The rule that we can learn from old-school bodybuilders is: to gain more muscle mass you have to get stronger. If you want to get stronger and bigger you might consider trying Reg Park's 5x5 workout routine. It is great workout routine for size and strength, that many classic bodybuilders used to build their powerful physique and they were much stronger than bodybuilders today. Old school bodybuilders also spent most of their time in the gym doing compound exercises and lifting heavy weights, which recruited more muscle fibers, and it is better for muscle growth. In order to only get stronger with this routine, you shouldn't be in caloric surplus which you need if you want to gain muscle mass and strength with this workout program. Exercises that Reg Park used to do in his training were squats, deadlifts, bench presses, and power cleans. Reg Park believed that training to failure is a mistake, he believed in gradual progression for gains that last longer. The aim of his workout routine is to do 5 sets of 5 reps with the proper weight and if you can do more than 5 reps with that weight then increase the weight on every training day. But, don't increase the weight if you can't lift it for five sets of five reps. First two sets in Reg Park's workout routine were warm ups and last three were primary work sets. For an example, if you lift 200 pounds on bench press it will look like this: 1 Set - 5 Reps with 160 pounds 2 Set - 5 Reps, 180 pounds 3 Set - 5 Reps, 200 pounds 4 Set - 5 Reps, 200 pounds 5 Set - 5 Reps, 200 pounds First two sets are great for building confidence and last three sets are the real deal. If you can lift more weight in last three sets, it is time to put 5 pounds more on each set. That is the point of Reg Park's progression, more strength with every training day which results in more muscle mass.