Your average heart rate and wattage, minus 5% is your Functional Threshold Heart Rate and Functional Threshold Power. The reason to subtract 5% from the test is because your LTHR & FTP is defined as the highest average wattage or heart rate you can maintain for 60 minutes at maximal effort.our average heart rate minus 5% is your Lactate Threshold Heart Rate. The reason to subtract 5% from the test is because your LTHR is defined as the highest average heart rate you can maintain for 60 minutes at maximal effort.