Intervals - how long?

"How do I decide on the right duration for my intervals?"  An interesting question posed by a friend at work. She is training for a triathlon and wondered if there was a formula (or guideline) to define the optimal interval length (perhaps, she speculated,  related to the duration of the ride one isplanning).

I suspect that there is an "upper limit" beyond which a longer interval will not provide additional training benefits. And one also needs to remember the importance of rest, that is the duration of the rest interval - too short and the muscles/CV system do not have time to recover and let you maximize the next effort, too long and you aren't stressing your performance to the max.

I did find this from the literature - as relates to sprint intervals: In order to enhance aerobic endurance and increase VO2max towards its upper, genetic limit, interval training should consist of 3-5 minute work bouts with a 1:1 work to rest ratio or less. The intensity should equate to 90-100% VO2max.

My advice would  be to set 3 to 5 minutes as the maximum length for your sprint intervals - where you are pushing your absolute maximum to train for that sprint at the end of a ride or preparing to respond with a chase to a early breakaway rider.

For endurance riding, the idea of stressing your CV system to improve still holds. But if you just want to pick up your average performance from 60% VO2max to 65%, then a half mile or even mile ridden at the faster pace qualifies as an "interval" and if you work in a half dozen during your ride, you will improve.

The final answer to "how long" the interval is, as in many things, "it depends" and more on the level of exertion of the interval than the length of your total ride.

Dick