heart rates

This is both an archetyping question and a physiology question

I want to record various heart rates in archetyped data.

One concept that seems important in athletics is a maximum heart
rate - now this concept seems to have clear semantics in
exercise circles, but I'm not sure whether a person actually really
has a 'maximum' heart rate. It's more the heart rate plateau that you will
get to if you are exercising at 'maximum' intensity. The maximum
heart rate is used to set training zones for athletes.

It can be estimated using various formulas (based on age, condition etc)
or can be measured using a heart rate monitor and putting the
person into an intensive workout.

I want to put a measurement like this into a persons record. I can
put a sequence of recorded heart rate events into a heart rate archetype
instance - I can even indicate a record stating the 'max' heart
rate of this sequence of events. However, to me this doesn't capture
correctly the semantics that this is the 'heart rate maximum' for
this person. I could quite easily have been measuring the person's
heart rate on a gentle walk on a tread mill and recording the 'max'
of this sequence of heart rates - not at all the same thing.

So, I guess my questions are:

Is the concept of a maximum heart rate medical nonsense?? (for instance
I can't find any snomed code corresponding to the concept)

If it is a valid physiological concept, how best to record it in an
openehr instance? Should there be a code in the 'protocol' indicating
that this is a 'maximum intensity' test? Is there a code I should put
somewhere else to indicate the type of heart rate measurement.
Should I be asking for a code like this to be put into snomed?

Andrew