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. 1. to transfer from one place, position, person, etc., to another: to shift the blame. 2. to put aside and replace by another; change or exchange: to shift ideas. 3. to change (gears) from one ratio or arrangement to another in driving a motor vehicle. 4. to change phonetically in a systematic way.