I will always emphasize shorter, more intense bouts of cardiovascular exercise, also called High Intensity Interval Exercise, for optimal metabolic response (burning fat), hormone regulation, and building of lean muscle tissue. But how do you know how long your exercise intervals should be? Here is a simple and easy guideline to follow broken down by experience/ability. Always keep the mind-set “less is more”. Never let your interval sessions last greater than 20 minutes at the most!
The goal with intervals is heart rate manipulation. You can calculate your heart rate range by taking 220 minus your age (i.e. 220-30=190). That would mean 190 is roughly your max heart rate (MHR). Take that number and multiply it by your goal intensity (i.e. 190 x .80[80%] = 152). When doing your intervals, constantly check your heart rate during the intervals to make sure you are within range. Once you become familiar with how your body feels at certain heart rates, you will be able to gauge your intensity.
Beginner: Stick with a 1:4-5 work:rest ratio. Sprint intensity 70-80% of MHR.
- This is for people brand new to the game. This means if you sprint for 20 seconds, rest for 80-100 seconds. Resting can be complete rest, but at this intensity try to stay at a 40-60% of your max for the remaining time following the sprint. Repeat this for 20 minutes.
Intermediate: Use a 1:3 work:rest ratio. Sprint intensity around 75-90% of MHR.
- Try to push yourself and get to new levels. I typically use bouts of 20-30 seconds for around 12-15 minutes at most.
Advanced: Push the limit. 1:1-1.5/2 work:rest ratio. Sprint intensity 90% or greater.
- This level is for those advanced individuals who are familiar with how their body responds to this type of exercise. Shorter bouts of sprints and recovery. I like to stay anywhere from 10-15 seconds worth of sprinting for no more than 10-12 minutes.
These above intervals are guidelines. Listen to your body and how you are performing. If you end your session thinking you should do another 10 minutes (exceeding your 20 minute mark), then you did not work hard enough! This protocol can be used for any type of cardiovascular equipment.
Coach Carsia 
For decades, swimmers have endured crippling interval sets that involve intervals lasting over two minutes with only 5-15 seconds rest. Thankfully, this kind of interval training is being questioned by a few experts and coaches, but is still popular. My question is this: If a swimmer is training for a race lasting two minutes, is this kind of interval training optimal for performance in such an event? I doubt it’s optimal for health.
I am not very familiar with the physiological or physical demands of swimming as I myself have not experienced the training required for it, but I can’t see how such short rest could prepare one for a competition. It does make sense to repeat multiple bouts to improve both anaerobic and aerobic work capacities within the same session, however the periods of rest must be trained appropriately, assuring the athlete is doing the following:
1. Improving their initial two minute swim time each time it is tested
2. Consistently producing the same work output and time results during the subsequent bouts in the same session
This has to be done by gradually decreasing the amount of rest in between each bout. And if a swimmer for a 2 minute race has the endurance to repeat the bouts after such short a rest, I doubt they have the power endurance to sustain a pace equal with that of the other swimmers.