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.
burden shifting - Legal Definition. The shifting of the burden of proof from one party to another at trial, after each party presents sufficient evidence to initially persuade the fact-finder of the truth or falsehood of a disputed fact or assertion, the burden shifts to the other party to disprove such fact or assertion. Link/Cite.