Joe Friel never figured out why he had precipitous drops in performance.
If anyone looks at the actual training of endurance athletes, there is a substantial volume just below the aerobic threshold over a long period of time. The type of intervals and particularly the rest period is very interesting. Older athletes need intervals, but the prescription is not so clear.
There is no
long term research showing the benefit of 2-4 HIIT sessions per week for older cyclists. What you do find is
6-12 week studies. The average being 7.9 weeks.
If I recall correctly, Friel recommends VO2 max intervals every 9 days. I am not sure if I still have his book, I'd have to look. He never defines LSD other than to trash it. He says the mistake of older riders is gravitating to long and slow. But what is slow and what is long. Most take that to mean, do HIIT over and over and not to really understand that a long workout just below AT is not easy. There are two mistake oldies make is not understanding how hard Friel's intervals are, these are VO2 max and are extraordinarily difficult and painful. Secondly, these intervals can be done about once every 10 days for an older rider whereas a young competitive rider might do twice per week in some training blocks/
Excessive HIIT risks burnout or overreaching with poor sleep and high cortisol levels, especially older riders. This is a real risk with no corresponding benefit in my experience. I am not talking about 50 year old elites but 60-70 year old average athletes.
Minutes 31-37 are good explanation of heart rate, power, and what level is sufficient to provoke improvement.
However, the health benefits, risks, and optimal design of HIIT are still unclear. Further, most of the research on the effects and benefits of HIIT has been done in younger and middle-aged adults, and as such, the tolerability and effects in older populations are less well-known.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8289951/