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 verb [I/T] (MOVE OR CHANGE). âº to âchange âdirection or move from one âperson, âposition, or âplace to another: [I] The âwind shifted to the âeast. [T] She shifted her âweight from one âfoot to the other. [T] He âtried to shift the âblame onto his âsister.