There’s a difference between “interval training” and “high intensity interval training,” which is important to know if you can’t do strenuous exercise.
When you hear the words interval training, this doesn’t necessarily mean that of the highly intense type (HIIT). I’m a certified personal trainer.
Interval training means alternating low intensity levels with moderate or medium, or even medium-high. When we talk of just “interval training,” the concept of strenuous or grueling is not included.
An example of interval training on a treadmill would be alternating 1-5 minutes of easy paced walking or comfortable slow jogging, with a faster speed (may add incline) that causes an increase in respiration and effort. (Don’t cheat by holding on.)
This change has a range: from a mere increase in heart rate and effort, to a feeling of “Gee, this is quite challenging, but just stick it out for a few more minutes.”
The work interval does not feel like, “This is brutal!” or, “I don’t know if I can do this for 30 seconds.”
In “interval training,” the work segment will not leave you breathless or unable to carry on a conversation, though at the high end, it will make conversation choppy with pauses in between words while you catch your breath. Again, this would be the high end, a moderate-high workload.
Interval training can sometimes be a better option than HIIT for novices, very heavy people or older individuals who don’t want to strain themselves or risk muscle or joint strain.
If you must start out with interval training minus the high intensity, this doesn’t mean you can’t eventually do HIIT, which brings on significantly more benefits.
Even if your knees don’t do well with the impact of running or rigorous stepping routines, HIIT can be done on a stationary bike or elliptical machine, sparing joints from hard impact.
In order of lowest benefits to highest, cardio exercise goes like this: 1) steady state or long duration cardio, 2) interval training, 3) high intensity interval training.
Few people cannot do #2. In fact, those who cannot do #2 probably can’t do #1, either.
Intensity levels are relative and are determined by using the rate of perceived exertion (RPE) scale.