Fast twitch muscle fibres
 

When you work out hard with heavy weights for low reps, you are usually working out your slow twitch muscle fibres. If you want to add more muscle and look more lean and defined, you can also work your slow twitch muscle fibre too but lowering the weight and increasing the reps to much higher e.g. 20-30 reps per exercise.

actually using heavy weight stimulates the fast twitch which is the the whiite fibers.. usually meant for building the nervous system to be able to handle controled heavy lifting.. these fibers consist of type a & type b fast twitch fibers ... and the red fiber are the slow twitch were these fiber and handle long resistance and they dont fatigue easy,, the way fast twitch fibers do.. these are endurance fibers usually for hypertrophy & conditioning training.. (for separation & tie-ins of muscle).. these are at a pace of 8 to 25 reps per set..