CHICAGO -- April Calderon recently pumped out 61 pushups in 60 seconds while balancing precariously on three medicine balls, a feat that has been submitted to Guinness World Records.

The key is "great balance, mental toughness and lots and lots of practice," said Calderon, a general manager for three Anytime Fitness gyms in central Florida. "But I'm really sick of pushups right now."

Pushups, one of the best measures of upper-body fitness, are worth the pain. Traditionally, they're done while in a horizontal position -- hands and feet on the floor. Beginners can drop their knees to the floor to reduce the load they have to lift back up.

But pushups, like many body-weight exercises, can also be made more or less challenging. They're functional; they stabilize the body and prevent injury. And pushups can be done anywhere.

"They're a true resistance and core exercise," said Michele Olson, research director of the Auburn University Human Performance Laboratory in Montgomery, Ala., who incorporates pushups into every class she teaches. "They work every single muscle that you can name in your upper body."

In addition to the triceps and chest, research shows pushups also recruit the shoulder muscles, biceps and abdominals, Olson said.

Offshoots of the pushup such as the plank, or holding the body in the "up" position -- also depend on the abs. "If they didn't, the back would collapse and sag," Olson said.

In addition to resistance training, pushups are used to rehabilitate the upper body after injuries because they can activate the muscles that stabilize the shoulder blade and the shoulder joint, said David Suprak, an assistant professor of kinesiology at Western Washington University.

Calderon started with traditional pushups, progressed to an unstable surface -- a rubber dome mounted on a platform, called a BOSU ball -- and then learned to balance on the three balls.

Unless you're trying to break a record, do pushups slowly and with control. More benefits come from fewer reps with good form than from cranking out as many as possible.

Also, balance the pushup with an exercise that recruits opposing muscles. Pushups rely on the chest muscles and triceps. Rowing exercises work the opposite muscles, the biceps and lats.

Or try a modified pull-up type of exercise, with your feet on the ground and face up. Then pull your upper body upward by pulling on a bar or bench, Suprak said.

Wall pushup

Place your palms against a wall, slightly wider than your shoulders with the fingertips pointing up. Back your feet away from the wall, about 20 inches. Lean forward, bending your elbows until your nose nearly touches the wall. Push back out to start. That's one. To make it harder, place your hands on a lower surface such as the edge of a desk. Used extensively in rehabilitation, wall pushups can decrease the amount of stress on the lumbar spine, Suprak said. But even these can irritate the shoulder for those who have a previous injury. "Makes sure your hands are low enough so the arm is not elevating above the shoulder level," he said.

Incline pushup

Find a weight bench, or if you're outside, use a park bench. Place your hands on the bench and your feet on the ground. Lower your chest to the bench and push back up.

Pushup with knees down

The more parallel your body is to the ground, the harder this exercise is. Beginners can start on their hands and knees; this position trains important shoulder stabilizers and lowers the stress on the lower back and the joints themselves, Suprak said.

Traditional pushup

Start in a plank position, or the "up" position of the pushup, with your body in a straight line and your hands just slightly more than shoulder-width apart. Tighten the abs as if bracing for a punch and slowly lower the chest toward the ground. Aiming at a target can help get the chest close to the floor, Olson said. Simply lay a water bottle or hand weight vertically on the ground. Slowly lower until the sternum touches the object. Keep the stomach off the mat and the back straight. Narrower hand placement benefits the triceps and the chest, Olson said.

Decline pushup

Elevating the feet makes the pushup harder. Use a low bench or a stair, which is about 5 inches tall, Olson said.

Minus a limb

Using a single limb -- one hand or foot -- instead of two, can increase the difficulty. Ground your right foot, stack the left foot on top of the right and lower down. Or, keep both feet on the ground and just use one hand. Be cautious. "A one-handed type of pushup causes a tendency to rotate around the lumbar spine," Suprak said. "There's a torque causing rotation of the body, and you're trying to stabilize. That can cause increased stress in the discs between the vertebra, he said.

Unstable platform

Medicine balls, stability balls, basketballs and BOSUs can all be used to decrease stability, working more muscles. Pausing at the beginning, end or middle of a movement will make it even harder. Some research shows unstable surfaces may increase the activation of the muscles that stabilize the shoulders, but studies are inconclusive.

Wobbly surfaces will, however, definitely increase trunk stabilization," Suprak said. And the chest can drop lower than normal if you're on three balls, so be cautious about going deeper, Olson said. "Basically, all your muscles are doing double duty; they're moving and stabilizing you, which can easily lead to injury."

Calderon, the pushup champ, sets up for medicine ball pushups by putting each hand on a ball. "Then it's one foot at a time until I'm stable," said Calderon, who is shooting for a new record: pushups using just two medicine balls.