Six weeks, forty gallons of milk, and a boatload of sweat later I went from weighing 220lbs to 252lbs. Having always struggled to build serious muscle mass, you won't find another workout program that will change your body faster. This is the most old school proven ways to gain muscle without steroids, creatine, or fancy equipment.
The workout I did is built around 1 set of heavy high repetition squats done 3x's per week for 6 weeks. On the first workout, you choose a weight that you normally squat for 10 reps, and just do 20 reps with the weight. Every workout you add 5lbs or 10lbs to the squats no matter how hard it was the previous workout. Along with the squats, throw in a a few sets of basic exercises like Bench, Rows, Dips, and Curls but the squats are the key to the program. How many sets of squats? Just one set of 20 reps, trust me you don't WANT to do more than one set of these bad boys. If you're not religious, you'll become religious after a set of heavy high rep squats!
Now to support the squats, it's recommended that you eat normally and drink 1 gallon of milk per day. Yeah, I did say a gallon of milk per day! Back in the 1930's when the 20 rep squat program was developed they had no nutritional supplements. Milk was all they had and it's actually one of the best things to build muscle. Most modern protein powder is derived from milk.
During the 6 weeks my strength went through the roof. I went from squatting 150lbs to 305lbs for 20 reps. This workout turns "before's" into "after's". It literally tells your body that it needs to grow BIGGER. The one thing you will have to consider on the 20 rep squats and milk workout is you'll nee a new wardrobe!