Sperm penetration tests check to see whether a man's sperm can move through cervical mucus and the fallopian tubes to join with (fertilize) an egg. This test is usually done when a couple is having trouble becoming pregnant (infertility). There are different sperm penetration tests.
The idea that a boundary is shifting can be applied to different areas. A shifting frontier of science would be referring to the fact that science is always changing and growing, and always with new areas. In wartime, a shifting frontier would be an international border which is constantly being renegotiated.
the part or edge of a surface or area that forms its outer boundary. the line that separates one country, state, province, etc., from another; frontier line: You cannot cross the border without a visa. the district or region that lies along the boundary line of another. the frontier of civilization.