Few believe it, but you don't need barbells, dumbbells, or machines to build muscle; in fact, weight-training equipment often inhibits the process.
Consider the pullup: It's the standard by which all body-weight exercises are measured. And even the most hard-core lifters will agree that there's no better muscle builder for the upper body—with or without weights. The reason for its effectiveness: It takes full advantage of the scientific laws of motion and leverage, placing your body in a position that forces your back and arms to lift your entire body weight. Call it applied science at its finest.
Now imagine if all body-weight exercises were as challenging as the pullup. You'd be able to build muscle anywhere, anytime—at home, on the road, or even in a public park. Physical science makes it possible. So with that said...the Five Laws of Body-Weight Training.
The Longer Your Body, The Weaker You Become