Prolonged muscle contraction is the most important stimulus for increasing muscle size, according to researchers from McMaster University in Canada.  They studied the effect of a single workout using either slow reps (6 seconds up and 6 seconds down) or fast reps (1 second up and 1 second down).  They used a light load of 30 percent of 1 repetition maximum.  The rate of protein synthesis was greatest during the slow repetitions.  Maximizing time under tension is the best way to make muscles grow.  However, because of the importance of neural control, slow reps are not the best way to gain strength, particularly in well-trained athletes.  (Journal of Physiology, 590:  351 – 362, 2012)