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.
Shifting agriculture, system of cultivation that preserves soil fertility by plot (field) rotation, as distinct from crop rotation. In shifting agriculture a plot of land is cleared and cultivated for a short period of time; then it is abandoned and allowed to revert to its natural vegetation while the cultivator moves on to another plot. The period of cultivation is usually terminated when the soil shows signs of exhaustion or, more commonly, when the field is overrun by weeds.