Phil,
Here are a few reactions.
It is entirely possible for your bike LTHR to be higher than your run LTHR. For individuals who are trained about the same in both, this is unlikely -- their run LTHR will be higher, in general. But if you are more experienced on the bike than on the run, then your tested run LTHR could be lower.
When you say "LTH 2x20 and I had 172 and 168 for each 20 minute session" do you mean that those were your averages? Did you warm up before each one to get your HR up a bit first?
Most LTHR tests will have you going all out for, say, 30 minutes, but only take the average HR for the final 20 minutes. If you take the average for the entire 30 minutes, then you are including a lot of time where your HR is just working its way up. So unless your HR was already elevated
(due perhaps to a warm-up with some intensity to get the HR up
), the 161 average on your 5K is probably LOW as a LTHR.
In general, I'd say: do a careful LTHR test for both run and bike.
(And indoor bike can be different from outdoor bike as well...
) The
protocol on BT is a good one, IMO. There are others. How do the results relate to training? They can be used to determine training zones.
(I know you know this.
) A lot of plans here and elsewhere will specify the intensity of a given session
(or portion of a session
) by zone. You can plug your LTHR into the calculator here on BT to get your zones, or use one of the standard formulas
(such as
Friel, but there are others, generally expressed as ranges of percentages of LTHR
).