heart rate recovery time

One study reported in the NEJM is based on the drop in HR from max exercise during a 1 minute recovery.
Recovery is 1.5 mph on a treadmill at 2.5 %.
Exercise was using the 'bruce' protocol.
The study found a relative risk of
6 for a recovery of 5 bpm
3 12
2 17
1 22
Risk is chance of dying within 5 years from a cardiac event. Bruce protocol used for stress.
Stage 1 is 1.7 mph at 10% grade (5 METs).
Stage 2 is 2.5 mph at 12% grade (7 METs).
Stage 3 is 3.4 mph at 14% grade (9 METs).
It goes higher, but most gym treadmills can't handle more grade.
Since the goal of the recovery test is to measure recovery, getting to max quickly can be achieved by speeding up the treadmill, as well as more grade.

It estimates vo2 max with VO2max = 14.8 - (1.379 x T) + (0.451 x T2) - (0.012 x T3). What are T, T2, T3?

Michael Lauer, author of the study explained it all in lay terms in 1999.

Its unfortunate that there are not studies for sub-maximal testing.