Back in 2005, when Dr. Martin Gibala and his colleagues at McMaster University first suggested that as little as six minutes a week of ultra-intense exercise could produce many of the same changes as prolonged, moderate cardio workouts, observers were skeptical. How could the fitness benefits of an hour of slogging be compressed into a few painful bursts?
These days, high-intensity interval training – HIT, as it's now referred to in gyms around the world – has gone mainstream, and its benefits have been confirmed in study after study.
But while much recent attention has focused on headline-grabbing, short-sprint intervals of as little as 10 seconds at a time, Gibala and other researchers have been experimenting with a wide variety of different HIT protocols using different lengths and combinations of intervals, looking for the most powerful – and practical – combinations.
The good news: There isn't just one "right" answer.
"You have this complex interaction between the amount of time you go for and the relative intensity," Gibala says. "There may be different mechanisms, but there are clearly a few different ways to achieve the same goals."
Here are some key factors to consider when including HIT in your fitness routine.
This is the universal key to effective HIT training. Whether you compress your effort into 30-second bursts or sustain it for three minutes at a time, you have to push into a zone of discomfort.
"It's a quick way to get fit," says Dr. Brendon Gurd, a muscle physiologist at Queen's University, "but it's not an easy quick way to get fit."
In July, Gurd and his colleagues published a study in the journal PLoS One in which they compared two groups doing a cycling workout alternating 60 seconds hard with 60 seconds easy for eight to 10 repetitions. One group did the hard intervals at 100 per cent of peak power, while the other group used a more moderate intensity of 70 per cent of peak power. Both groups made gains in muscular and metabolic health, but in the most important health marker of aerobic fitness, the high-intensity group gained 27.7 per cent in three weeks while the moderate-intensity group gained just 11.0 per cent.
In another PLoS One paper, published last month by a team at the Mayo Clinic in Minnesota, researchers combined the results of 37 different studies on interval training and aerobic fitness. In addition to the expected finding that intervals are highly effective, they noted that the nine studies with the best results tended to use intervals lasting three to five minutes.
Similarly, a Norwegian study published in April (of which Gibala was a co-author) found that four intervals of four minutes each with three minutes of rest, three times a week, produced big gains in fitness.
These workouts are quite different from Gibala's original protocol of four to six all-out 30-second sprints with four minutes rest.
So which is best?
The most important differences here aren't physiological, Gibala says. Instead, it comes down to which workouts are most feasible. The all-out 30-second sprints demand an extremely high intensity that's hard to replicate outside the lab; on the other hand, inexperienced exercisers have difficulty sustaining the focus required for intervals that last more than a few minutes.
The sweet spot for many people, Gibala and his colleagues have found, is the protocol used in Gurd's study, alternating one minute hard with one minute easy.
Even the best workouts suffer from an inevitable law of diminishing returns. "If you only do one workout over and over, you'll eventually get a plateau effect," Burd says.
That's not necessarily a problem if you've found a workout that you're comfortable with and that keeps you fit. But if you're looking to keep progressing, including a mix of different workouts will force your body to keep adapting to different stimuli.
In fact, research at Western University has shown that even though short intervals and long runs produce similar increases in fitness, they do so through the different mechanisms. The intervals stimulate greater gains in the efficiency of the muscles, while the long runs produce more adaptation in the heart itself.
That suggests that, if your schedule and motivation permit, including both types of workout in your weekly routine is ideal.
"The varied approach to training is always going to be the best way, continually hitting the system in many different ways," Gibala says. "And indeed, that's what highly trained athletes do."
In the end, the most effective workout is the one you're willing to do – a truth that presents a challenge for the unavoidable intensity of HIT training.
But Gurd noticed an interesting detail in how his volunteers responded to the medium and hard workouts in his study. In the heat of the moment, the volunteers rated the hard workout as highly unpleasant compared to the easier one. But those feelings dissipated within minutes, and the two groups ended up reporting equal overall enjoyment.
"I think what happens is that you very quickly forget how bad it felt, and you start to feel, 'Man, that was hard, but I made it through it,'" Gurd says. "So those feelings of negativity are replaced by, 'That was awesome!'"
Whatever HIT workout you choose, that's the pattern you need to remember. If you're doing it right, it will feel bad and then it will feel good – and it's the good feeling that lasts.
Standard high-intensity interval programs generally involve repeating the same workout three times per week.
But you can mix and match different workouts to vary the stimulus and keep things interesting.
Monday: 10 x (60 seconds hard, 60 seconds easy)
Wednesday: 30 minutes at a comfortable pace
Thursday: 6 x (30 seconds hard, three minutes easy)
Saturday: 4 x (four minutes hard, two minutes easy)
The workouts can be adapted for running, biking or other cardio activities. Precede each HIT workout with a five-minute warm-up.
Alex Hutchinson blogs about exercise research at sweatscience.runnersworld.com.