Muscle Fiber Types and How to Train Them

Within skeletal muscle there are three types of fiber: Type I that are slow twitch, and Type IIa and Type IIb, which are fast twitch.
Type I muscle fibers have the highest oxidative (aerobic) capacity and the lowest glycolytic (anaerobic) capacity. They contract slowly and are able to keep a steady pace for long periods of time without fatigue. Type I muscle fibers are mainly used in endurance activities such as long distance runners or cyclists. Individuals that participate in such activities usually have an ectomorphic or slim, linear body type.
For a marathon runner to perform better he would need to focus on his linear strength endurance by strengthening his Type I muscle fibers. As for as weight training do twelve or more repetitions with two to three working sets. I would not use weight training but maybe twice a week if training for a marathon. I would focus more on getting better at the sport I am participating in, in this case long distance running.
Type IIa muscle fibers are fast twitch or intermediate fibers. They are larger and much stronger than type I fibers. They are more prone to fatigue than type I fibers but can produce high force output for long periods of time. They also have a high capacity for glycoltic activity. Those having mainly type IIa fibers would have a mesomorphic or muscular body type.

Bodybuilders would be a good example of someone with mostly type IIa fibers. One way for bodybuilders to increase their muscle mass is with resistance training. 75% of their 1RM doing 6 to 12 repetitions for 3 to 6 working sets with one minute rest periods. Doing high intensity training such as this will a also increase growth hormone levels.

Type IIb muscle fibers have almost no resistance to fatigue or injury but are extremely strong. They are ideally suited for short fast bursts of power. Those who perform in activities that require maximal and near-maximal lifts and short sprints would be using mainly type IIb fibers and would most likely have a mesomorphic body type. For someone who wants to increase their 1RM they would need to train with heavy weight. 1 to 4 repetitions and 3 to 5 working sets.