The amount of force a muscle generates is proportional to the amount of muscle growth you'll be able to create. Force is defined as mass (the weight you use) multiplied by acceleration (the speed at which you push a weight against resistance). To generate more force, progressively increase your poundages while lifting explosively -in this context, you actually increase speed during the second half of the rep.
When you lift heavy a message is given to the brain to get stronger to face the increasing load
Light training with higher repetition will lead to losing muscle mass and heavy training with fewer explosive fast repetitions will lead to bigger mass.