You don't need a fancy gym with weight training machines or barbells to build muscle strength. You can get stronger by using the physics of your own body force, and you can do it anywhere, anytime.
The farther you lengthen your body, the more difficult it is to move. To apply this resistance concept, raise your hands above your head or to the side while you do lunges, crunches or squats.
Step2
The farther you move, the harder your muscles work and the stronger they become. To apply this concept, move your hands and feet farther from the floor. When doing push-ups, put your hands or feet on a stair or on books so your body has farther to move. When doing lunges, put one foot on a stair. If you're doing a sit-up, put a rolled towel under your back. To make the sit-up even more difficult, raise only a quarter of the way, lower down and then raise fully.
Step3
Pause for four seconds when you reach the down position of an exercise. This pause forces your muscles to use more strength to get back up because they don't have the momentum and bounce of the return position if you stop briefly. For example, when you do a push-up, pause for four seconds and hold before you push back up.
Step4
Add rotation to your exercises. A simple twist of the torso when doing a push-up, lunge, or crunch will work more muscles than doing that same movement without rotating.
Step5
Hold one foot in the air and force your body to stabalize itself while doing squats, lunges or deadlifts. Your muscles will have to work harder to stablize your body and you'll build more strength than doing the exercise on both feet.