Stock Index Futures Lower

September SP500 is trading at 2179.00, down 6.25. September Nasdaq 100 is trading at 4808.00, down 9.00 September Russell 2000 is trading at 1244.10, down 2.90. September Dow Industrial futures are trading at 18475, down 53. Stock index futures are very choppy and near the session lows. Market chatter about corporate news and the upcoming speech from Federal Reserve Chair Janet Yellen at the Jackson Hole Symposium on Friday has kept buyers and sellers active.