There are several theories supporting the origin of baseball's traditional seventh inning stretch. Baseball historians recognize two theories as being more likely than others, and it seems plausible one supports the origin of the practice, and the other the establishment of the practice as a tradition.
"The (Cincinnati) spectators all arise between halves of the seventh inning, extend their legs and arms, and sometimes walk about. In so doing they enjoy the relief afforded by relaxation from a long posture upon hard benches," wrote Harry Wright in 1869. Wright was a player for the Cincinnati Red Stockings who was writing home about his baseball experience. Wright's letter was discovered decades after many researchers declared President William Taft the originator of the seventh inning stretch.
The frequently written story about President Taft and the seventh inning stretch is well documented; he attended the Washington Senators' opening day game of the 1910 season, stood to stretch in the middle of the seventh inning because his back was sore, and other fans in the stadium stood with him. The story supports the theory that Taft popularized the seventh inning stretch in Major League Baseball, because the practice became customary subsequent to that particular game.
Is it a coincidence that William Taft in 1910, and Cincinnati fans in post-Civil War 1869, decided the middle of the seventh inning was a comfortable time during a game to stretch? What did fans do during the seventh innings of all the games played from the time of Harry Wright's letter, to the day of the President's sore back? The statement by Wright infers a routine practice of stretching, not a one game occurrence, and still, the legend of the seventh inning stretch appears to have begun forty years hence.
There is no indication President Taft chose to stand in the seventh inning for any reason other than he was uncomfortable sitting. We can credit him with giving us one of baseball's oldest, and favorite, traditions; knowing our fellow fans had the same idea in the nineteenth century.