Since writing my previous piece on HIT in which I wrote
about the benefits to physical fitness(http://www.ll.dlpa.bru.nihr.ac.uk/brublogs-5072.html),
I have focussed my attention on metabolic responses and the effects it might
have for those with or at risk of type 2 diabetes. As a side, I've also been
interested in its practical application; however great HIT might turn out to be
in the lab, will people do it on a day to day basis?
What I have found over the past 18 months of research is
that, ultimately, HIT works. It's good for fitness and it's beneficial for
glucose regulation. It's not necessarily better than traditional continuous
exercise, but it's certainly better than nothing. I also believe (and I have no
money to make from saying this) that, if sold properly (unlike in this article
I was asked to contribute towards - I did not say it had to hurt; https://www.sciencenews.org/blog/scicurious/high-intensity-interval-training-has-great-gains-%E2%80%94-and-pain),
HIT, or to be more accurate "interval training", is the best way to
get a non-exerciser to exercise.
What is HIT?
First I should explain what HIT is. The academic definition
puts it simply; "short bursts of vigorous intensity exercise interspersed
with periods of recovery or absolute rest". If you try to explain this in
more detail however, you realise how nuanced this form of exercise training is
and the range of work it encompasses.
Firstly, "vigorous intensity" is categorised as
any physical activity that elicits a heart rate of 77% or greater of an individual's
maximum. This sounds high, but in fact can range from work that feels
relatively easy to all-out maximal efforts. How "hard" the intensity
feels depends on factors such as age and fitness. Things are complicated
further when you consider that even when exercising at their maximum HR,
individuals can keep on working at higher speeds, gradients or watts. In the
HIT research, effort intensities range from the lower end of the vigorous
spectrum to all-out maximal efforts.
Next you have to work out how long to sustain the effort
for. Intuitively, the higher the intensity, the shorter the effort. In the literature, intervals range from 6
seconds to 4 minutes.
The duration and intensity of the recovery periods also need
to be determined. The extent to which the exerciser recovers between intervals
affects the type of adaptation that the activity will stimulate.
Finally, the number of intervals completed can be decided
upon. This is generally dependent on the desired energy expenditure or overall
time commitment the work out requires. Deciding upon a prescription for a
certain audience requires consideration of a number of factors including
ability and desired outcome.
HIT and type 2 diabetes
Ideally, evaluating the effect of HIT on markers related to
type 2 diabetes would pool results from studies employing the same HIT protocol
investigating the same outcomes. However, HIT has only recently been applied in
a health setting and there is not enough information available to come to a
conclusion regarding the specific effects of specific protocols. I therefore
did my best to combine the results from the available studies, which used
different HIT protocols, and run some analyses that would help to tease out
which might be the most influential components.
While I found that overall, the various HIT programmes that
had been tested improved insulin sensitivity1, reduced HbA1c2
and, consistent with other reviews, physical fitness, I was unable to determine
what it was about HIT specifically that stimulates these improvements . There
is clearly something about this mode of exercise that enhances the body's
adaptations to physical activity because equivalent benefits to
moderate-intensity activity are observed following a substantially lower volume
of exercise. The mean "active" duration (i.e. not including rest
periods) of the HIT work-outs I analysed was 21 minutes compared to 44 minutes
of moderate-intensity exercise and yet greater or equal benefits were produced.
Taking the HIT programmes alone, I looked at whether the
magnitude of the improvements observed were related to the interval intensity,
total exercise volume or duration of training programme. In other words, do you
get better effects with higher intensities, longer exercise sessions or more weeks
of training? I found that none of these factors predicted outcomes. There are a
number of possible reasons for this. First of all it is quite likely that the
variation in the studies I included diluted the effects each factor was having;
its not to say that there is no relationship, but I was unable to see one.
Secondly, it could be something else; the change from high to low intensity for
example, which predicts how well HIT works.
The article can be accessed here: http://www.ncbi.nlm.nih.gov/pubmed/26481101
What this means is that currently we know that HIT can
work, but we don't know why it works.
Practically speaking though, that's what is important. Other
research groups are investigating the mechanism of action of HIT and this will
help to optimise HIT protocols for different populations. For now though, we
can tentatively recommend HIT in the prevention and management of type 2
diabetes.
Working alongside healthcare professionals for the past 2
years has led me to be sceptical about the uptake of HIT by a clinical
population. Indeed, participation in any physical activity by the population as
a whole is remarkably low and now I want those who are suffering most from the
consequences of inactivity to exercise hard?!
The media attention HIT has received, I feel, has appealed
largely to Lycra-clad gym-goers; a minority population. It has focussed on
all-out, maximal effort HIT the intervals of which cause the participant to
grunt and groan and leave them gasping for breath. Yes, HIT needs to feel hard,
but as alluded to earlier, 'hard' can start at 'not comfortable'. Then of
course you've got the rest periods; you get to stop! Perhaps placing more
emphasis on the recovery periods will attract a wider range of people to HIT.
Admittedly, to date, my patient contact has been limited to
a few willing study participants (not entirely typical of the general
population), but what I have consistently observed is that once someone has
tried HIT, they realise that it is not as unpleasant as they thought and, in
many cases, easier overall than continuous exercise of a lower intensity. In my
opinion, the biggest challenge with HIT is selling it right. One of my ideas is
to drop the 'H' and call it 'interval training'. This way you remove the focus
from the high-intensity part and emphasise the change in intensity
between intervals.
As part of my follow-up project to the meta-analysis, I will
be asking participants to rate how hard they find HIT compared to continuous
exercise. By the end I should have some idea as to how people rate each type of
exercise, as well as how effective they are in controlling blood glucose.
In conclusion, there are still many unanswered questions
with HIT, but what we know is that as long as heart and breathing rates have
risen, and/or 'hard' effort has been exerted during the workout, HIT is going
to have some kind of beneficial effect. The difficulty is breaking the barriers
associated with HIT and convincing people it is safe and achievable.
__________
1 Insulin sensitivity reflects the bodies ability to utilise
the insulin it produces. It is a term used widely in research as it is the
physiological state which precedes type 2 diabetes. Insulin resistance has not
been employed as a clinical term due to the difficulty of its measurement and
lack of normal range values.
2 HbA1c is glycated
haemoglobin, which is a measure of an individuals' average blood glucose over a
period of weeks.
