I read a report in a runner's magazine about what kind of muscle fibers are present in people who have chronic low back pain compared to normal, healthy adults. All of the subjects were men. Why aren't women included in these kinds of studies?

There are gender differences for muscle performance and muscle fiber-type. To help keep the research "pure" scientists may choose to include only one gender type. This allows them to compare apple with apples instead of apples with oranges. In some studies, the topic under study isn't different from men to women, so both are included.