HIIT (High Intensity Interval Training) as you
describe is a very acceptable, if not a prefered method of
training. According to the Tabata
.. adequate high-intensity intermittent training may improve
both anaerobic and aerobic energy supplying systems significantly,
probably through imposing intensive stimuli on both systems.
Can HIIT be dangerous? Yes, it is a more advanced exercise
technique in that it is not for the beginner who is in poor shape.
Nor is it for someone with heart problems without proper
Can intervals be safe? Yes, provided that you exercise within
your tolerance. HIIT requires that you exercise intensely for short
intervals followed by short rest intervals as you described. You
must rest during the rest periods so that you can muster the
ability to hit another intense interval.
The rest period can be active or
passive. In an active rest period you would continue a low
intensity exercise like walking. In a passive rest period you would
just try to recuperate and reduce your heart rate. (Personally, an
active rest interval works best for me.)
I don't have access to the original Tabata study, but in the
abstract it simply mentions "rest" periods without specifying
whether it was active or passive. If you have any concerns, just
walk during the rest period. Other
studies show that active rests are superior to passive rest
intervals in that they provide a better quality of work interval
and produce lower lactate levels.
The ideal for safety and effectiveness when doing interval
training is to wear a heart rate monitor and exercise into your
target range during your exercise interval (but
avoid exceeding your maximum heart rate), and then reduce your heart
rate to your target recovery range during your rest period.