Noted researcher and fitness expert Brad Schoenfeld recently completed a study in which he looked at the effects of light loads and high reps versus heavy loads and low reps.
Two groups of trained subjects performed three sets of seven exercises three times a week. The first group used light weights and performed 25 to 35 reps per set. The second group used a heavier load that allowed them to complete eight to 12 reps.
After eight weeks, both groups experienced significant and similar levels of muscle growth. However, the group who pushed heavier weights gained far more strength. The high-rep groups did enjoy greater gains in endurance, but ultimately building more strength will bring about greater changes to your body more quickly. Progressing in the loads you are able to push is a more potent stimulus for change than 30, 40, or even 50 reps of the same weight.