HRM or feel?

Couple things...  If you are over your AT (which some interchange with LT... and some don't) without it hurting pretty bad, then your AT/LT is wrong IMO.  How did you determine it?

I tend to keep an eye on the HRM (I wear it all the time), but not a slave to it.  If I creep to Z3 during a Z2 run, I don't worry about it too much.  But I will back down if I hit Z4.  But I don't need a HRM to tell me to back off if I get going that hard.

AND, a Z5 run (over my LT) hurts no matter what the temp is.  That's a big effort.  Unless I'm doing sprints or high intensity intervals, I see no point to even be there.

wtf alarm for what?? max hr nooo..im hopin min hr??

You can set most HRM to go off at certain levels.

OK, good start.

I used to follow the Maffeton theory for a long while, but gave it up for the LT method.  Maffetone gave me MUCH too low heart rates IMO.  Something similar to what you seem to be experiencing.

However, I would still question your field test.  Was it an ALL OUT effort?  Was it a run field test?  Was it puke worthy?  I only question because if your LT is 158, going over that should feel VERY hard.  Most can't do if for more than a 5k.  Elite runners can do it for maybe an hour (according to the definition).

If you think the test results are valid, fair enough.  I would go by feel, but wear your HRM.  If you find that you average a higher HR than your LT test, then use the new average as your LT.

I know everyone is different, but my Maffatone number (the "number" where you start going anaerobic from aerobic) is 145.  That is NOT your LT (the point where your body can no longer metabolize the lactic acid your muscles generate while working out.)  That number should be higher in most cases.  My LT for running is 180.  I can keep a running HR in a 10k in the high 170's with an excess of 180 during the sprint finish.

OK, then if you are using the Maffetone number as your AT (aerobic to Anearobic), then yes, you should be able to go over that comfortably.  That is like Z2 using the LT method.

Then it depends on what you are trying to achieve.  I do almost 90% of my training in Z1/Z2  That's 123 to 163.  But I'm working on an IM, where base trianing is important (it's important for all lengths, actually).

If you are interested in HR training, these are good resourses..

http://www.d3multisport.com/articles.php

they are all worthy reads.

Good luck!

ETA:  I plugged in 174 into the calc.  I see the 158 and agree with it.  I think that going up to 166 (high Z3) is ok occasionally too.  If you can do 166, does that feel hard enough?

Mileage, frequency and consistency.  I'd take the 30 in 5 over the 25 in 3 any day.

Yup, what he said ......

To the OP, living in FL and running in extreme heat and humidity can easily push my HR above my Z2 base training range.  I wear my HRM all of the time but when it's really hot I'll set the low alarm at the bottom of Z2 and the high at the top of Z3 and then just go by feel. 

I have days in the summer when the heat gets to be stifling and I just shut it down and walk.  I also have some routes that keep me mostly in the shade for really hot days.  

Mark