I would want to have my VO2max, efficiency, HR @ Lactate threshold, and HR @ OBLA (onset of blood lactate accumulation) tested.
By far and away the most important though is the VO2max test because it makes transparent the power output that you could theoretically sustain (you can come up with an FT Heartrate in field tests).
The following is entirely based on reading what other people had to say, and making some assumptions as well, hence if its' misleading or wrong please correct me. Anyways…
1) it seems to be established that the body can liberate 20,000 kj of energy per minute per litre of oxygen consumed when fueled with the fastest burning carbs.
2) If your absolute VO2max comes back from the test as 5 litres a minute then in theory your VO2max power output could be:
((20,000 x 5) * efficiency) / (1min*60 seconds)
3) Most cyclists appear to be 21-23% efficient, hence using a number of 22%
((20,000 x 5) * .22) / (1min*60 seconds) = 366 watts at VO2 max
4) Your threshold power output is going to be this number, multiplied by the percentage of VO2 max you can sustain for a given duration.
5) There is a relationship you can use to estimate this, from threshold HR, based on some empirical analysis done in the past...
(thresholdHrInPercent - .37) * (1 / 0.64)
Again for example, at 90% of max HR...
Sustainable % of VO2max = (((.90 * 100) - 37) * (1 / 0.64)) / 100 = .828
Sustainable power = 366 * .828 = 303watts
5) For these reasons, the importance of a high VO2 max is plain to see, as is the importance of doing everythign you can to improve it.
I have had a VO2max test, the absolute number was a little over 5L/min, yet I can only sustain 290 watts for 20 mins 9averaging 90% of MHR). What that means in light of the above, I'm not sure. If the issue is poor efficiency then I will soon find out because I have signed up to be part of a Phd research study on cycling efficiency. It will be very interesting to see my real world efficiency numbers.