High intensity interval training (HIIT) has become enormously popular within the health and fitness industry in recent years. Sydney's eastern suburbs are no exception. Take a stroll along the iconic Bondi to Coogee coastal walk early in the morning and you'll see all sorts of HIIT sessions taking place. It is a great thing to see our community so engaged in physical health. When I talk to my patients about the popularity of HIIT, there are two themes that seem to emerge. The first is from those who are hesitant to engage in HIIT. They tell me that HIIT "pushes people too far" and is more likely to cause injury than conventional exercise approaches (steady state aerobic or conventional weight training). The second view is from proponents of HIIT who argue that it is great because "you get the benefits of a long workout compressed into a short, intense session". There is a view that HIIT is a more time efficient form of exercise. Let's turn to the science and published research to see if these popularly held views are supported by evidence?
DOES HIIT "PUSH PARTICIPANTS TOO FAR"?
Surprisingly, there has been very little research carried out to address the question, "Am I more likely to get injured doing HIIT than a more conventional continuous aerobic and/or moderate strength training?" The existing research is comprised of a low number of qualitative trials, in which Crossfit participants completed surveys about their injury experiences in relation to Crossfit participation. A systematic review by Claudino et al (2018) examined 31 studies of which only 4 met inclusion criteria for a meta-analysis. What does this mean? It means that most of the research carried out on Crossfit training is of insufficient quality to draw meaningful conclusions. We simply don't have data to quantify the risk of musculoskeletal injury during Crossfit specfically, or HIIT generally (relative to more conventional aerobic training and / or conventional strength and conditioning training). In the absence of solid evidence pointing to a concrete, modifiable risk of injury during HIIT, we must make a judgment based on what we do know.
DOES HIIT GIVE YOU MORE "BANG FOR YOUR BUCK"?
So what do we know about HIIT versus others forms of exercise training? A popularly held view is that HIIT is better for you than a typical cardio session because "you can burn more calories and get fitter in less time". There is also a view that HIIT confers a longer "afterburn" compared with steady state cardio training. A review by Milanovic (2015) however, concluded that increases in fitness (VO2max) were only marginally greater for HIIT participants compared with endurance/aerobic performance. For equivalent doses of exercise both HIIT and conventional steady state aerobic training significantly improve cardiorespiratory fitness, HIIT slightly more so than conventional cardio training. This evidence challenges the popularly held view that HIIT is a more efficient approach for improving fitness.
With regards to the view that HIIT produces a greater "afterburn" of calories than an equivalent dose of conventional steady state aerobic exercise, Greer et al (2015) reported that HIIT sessions (and conventional weight lifting) produced a greater post exercise calorie consumption compared with steady state aerobic exercise. This could be interpreted as a tick in the box for HIIT and conventional weightlifting. However, the authors noted that the caloric difference was small and could be cancelled out by extending an aerobic exercise session by as little as five minutes. They also noted that their study was the only trial they were aware of where the afterburn was less for the steady state aerobic group compared with HIIT (for equivalent caloric workouts). Again, the evidence doesn't support the popularly held view that HIIT produces a greater caloric afterburn than conventional strength or aerobic training.
WHAT DO WE MAKE OF ALL THIS?
Put simply, we don't have evidence to support the popular view that HIIT gives you more "bang for your buck". Nor can we argue that HIIT is more risky than conventional strength or aerobic training. We have roughly equivalent evidence for the benefits, and an absence of evidence about the relative risk of injury for each exercise approach. Although we don't have compelling evidence for these popularly held views, we have no reason to call for a cessation of HIIT.
In fact the benefits of HITT arguably stretch beyond these (possibly) erroneous views. HIIT sessions are high energy, varied and typically require little to no planning from the participant- exercisers can simply turn up, be put through their paces and go home or head off to work feeling like they've done their bit for their physical health for the day. HIIT sessions appeal to those who are easily bored by repetitive exercise and those who have little interest in designing, structuring or progressing their own exercise program. Perhaps HIIT's greatest strength though, is the sense of social cohesion and accountability that tends to develop when HIIT is delivered (as it most often is) in a group setting. Group HIIT may boost intrinsic motivation, while helping develop a sense of camaraderie and healthy competition to push one's self. This arguably drives adherence and helps to make exercise an important, ongoing component of one's life.
On the flipside of a group culture of pushing one's self however, is a potential for pushing one's self too far. Although we don't have evidence to support or reject the idea that HITT is more injurious than other exercise approaches, the notion of an increased risk of injury within a group culture that promotes maximal efforts in fatigued states is plausible. In deciding whether or not HIIT is better for you than conventional aerobic or weights training, we need to weigh the benefit of greater social cohesion and its likely positive effect on exercise adherence against the roughly equivalent fitness / caloric benefits and an unquantifiable yet plausible risk of injury. In the end, the decision to participate in HIIT or conventional forms of strength and fitness training will boil down to personal preference. Do you enjoy the pain, jelly legs and exhaustion of high intensity exercise? By all means go for it. But keep in mind that it unnecessary to go through all that pain to get similar benefits.
HOW TO PROCEED?
Whether we are talking HIIT, conventional weight training or conventional steady state aerobic training, I hypothesise the risk of injury increases when an exercise participant engages (or feels compelled to engage) in repetitive, high intensity efforts near to (or to) the point of failure (the point where one is too fatigued to complete another repetition of a given exercise). I hypothesise that this risk is greater if;
the exercise participant is fearful that he/she is exercising beyond a safe limit
the exercise participant is a novice at the particular exercise in question
the exercise participant is experienced at a given exercise but has had a period of greater than a few weeks break from performing it
the exercise being performed is technically complex
the exercise participant repeats efforts to failure across multiple exercises within a training session
I think there is tremendous potential for a group fitness regime that is able to mitigate these risks while simultaneously harnessing the camaraderie, competition, social cohesion and exercise habit forming power of (group) HIIT. I would hazard a guess that there probably already exist specific gyms and trainers who have managed to strike a balance between the mitigating of these risks and the achievement of health and fitness goals. It is difficult though for consumers of fitness services to vet gyms and trainers. In the meantime, regardless of whether or not you are a HIIT enthusiast, I think it makes sense to be mindful of the risks outlined above, and to be sure to communicate your goals, preferred style of exercise and your sense of your own physical limits with your trainer.