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.
shift, displacement(noun) an event in which something is displaced without rotation. transformation, transmutation, shift(noun) a qualitative change. shift, work shift, duty period(noun) the time period during which you are at work. switch, switching, shift(noun) the act of changing one thing or position for another.