Weights aren’t necessary to build muscle, but, resistance is. And, to continue building muscle without weights, some form of resistance must progressively increase. If the average person (not a fit person!) began suddenly training with bodyweight exercises, for example, they would soon see that their muscles begin to grow in size.
However, there is a limit which can be soon reached. Take push ups for instance. There are incredible people out there who can do thousands of push ups, which shows phenomenal physical endurance capabilities. But, they aren’t built anywhere near the size or Arnold Schwarzenegger. (And, I bet Arnie couldn’t do 500 push ups!)
Why is this?
Like I say, building muscle without weights still requires progressive resistance. Some form of weight has to be used. Something progressively heavier and heavier, or muscle will simply stop growing once it has reached it’s maximal size with a particular weight… it will simply gain endurance at that size.
Diet is another aspect which is often forgotten amongst people looking into building muscle without weights. People see special muscle building diets as something only iron-pumping bodybuilders do, which isn’t true. Anyone looking to build muscle must add progressive resistance in their training, and also eat more food than they would normally.
If they fail to gain muscle, or muscle gains slow, then more food is needed and/or more weight is needed. It’s as simple as that.
Make sure you don’t eat junk food. Most of your calories should come from healthy whole foods, which will be a heady mixture of good protein such as eggs, fish, beef, chicken etc. And also good fats (fish oil, dietary fats) and carbs such as rice, pasta bread and oats.