page 4 - Inactivity

You can't easily get tested to see if you have the gene variations that make blood pressure respond to regular exercise. But that shouldn't keep you off your feet, says Hurley.

"You may not respond with lower blood pressure, but you may improve more than others in other areas such as insulin sensitivity," he explains. "We've never seen a case where someone doesn't have the right genotype for any benefit."

6 Muscles

It happens earlier in women than in men, but in both genders muscle starts to wither away if it's not used.

"Muscle atrophy for people with average activity levels starts at age 40 for women and in the late 50s for men," says Hurley. The muscle wasting probably starts even earlier, he adds, but so slowly that it isn't easy to detect.

"For every decade after about age 50, you lose some 6 percent of your muscle mass, which comes with a 10 to 15 percent loss of your strength," says Hurley.

But anyone can build muscle back up with strength training exercises. (11) "After two months of training, we see a 40 percent increase in strength," says Hurley.

"The earlier you start, the better," he adds. "But even people over 100 years old can partially reverse some of the loss that occurs with aging."

The trick is to not just use the muscle, but to overload it. "You have to make it work harder than it's accustomed to," explains Hurley.

"If you overload it in a gradual, progressive way, you can make the muscles bigger and stronger by making each muscle fiber thicker."

If you think of strong muscles as a luxury, think again. They can ward off the frailty that makes older people lose their independence, either because they can't take care of themselves or because they fall and fracture a hip.

"Regular exercise is the best way to stay out of a nursing home," says the Cooper Institute's Steven Blair.

7 Bones

Fragile bones--osteoporosis--cause more than 1.5 million fractures each year in the U.S. But bone starts to disintegrate decades before it cracks.

"Bone is like any other part of your body," says Harvard's I-Min Lee. "If you stress it, it responds."

If you don't, the ongoing balance between bone buildup and breakdown shifts towards a net loss. But strength training can make a difference.

"The research doesn't consistently show that you can increase bone, but you can prevent loss," says Hurley.

The best studies randomly assign people to strength training or a non-training "control" group. (12) "In postmenopausal women, the control group loses almost 1 percent of their bone mineral density in a year," notes Hurley. "But the group that trains either stays the same or has a slight increase."

Whether you actually gain bone or hold on to it may depend on how much you stress, or overload, the bone.

"We reported a 3 percent increase in density in the femoral neck bone at the top of the thigh, we think because the leg press--an exercise that strengthens the major leg muscles--has the greatest load," says Hurley. "We didn't see an increase where the load wasn't very big."