Pullups work every major muscle in your back at once, along with your shoulders and every pulling muscle in your arms. But, to do pullups at home, you need access to a pullup bar. If you rent, have steel door frames or simply don't want to drill holes in your doorway, a conventional screw-mount pullup bar won't work. But there's an alternative: doorjamb chinup bars, also known as leverage-mount pullup bars.
Most doorjamb chinup bars are built around a sturdy horizontal bar that sits on top of the door frame, on the opposite side of the doorway from where you do the pullups. The handles you pull up on curve down from this bar and protrude through to the near side of the doorway. Most leverage-mount pullup bars have at least two handles to choose from: neutral-grip handles that protrude straight toward you, or a horizontal bar you can grasp with a narrow or wide grip, palms facing either toward you or away. A crossbar on the near side of the doorway establishes leverage so as you suspend your weight by the pullup handles, you're actually tugging the bar on the opposite of the doorway into a more solid position atop the door frame.
Most doorjamb chinup bars are straightforward to install. Simply slip the mounting bar through the doorway and rest it on top of the door frame as snugly as possible against the wall above the trim. Tug gently down and toward you on the pullup handles, wedging the entire mechanism securely in place.
Using your doorjamb chinup bar properly helps reduce the risk of it slipping off the door frame. Grip the bar handles and pull yourself up smoothly. If you can't reach the handles from the floor, use a chair or platform to stand on. Don't jump onto or swing on the bars; the sudden jerk might pull the bar out of its proper leverage position.
Keep in mind that, like any other piece of equipment, the bar has at least a minimal potential to fail unexpectedly. Clear any obstacles that pose a risk of injury out from beneath you, and think in terms of landing safely if you should happen to fall. Always inspect your chinup bar thoroughly before each use. The connectors between the individual bar pieces might loosen with repeated use. Re-tighten them as necessary to reduce shifts between the bar parts and lessen the risk of it suddenly failing beneath you.
Pullups and chinups -- a palms-toward-you variation on pullups -- are extremely challenging. You might have purchased a doorjamb chin-up bar because you want to build back, shoulder and arm strength but aren't strong enough to do full pullups on your own yet. You can do self-assisted pullups by placing a chair, bench or other sturdy platform beneath the pullup bar. Stand on the platform and pull yourself up, using as much back and arm strength as possible to do so. Push with your legs, as necessary, to make up for whatever back strength you're lacking -- you'll get stronger with practice. Be prepared to land on, or support your weight on, the platform if the bar fails. If you can't do this safely, move the platform slightly forward or back so you can still use it to push off but have room to land safely on the floor.