The issue of Presidential health is an important one, as the stresses on the Chief Executive, are, and have been, massive over time, and the job ages all Presidents noticeably.
But separate from general health, there is also the issue of how “fit” or “unfit” Presidents have been while in office, and those who have been active in athletic activities before and during their White House years.
The list of truly “fit” Presidents has favored the younger Presidents over time, but there are also cases of other Presidents who have made being fit an important part of their image as Presidents. These Presidents participated in sports, and even when having health issues over time, they still emphasized the active life.
So the truly “fit” Presidents would include:
John Quincy Adams
Dwight D. Eisenhower
John F. Kennedy
George H. W. Bush
George W. Bush
All of the above 12 Presidents did a lot of exercise throughout their lives, and some were in the military as generals (Washington, Jackson, Eisenhower).
Those Presidents that would qualify as particularly unfit would include:
Martin Van Buren
Chester Alan Arthur
William Howard Taft
Warren G. Harding
Lyndon B. Johnson
All of the above 10 Presidents had major issues with weight, particularly Taft and Cleveland.
Of course, “fitness” has nothing to do with greatness in the White House, as Franklin D. Roosevelt proves! But FDR also had massive upper body strength, despite the polio that prevented him from walking.