Progressive means that as soon as you get used to lifting a certain amount of weight, you add a little more. Over time, you'll get stronger because your muscles adapt by getting larger to handle the added weight.

Our bodies adapt to whatever stress we regularly put it through. When you lift weights, what you're doing is actually tearing the muscle tissue in a healthy way. When you eat the proper nutrition, mostly protein, they'll actually repair themselves. In fact, they get stronger and bigger without lifting more weight.

As you continue to lift more weight, your muscles will keep adapting to this increased overload. So they get bigger.

But one thing you may not know, is, lifting weights actually causes muscle fiber damage. And this microscopic damage needs to heal properly if you're going to gain any muscle.

There are type A and type B fibers in your muscles.

Let's say the higher repetitions might tear the type B so heavier weights will tear them and bring them back stronger. Some people are fast switch and some people are slow switch.

Fast switch responds to high reps, mostly between 8-12 repetitions. Slow switch will respond to slower reps between 4 and 6.

Every body has a different buildup. Let's say, for example, my bicep is 90% fast switch and 10% slow switch. I would need to do mainly high repetitions for my bicep.

You, on the other hand, might be 90% slow switch and 10% fast switch so the approach you would want to take is lower reps.

So, in order to build muscle mass, you first have to tear it slightly.

The degree of tearing is directly related to how hard and how long you exercise and what type of exercise you do. Movements in which muscles forcefully contract while they are lengthening will cause the most muscle growth.

These are called "eccentric" contractions, and they provide the resisting action of the muscle such as lowering weights and performing the downward movements in exercises.

When you get the proper nutrition, your body repairs this muscle tissue damage and makes it stronger, for the next time you have to handle this weight.

That's why you have to first cause slight muscle damage, for new muscle mass to be built.