NIU left after the 1985 -- 86 season .
The first major expansion since the 1950s took place in the mid-1970s with the addition of Central Michigan University and Eastern Michigan University in 1972 and Ball State University and Northern Illinois University in 1973 .
Northern Illinois University left after the 1985 -- 86 season .
The first major expansion since the 1950s took place in the mid-1970s with the addition of Central Michigan University and Eastern Michigan University in 1972 and Ball State University and Northern Illinois University in 1973 .