Muscle fibers have 4 distinct types based on how rapidly they fatigue. Essentially slow twitch fibers have a high resistance to fatigue so they can keep going, great for steady state training, however they don't generate a lot of force.
Fast twitch fibers are a lot bigger in terms of the motor unit (bunches of fibers connected to a single nerve), so when they “fire” they recruit a lot more individual muscle fibers resulting in stronger contractions. The trade off is that they tire very quickly, so is not sustainable.
To put it in context
- a marathon runner uses predominately slow twitch fibers, long, slow, sustainable efforts.
- a body builder uses a combination of all muscle fibers as they want everything to grow bigger. They train for maximal motor unit recruitment, aiming to get as much as the muscle working as possible.
-a 100 meter sprinter uses fast twitch fibers to develop dynamic and explosive actions.
Our muscle fibre make up is genetically predetermined, everyone has a combination of fibers, and the central nervous system is very efficient at using just the right amount of fibers to get the job done.
A big part of getting fitter is increasing the amount of motor units used, or recruited. These neural adaptations are very important in any sport or exercise, and are very specifc to each individual movement, in some instances taking years to develop.
Type 1 - Slow Oxidative - slow fatigue rate so can last long times, mainly used in aerobic steady state exercise such as walking, cycling, light jogging. Recovers rapidly, within 90 seconds.
Type2 - Fast Oxidative - is able to handle more intensity, stronger contractions such as in resistance training. Fatigues rather quickly
Type2ab - Fast Oxidative Glycolytic (intermediate). Has a combination slow/fast fatiguability, used in moderate to high intensity training. Provides a very good all round base of fibre recruitment.
Type 2b - Fast Glycolytic. Used in maximal, high intensity bursts requiring a substantial amount of force production. Fatigues very quickly and takes up too 4-10 days to recover if exhausted
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