Type IIa Fast Twitch Fibers

. Type IIa: Fast Twitch Fibers.2-
The next category of muscle fibers is fast twitch fibers, divided into type IIa and type IIb. Fast twitch fibers are known for their ability to rapidly transmit action potentials and generate a high crossbridge turnover rate (responsible for quick muscle contractions)
These fibers also possess a high activity level of myosin ATPase and show a rapid rate of calcium release and uptake by the sarcoplasmic reticulum (Katch et al, 2000). Due to these properties, these fibers generate an explosive burst of power for a short period of time. This makes them most suitable to stop and go activities such as basketball, soccer, and hockey, as well as max output activities such as weightlifting, and many track and field events.
These fibers rely heavy on the glycolytic energy system (using the method of anaerobic glycolosis to produce ATP). Type IIa fibers are in the middle of the muscle fiber spectrum, as they are less fatigue resistance, produce more muscular force, and contract at a faster speed than slow twitch fibers.



Type IIb: Fast Twitch Fibers.3-

The type IIb fibers are the most fatigable out of all the fibers but also generate the most power and force, and therefore are the fastest twitch muscles fibers. These types of fibers are recruited in activities that require an all out burst of power and only act for an extremely short period of time, as the total length of their contractions usually last only 7.5 milliseconds.
In terms of general recruitment, they are also the last to be recruited. For example, upon normal activities, slow twitch fibers are recruited first, followed by type IIa when the type I can no longer suffice, and then finally the type IIb, which are recruited to produce maximal strength..
Difference between muscle fibers
In regards to physical appearance, type IIa are pink in color, have an intermediate diameter, capillary level and mitochondria volume. The type IIb fibers are white in color, have the largest diameter and have a low capillary and mitochondrial volume. Most strength athletes possess a higher % of fast twitch fibers, as do those in short duration, quick moving activities.
The primary reason why fast twitch fibers are not resistant to fatigue is because they rely on anaerobic glycolysis to produce ATP. During this process lactic acid begins to accumulate and a condition called acidosis occurs which brings about muscular fatigue.