Middle age is not too late to increase cardiac fitness, even after years of inactivity. That's the conclusion that three studies — all done independently — have reached.

The most remarkable of the studies, "Reversing the Cardiac Effects of Sedentary Aging in Middle Age," went where no previous exercise study has gone, lasting two years rather than the more-customary three to four months. And for precise results, the researchers probed into their subjects' heart ventricles.

The exercisers, who had an average age of 53 when they started working out after years of sedentary living, increased their aerobic fitness by 18 percent. They also improved their cardiac compliance, or elasticity, by 25 percent. Loss of elasticity is a major cause of heart failure.

"A 25 percent increase in cardiac elasticity is huge," said Ben Levine, senior author and director of the Institute for Exercise and Environmental Medicine in Dallas. "It allows the heart to fill more easily and pump more blood."

Levine's subjects began with low-intensity "base training" workouts three times a week for 30 minutes each. Later, they added interval training to their regimen, and one weekly longer workout of at least 60 minutes. During peak training, they worked out four or five times a week for about 180 minutes in total — 30 minutes more than the minimum standard of 150 minutes per week recommended by many fitness guidelines.

Some would consider this program too time-consuming for many busy adults. Levine disagrees.

"Exercise is so important that people should think of it as part of their personal hygiene, like brushing their teeth," he said. "Our program isn't difficult to incorporate into your life. You should do one fun activity for at least an hour on the weekend, and one hard activity for 30 minutes after your second cup of coffee another day. Then, on another two or three days, exercise for 30 minutes while you're watching TV."

Another new research report, from Mayo Clinic Proceedings, tracked changes in fitness and mortality among more than 6,000 men and women who were, on average, in their late 40s at the outset. Those who maintained or improved their fitness over 4.2 years had a 40 percent lower mortality rate than those who lost fitness due to insufficient activity.

In addition, a study published in the American Journal of Preventive Medicine investigated the walking habits of 139,000 Americans who were nearly 71, on average, when first monitored. Thirteen years later, those who reported little to no weekly walking had died at a rate 26 percent higher than those who walked regularly, but for less than two hours a week. Those who walked two to six hours a week had a mortality rate 36 percent lower than the under-two-hour walking group.

"Exercise supplies many benefits that can be achieved even if it is started later in life and at low doses," Levine said.