Many people think that standard push-ups isolate muscles, which they then take advantage of by training harder and higher rep counts. You will indeed work out those specific muscles by doing these, but the isolation focus does nothing to give your whole body a balanced workout. Instead, when you train these muscles, you should focus on large muscle groups and smaller stabilizer muscles (muscles located below the larger muscles). In other words, if you want to gain lean muscle mass and definition, you need to be performing workouts that stress large groups of muscles versus small isolated ones. This will give you the best results possible.