High intensity interval training has been proven to make you much fitter much more quickly than traditional aerobic exercise. But how many intervals, how fast and how often should you do?

1. HIIT works, but how many intervals should I do?
HIIT (high intensity interval training) has had lots of press over the
last 2 years, from the BBC Horizon programme “The truth about
exercise” to the well publicised Andrew Marr incident. Many accredited
studies have shown that HIIT will increase aerobic fitness (VO2 max)
as well as providing all the health benefits of longer duration exercise.
Its time efficiency and effectiveness have made it very popular; but
what are the optimal number of intervals?
In the BBC Horizon programme, Dr Michael Moseley was tested doing
a single 3 minute routine, containing several 30 second, maximal
bursts. Some complete just one sustained, flat-out 3-4 minute burst,
whereas others practice a more conventional training routine of more
intervals – as many as 10, all close to maximum heart rate. Each
interval is followed by periods of slower recovery.
Frequency of training also varies greatly. Some may train every day,
others a few times a week with days off between exercise. Dr Mosley
Was completing just two sessions a week.
Although scientific studies have proved that intense interval training
can increase your aerobic fitness up to 10 times faster than moderate
exercise, no studies had been done to determine the optimum amount
of intervals. Until now. A study by Mcmaster University in Ontario
tested 17 young men and women on different HIIT regimes. 10 of the
17 were asked to exercise on 2 separate days a week. On the first day
they did a standard HIIT session of four 30 second intervals of
excruciating effort alternated with one minute of recovery. On the
other day they did a single, non-stop interval of around 4 minutes,
after which they had used the exact amount of energy as in the first
session. Blood samples were gathered after each session.
The remaining 7 volunteers did the non-stop, single four minute
interval for 6 weeks, completing it 3 times a week. Again blood
samples were taken.
Although there were gains for the single interval training group
immediately after exercise, these did not translate into lasting
improvements over the 6 week study. Only the group who were doing
multiple intervals gained in the medium term.

2. In another study by the University of Science and Technology in
Norway, volunteers were asked to perform 24 HIIT workouts
over three or eight weeks. This meant that some of the subjects did
sessions every day (sometimes twice on the same day), while others
only exercised three times a week. At the end of the study, those
exercising 3 times a week had improved aerobic fitness by 11%, while
those who were at it daily showed no improvements; in fact some of
them showed a small decline. Only after they stopped exercise did the
daily group show signs of improvement, with aerobic capacity creeping
up by 6% some 12 days after cessation of activity.
The conclusion of both of these studies is that the best gains come
from more standard interval training. Several intervals interspersed
with recovery are best. So try to do 60-90 seconds off full on
swearing-inducing effort with 60 seconds of slow recovery. Repeat at
least 4 times, but 8 is better. The less recovery you have the faster
you will improve lactic tolerance. The longer recovery you have the
more you will improve speed and power.