HOW QUICKLY TO PERFORM A REPETITION

  In its simplest form, a repetition is made up of two phases; the eccentric phase and the concentric phase.

  The concentric phase is when you are moving a weight against gravity.  The muscles that are your primary movers are both shortening.  During a bench press, the concentric phase occurs when your are pressing the weight upwards. 

  This phase is performed rather quickly, though not too quickly.  Typically the concentric phase should take you between 1 and 2 seconds, however, it may take longer on heavier sets or on repetitions at the end of your set.

  The eccentric phase is when the muscles, which are your prime movers, are still contracting to support the weight, but they are also lengthening.  During a bench press, the eccentric phase occurs as you lower the bar towards your chest.  Your chest, triceps, and anterior deltoids are contracting to keep the weight from crashing down on you, but they are also becoming longer as the bar draws closer to your chest. 

The greatest amount of muscular damage occurs during the eccentric phase.  As a result, this phase is responsible for a large portion of your strength gains and muscular development.  To maximize your results take a minimum of 3 to 4 seconds during this phase of a lift.