Athletic Threshold

Do you mean the anaerobic threshold?   This is the level effort that the muscles cannot flush the byproducts of anaerobic energy conversion (like lactic acid) from the muscles and they begin to build up.  Here is a more complete explanation, http://www.rice.edu/~jenky/sports/anaerobic.threshold.html

The best measures of the the AT involve taking blood samples while exercising so this means going to an exercise lab.   Some empirical tests like the Conconi test try to measure the AT point using heart rate.   http://www.brianmac.demon.co.uk/coni.htm

Good luck.

TW

You can also find your AT by doing a simple, but tough 20-min run test.  There's an article with all of the "how to's" on this site.  A quick search of the articles for "field test" should find it.  You'll notice that your AT test and results will differ for the bike and run, but do the run test first -- it seems to me to be the least complicated. 

Good luck!