Stock futures muscled higher ahead of Tuesday's open, tracking action in currencies and overseas markets. Dow futures were ahead 45 points and rising. Nasdaq 100 futures climbed 13.5 points and S&P 500 futures were up 5.75 points. The stock market today follows two days of advances, positive reversals that lifted the major indexes near highs, but also flashed some mixed signals. The gains occurred in lower volume, while last week's losses saw