The most patriotic moments at Yankee Stadium can also be the most confining.

Seconds before “The Star-Spangled Banner” and “God Bless America” are played, police officers, security guards and ushers turn their backs to the American flag in center field, stare at fans moving through the stands and ask them to stop. Across the stadium’s lower section, ushers stand every 20 feet to block the main aisle with chains.

As the songs are played or sung, the crowd appears motionless.

The national anthem has long been a pregame staple at sporting events. But after the Sept. 11, 2001, terrorist attacks, Major League Baseball directed teams to play “God Bless America” before the bottom of the seventh inning at every game. Baseball scaled back the next season, telling teams they needed to play the song only on Sundays and holidays, which is still the case.

Only the Yankees continue to play “God Bless America” at every home game. They are also the only ones to use chains to prevent fans from moving during both songs, which concerns some civil liberties advocates.

Howard J. Rubenstein, the spokesman for the Yankees’ principal owner, George Steinbrenner, said the policy was an expression of patriotism.