I like it somewhat less when I do ETT's on my athlete patients. It takes much longer if you really want a 'max effort' test and that will often put me behind with other patients. Plus, the more fit they are the less likely it is that I'll hold my record of being in the top 3 for people who have gone the longest on the treadmill.
:)

The formulas are more accurate for the population-less fit, and far less likely to be lifelong athletes. Another formula is 200-1/2 age. Both are often inaccurate in being too low if you have been active doing aerobic activities your entire life.

Your max HR is highest in youth and degrades variably with age and somewhat more so with inactivity. My kids wore my HR monitor many, many years ago when they were doing JR OLY cross country, and were playing various sports. My 7 year old had a NORMAL max HR of 235 and my 10 year old 230.

I'm now 56 and my max measured HR by 5K races at the end of sprinting like my hair was on fire and a true very hard effort was 160, pretty close to the formula. N=1. But over the 20+ years that I've done stress tests on my thousands of patients, it's pretty close...

To the OP, max HR is less useful for training. The more important number is the tempo/threshold HR. There are various field tests you can do but as others have alluded to, best to test with a hard effort 10K. You can use Daniel's/McMillian or others charts to get guides of training zones after that.

Dale