People who get fit in middle age reduce their risk of developing heart failure, a new study indicates.

Heart failure is defined as the inability of one or both sides of the heart to pump enough blood to meet the body's needs.

US scientists monitored the fitness levels of over 9,000 men and women who took two fitness tests eight years apart. All of the participants were middle-aged, with an average age of 48. They were followed up over a total period of 18 years to see who developed heart failure.

Not surprisingly, people who were not fit at the beginning of the study had an increased risk of developing heart failure after the age of 65.

"However, those who improved their fitness reduced their heart failure risk, compared to those who continued to have a low fitness level eight years later," the scientists explained.

They used a measurement known as metabolic equivalents (METs) - this measures how well people do on a treadmill.

The study noted that for each MET improvement in fitness, the risk of heart failure among the participants fell by 20%.

"For example, if a 40-year-old went from jogging 12 minutes per mile to running 10 minutes per mile - an increase of two METs - he or she reduced heart failure risk at a later age by 40%," they explained.

The scientists pointed out that as more people are living with heart disease and are surviving heart attacks, the prevalence of heart failure is set to increase.

"Improving fitness is a good heart failure prevention strategy - along with controlling blood pressure and improving diet and lifestyle - that could be employed in mid-life to decrease the risk of heart failure in later years," the team from Texas said.

Details of these findings were presented at the American Heart Association's Quality of Care and Outcomes Research Scientific Sessions 2013.