Zummack represented her nation at four World Sprint Speed Skating Championships for Women and at two World Single Distance Speed Skating Championships. At the sprint world championships she competed in 1990 (finishing 17th overall), in 1995 (finishing 20th overall), in 1999 (finishing 17th overall) and in 2001. At the 1996 World Single Distance Speed Skating Championships she competed in the 500 metres event (13th) and in the 1000 metres event (19th). At the 1999 World Single Distance Speed Skating Championships she competed in the 500 metres event (11th). She also competed at other international competitions, including at ISU Speed Skating World Cups.