Carbonfiberboy 

My wife and I are doing a series of indoor rides during which we try to stay just a little below AeT, one's aerobic threshold. My understanding is that breathing rate increases rather abruptly at AeT, plus there are recommended percentages of HR (85% of LTHR), and power (75% FTP) which are supposed to define one's approximate AeT. We're both using our FTP and LTHR from last year, which may not be particularly accurate, but hopefully great accuracy is not required.

Working to my AeT seems relatively simple. My breathing rate increases around, actually a little above, 75% FTP, and quite close to 85% LTHR. For my wife, trying to work just below AeT is more frustrating. She's in quite good aerobic condition. Her morning resting HRs are usually between 45 and 48, however her lungs are small and her bronchia are narrow - on a good day she blows ~300 on a peak flow meter, where I blow 800. Her breathing rate increases at ~80% LTHR and ~60% FTP

So what to do? Go strictly by breathing rate or let rate rise a little to get a slightly harder workout? What we're both trying to accomplish is exercise that is strictly aerobic, meaning no lactate generation. We aren't interested in investing in a blood lactate meter, but maybe there are other signs?

Thanks for any suggestions.