Gain Muscle Mass
There are some solid methods to gain muscle mass that have been established throughout history. Let's try to NOT re-invent the wheel here by exploring some of the ways that you can quickly and effectively gain muscle mass, all without a bunch of goofy, off-the-wall or esoteric (and expensive) ways that the “fitness industry” wants you to buy.
One thing you have to keep in mind above all else when you're trying to build muscle mass is that your muscles are resistant to change; in fact, your entire body loves the status quo.
What you need to do is coax your muscles to get bigger. The ONLY way to do that is through progressive resistance weight training. What does this mean? It means that in order to build size, you must continuously make the muscle work harder through increased weights.
For example, if you're trying to build mass in the chest, you will exercise with the bench press. To force your pectorals to gain in size, you have to add weight to the bar! Now, this doesn't mean that you have to add plates each and every workout. No, in fact, you may have to decrease weights once in a while.
But over time, you should see a gradual increase in the poundages you use for the bench press.
This is the ONLY way you will EVER gain significant muscle mass.
That is a fact. There is no way around it.
Remember when I said that the body was resistant to change? Variety can help you gain muscle mass, too. As an example, I almost never do the same workout. I may employ the same exercises, but I vary the sets, reps, weights, and/or tempo. I may vary the angle, too. Or, I may superset one day, and do straight sets the next workout. I also employ rest-pause, drop sets, giant sets, partial reps, and the like.
All of this really works. Typically, in a mass-building phase, you perform low reps (in the range of 6-10). However, you should change things up a bit by increasing the number of reps you do. You can either end a segment of your workout with a set of 15-30 reps for a body part, or you could employ an entire workout using high reps. Or, like I would suggest, do both.
Your growth phase should last between 3 and 6 weeks. At the end of this time, take a week-long break. Don't do any weight training. In fact, the only activity you should do would be something aerobic and fun (try hiking, swimming, or sprinting).
This gives your body (and mind) a break from the stress and allows you to recharge your batteries. Lifting heavy puts a tremendous strain on your muscles, connective tissues, and central nervous systems; you want them to recover from this overload and the best way is to REST!
Something I just alluded to was overload. Your muscles will not grow unless they are forced to adapt to the weights, volumes, and intensity you subject them to. This means that, over time, you have to workout at a high intensity if you want to gain muscle mass.
High intensity does not mean single reps, nor does it mean “going to failure” every set of every workout.
Once a week per body part, at most, should you work out at very high intensity. By this, I mean that your last rep of your last set on a given body part should be near-maximum effort. You may be able to get one more rep if your life depended on it, but that's about it. Forced reps are an extreme way to over-deliver on the high intensity spectrum; only once in a great while should you employ them.
Using these two methods — progressive resistance and overload — you should be able to gain all the muscle mass you could ever want (assuming your diet is structured correctly and you get sufficient rest).
If you really want to blast your muscles into hyper-growth, try the Hardgainer's Manifesto.