With a potentially epic showdown coming on Sunday against the Ravens, the Steelers are trying to get some of their guys a little extra rest. On Wednesday, four starters missed practices for reasons other than injury: running back Willie Parker, receiver Hines Ward, defensive end Aaron Smith, and cornerback DeShea Townsend. Starting center Justin Hartwig missed practice on Wednesday with a knee injury, and safety Troy Polamalu was absent due to a calf injury suffered on Sunday during pregame warmups.