How much High Intensity Interval Training (HIIT) is too much?

It’s no secret that the exercisers of today are more time-poor than ever before. Long gone are the days of slogging it out on a treadmill for hours and spending an entire afternoon in the weight room to hone the perfect body. Today, our members want to achieve the maximum gains – in the fastest time possible.

Enter HIIT. We’ve all witnessed this training mode become a mainstay of Millennial workouts in recent years, thanks to the rapid changes it improves both fitness levels and body composition. Short bursts of intensity have generated sensational results for our members, and are perceived as being far more effective than longer, steady-state CV sessions. The hype around HIIT has many members opting to replace all of their sessions with HIIT training, with an underlying belief that the more frequent and intense the session, the faster they will see the results.

However, despite feeling as though they’re pushing their body to maximum performance, it turns out that intensity enthusiasts may be running themselves into the ground.

Too much of a good thing

Landmark new research reveals there is a specific tipping point when too much HIIT leaves people unable to reach the 90%+ heart rate zone – the hallmark of a typical HIIT workout and a key ingredient for producing transformative fitness effects – and more prone to injury.

The study findings – presented by Penn State University Associate Professor Jinger Gottschall at the 2018 American College of Sports Medicine conference this month – show that the sweet spot of HIIT lies at 30 - 40 minutes per week with the heart rate above 90 percent maximum is the recommended total.

For the most part, the stress HIIT puts the body under is beneficial – as it’s this stress that drives change. But the body is only able to handle a certain amount of stress at once. As it transpires, too much HIIT can actually bring more harm than good.

“Fitness fans love HIIT and are motivated by the results,” says Professor Gottschall. “These results come from going as hard as you can for a short period of time, resting, and then repeating. It’s a formula that allows you to keep reaching your maximum training zone, again and again, shocking your body each and every workout.

“But our research findings tell us there’s only so much HIIT a regular exerciser can do in one week before these fitness results are compromised.

“In other words, we’ve been able to establish that any more than 40 minutes of HIIT per week doesn’t help achieve transformative effects, it actually hinders.

Diminished Returns

Not only does this reduce your training’s effectiveness, says Gottschall, but it can also actually have a negative impact. The risk of injury intensifies –. Depending on fitness level and other training session intensities, if exercisers spend more than 40 minutes a week with their heart rate above 90 per cent max (as it is during the majority of a HIIT workout), injury incidence and over-training symptoms increase. “For example,” adds Gottschall, “when you are fatigued, the core cannot properly support the trunk, increasing the likelihood of injuries to the shoulder or lower back.”

So, rather than an ‘all or nothing’ approach, combining HIIT with moderate-intensity cardio can bring the best of both worlds. “Moderate-intensity cardio can be performed for longer periods of time and thereby train endurance systems,” Gottschall adds. “This intensity has been tested in countless studies and provides multiple benefits in terms of cardiovascular health, cholesterol levels, triglycerides, and fasting glucose.”

What does this mean for clubs?

“There aren’t currently any recommended guidelines concerning the maximum amount of HIIT people should do in a week to prevent an injury,” says Bryce Hastings, Les Mills Head of Research.

“Every day, thousands of people all over the world are doing HIIT classes, such as LES MILLS GRIT^™, so we have a responsibility to inform them how to workout safely and effectively at this intensity.

“This “magic prescription” is one that clubs and instructors can share with members to ensure they get the most out of their training. When we educate our exercisers on how to maximise results whilst simultaneously reducing risk of injury, we are more likely to increase exercise adherence and thus boost retention.”

Recommended weekly workout schedule for your members

Recreational exercisers who exercise for enjoyment and good health

• 2x cardio workouts (30-45 mins)
• 1x strength workout
• 1x flexibility workout
• 30 minutes of HIIT

Intermediate exercisers keen to improve fitness

• 3x cardio workouts (30-45 mins)
• 2x strength workouts
• 1x flexibility workout
• 60 minutes of HIIT (which should drive you to train at 90 per cent of your maximum heart for 30-40 minutes)

Advanced exercisers keen to push their fitness to elite level

• 4x cardio workouts (30-60 mins)
• 2x strength workouts
• 1x core workout
• 60 minutes of HIIT (which should drive you to train at 90 percent of your maximum heart for 30-40 minutes)

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