This progressive increase in resistance is the only way to build muscle mass. The greater the amount of weight (overload) you place on a muscle, the larger that muscle will become, provided you get the proper nutrients needed for muscle growth to occur.
After all, what you're doing is stressing the muscle, forcing it to respond by adding more muscle and strength. If you continually lift the same amount of weight, your muscles have no reason to get bigger or stronger.
It can already handle the weight you're using. So to build muscle, you have to keep the muscle stressed, by forcing it to lift more weight.
3 ways to increase the amount of resistance you place on a muscle:
1. Do more reps with the same amount of weight each time.
2. Do the same number of reps but use more weight
3. Use the same number of reps and same amount of weight, but shorten the rest periods in between sets.
The most common ways to add more resistance, in order to build muscle, is with the first 2... using more weight and/or doing more reps.
One of the easiest ways to increase the amount of weight you lift is to decrease the number of reps you do. You keep increasing the weight until you use a heavy enough weight so that you reach muscle failure between 4 and 8 reps.
When you increase the weight each set while performing the same number of reps, you want to go easy for the first few sets. They're just warmups and will not really help to build muscle.
Only your last 2 heavy sets, the ones with max poundages, are the ones that build muscle. All other sets just help you get ready for these heavy sets.
Again, overloading a muscle group is the only way to gain muscle, increase its size, and tone. So, for your heavy sets, you want to reach the point of failure with low reps and heavy weight.
The more weight you use and the harder you work, the greater the muscle gains. This method is very intense, so you want to ensure proper rest and nutrition.
Overload will help you gain muscle indefinitely, as long as the environment for constant muscle growth is met. Again, it takes intense training, proper nutrition, and plenty of rest.
If one of these conditions is not met, you will not gain the muscle mass you could.
Now that you know what causes muscle gains to occur, you should be well prepared the next time you enter the gym to workout. You'll know that merely lifting weights is not good enough.
If you can already handle a certain amount of weight, it would do you no good to keep lifting that weight. You would not build muscle that way, since your body can already handle it.
You have to keep getting better and stronger, by increasing the amount of weight you lift or the number of reps you do.
But be aware, this process takes time. Be committed to slow, constant growth and you'll soon start to build muscle faster than you ever thought possible.