Concept of Heart Rate Reserve

As a large part of the training for endurance athletes needs to be conducted in low training zones  it may be useful to apply some variation of intensity also in the  lower training zones.

A useful methodology for Heart Rate zones can be the application of the concept of Heart Rate Reserve (ref Karvoonen heart rate methodology).

Heart Rate Reserve is defined as the difference between Maximal Heart Rate and Resting Heart Rate

HR reserve = HRmax - HRrest

Heart Rate Zones can then be defined by adding a % of Heart rate reserve to HRrest.

Example

Athlete with HRmax of 200 and HRrest of 50

Heart rate reserve is 150, Heart Rate zone 2 could be defined as

Zone 2 (from 60% to 70% of HR reserve): 140-155 

and can be further split into: 

Zone 2a (from 60% to 66% of HR reserve):  140-149

Zone 2b (from 66% to 70% of HR reserve):  149-155

Try doing some endurance runs alternating between the different parts of the zone opposed to running at constant pace.   It creates more variation - run a large part in the lower part of the zone whilst using the higher part of the zone from time to time to maximize fat metabolism.