DThe older we get, the more likely we are to decrease the intensity of our workouts. Exercise that is too easy will not allow you to reap all of the benefits. However, if we exercise too hard, we might not be able to adequately recover during the workout and may have to stop short of completing it.
The RPE scale (pictured above) is very useful to help you gauge the intensity during a HIIT session rather than measuring heart rate because it is much simpler and doesn't require any math or pulse counting. For the same reasons, this is also a great method you can use with your trainer. By familiarizing yourself with the RPE scale, you can communicate quantitatively how you are feeling.
It's important to note that intensity, duration, and recovery are interdependent. The harder the exercise interval, the shorter you will be able to do it and the longer recovery period will be needed.
During your HIIT workout, there should be times of "active" recovery. This means that while you may be out of breath, you should keep moving (not standing still, sitting, or lying down). In fact, if you need to do any of those things, that is a pretty good indicator that the high intensity part of the workout was too intense. Therefore, keep in mind that your optimal Base Level during the recovery phase should be around 11-13 on the RPE scale.
To increase your intensity during the intervals you can either:
For an example of how RPE works, see the simple HIIT workouts listed below.
Beginner HIIT Workout
Start easy and gradually increase intensity so that you achieve an RPE of about 11-12 (your base level) by the end of the warm-up
Stage 1 (9:00) 3 Hills:
Stage 2 (4:00) 3 Sprints
Cool Down (3:00)
Decrease intensity to achieve an RPE of 9-10. Heart rate should come down significantly.