aerobic fibers. You can see in the graph below how these fibers might play out in the land of track and field (source ACSM).

Now thinking of cycling, the PCr system (FTIIb) is used for the closing sprints, or high end efforts and can last about 15 to 30 seconds. The Anaerobic system (FTIIa) is used for intensities lasting up to approximately four minutes for a world class athlete. For mere mortals, it is generally up to about two minutes. Then there is the powerhouse of it all – aerobic metabolism and the Slow Twitch muscle fibers (ST). The downward slope of the line, in the above graph, indicates that as the event goes on more work has to be performed by aerobic metabolism or the Slow Twitch muscle fibers. Now let’s look at the reason the systems and muscle fiber types work the way they do and how it transfers to our power zones by looking closely at the table below (source NSCA).

Property	Slow Type I	Fast Type IIA	Fast Type IIB
Contraction Time	Slow	Fast	Very Fast
Size of Motor Unit	Small	Large	Very Large
Fatigue Resistance	High	Intermediate	Low
Energy System	Aerobic	Long Term Anaerobic	Short Term Anaerobic
Force Production	Low	High	Very High
Mitochondria Density	Very High	High	Low
Capillary Density	High	Intermediate	Low
Oxidative Capacity	High	High	Low
Glycolytic Capacity	Low	High	High
Fuel	Triglycerides	Creatine phosphate, glycogen	Creatine phosphate, glycogen

In essence, this table is telling us that the Slow Twitch system is relying on fat as a primary source of fuel, for most of us we have a fairly unlimited supply. While the Fast Twitch system is relying on various forms of carbohydrates (depending on body size, intensity, and nutritional state most of us have about 30-90 minutes of fuel stored). In addition the Slow Twitch system relies on metabolic pathways that have a strong delivery and removal system in the form of capillary density. The slow twitch system also has more mitochondria – or the powerhouses of muscular contraction. Thus, the longer you can rely on your Slow Twitch or aerobic system, the longer you can sustain efforts at a reduced cost to the body (relying on fat as a fuel instead of carbohydrate). As you examine the table closely row by row you can see the advantages the Slow Twitch fibers have for sustained work at the right intensity.

As always, there is a time and place for developing a system. This is where we come back to the specificity of training. Your power zones train specific systems. Zones 1 and 2 primarily develop your Slow Twitch fibers (aerobic capacity), as you go up in the zones the pendulum swings to developing more of your Fast Twitch fibers (anaerobic capacity). Ultimately, you have to develop the engine for the event you are training for, BUT the longer you can be aerobic and utilize the Slow Twitch system (in general) the better your results. Remember, you are building the Slow Twitch system one mitochondria at a time and this takes patience. Patience transfers into results! You will notice through periodic performance testing that your power zones will go up. This is because you are using your zones to develop patterns of muscular contraction and fuel utilization. So stop twitchin’ and use your power zones to your advantage to train the muscle fibers that you want to develop.

On your next exercise session try this aerobic boosting workout.

Train Smart, Train with Science, Get Fast,

Joey Adams, M.S.
CycleOps Master Training Specialist
Owner of Intelligent Fitness @ intelligentfitness@comcast.net
Copyright shared with permission to Intelligent Fitness customers