From the article:

Earlier research by UT Southwestern cardiologists showed that left ventricular stiffening often shows up in middle age in people who don’t exercise and aren’t fit, leaving them with small, stiff chambers that can’t pump blood as well.

However, the researchers also found that the heart chamber in competitive masters-level athletes remains large and elastic, and that even four to five days of committed exercise over decades is enough for noncompetitive athletes to reap most of this benefit. In the current study, researchers wanted to know if exercise can restore the heart’s elasticity in previously sedentary individuals – especially if begun in late middle age.