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Get your pre-holiday HIIT

01.12.2025

Fitness

Short on time but want lasting results? Research reveals that High-Intensity Interval Training can deliver rapid strength, lean muscle, and fitness gains – and help you hold onto them even when you break.

If HIIT isn’t part of your routine yet, now is the perfect time to start. Research shows a well-structured HIIT workout is the ultimate one-stop solution for fast, all-around fitness results. It delivers impressive strength, lean muscle, and aerobic gains – quickly. And here’s the clincher: this fascinating study suggests the strength and lean muscle gains you get from HIIT could have more sticking power than the gains acquired by other types of training.

The strength and lean muscle gains you get from HIIT could have more sticking power.

Researchers set out to discover whether short-term, single-mode exercise could boost fitness before a period of reduced activity. Their focus? Finding the best way for middle-aged men to prepare for surgery and stay strong during recovery.

Participants completed three morning workouts per week for six weeks. One group did resistance training, another endurance, and the third HIIT. HIIT sessions lasted just 13 to 23 minutes, while the other workouts were significantly longer. After six weeks, all participants took a 2.5-week break from structured exercise.

Close-up of one person in a blue workout top with arms raised behind the head, sweating after exercise. Another person in dark workout gear is visible in the background.

Throughout the study, researchers measured everything from muscle fiber characteristics and body composition to strength, aerobic capacity, energy expenditure, and glucose regulation. The results were striking: all participants gained strength and muscle fiber size, with some improving lean mass and aerobic fitness. But only the HIIT group achieved strength and fitness gains across the board.

“This highlights how HIIT is a winning way to get strength, lean mass gains and aerobic fitness in one hit,” says Bryce Hastings, Les Mills Head of Research.

HIIT is a winning way to get strength, lean mass gains and aerobic fitness in one hit.

Even after the 2.5-week break, aerobic capacity dropped, but strength and lean mass held steady for those who had done resistance training or HIIT.

“It indicates that with HIIT – and resistance training – you can keep the strength and lean mass gains even after you’ve stopped,” says Hastings.

This is good news if you’ve got a holiday on the horizon. Just like surgery patients, you want to feel fit, strong, and healthy before your break—and now you know HIIT can get you there fast. And what’s more, you can enjoy a relaxing and restful break without worrying that your strength gains are depleting.

You can find more insights into how to maintain your fitness levels while taking a break from exercise here.

JUST IN TIME FOR THE HOLIDAYS:

NEW LES MILLS GRIT WORKOUTS LAUNCH ON LES MILLS+ IN MID DECEMBER

Four people running outdoors on a path surrounded by greenery. Three wear bright red sports bras and shorts, one wears black shorts and a dark top with red shoes.

MORE HIIT HYPE

It's not just your strength and fitness that can benefit from HIIT. Findings show HIIT workouts can supress ghrelin levels more than other types of exercise, which can lead to less hunger pangs – helping you to maintain a healthy weight. Norwegian researchers have found HIIT may help you live longer too. And if that isn’t reason enough to jump into a heart-rate spiking sweat fest, check out these five reasons to give HIIT a go.

THE HIIT SWEET SPOT

Research shows the optimal weekly dose of HIIT is just 30-40 minutes with your heart rate about 90 percent max. This means you can’t rely solely on HIIT to safely fuel your fitness – it should be teamed with longer-form endurance training for the optimal balance.

THE HIIT CATCH

Research shows the optimal weekly dose of HIIT is just 30-40 minutes with your heart rate about 90 percent max. This means you can’t rely solely on HIIT to safely fuel your fitness – it should be teamed with longer-form endurance training for the optimal balance.

HOW TO GET YOUR HIIT

Try LES MILLS GRIT™, a science-backed HIIT workout shown to deliver rapid fitness transformation. Another unique spin on HIIT is LES MILLS SPRINT™ a low-impact workout that will significantly improve cardiovascular, metabolic and musculoskeletal fitness.

FIND A CLASS WORK OUT AT HOME

ERIN EXPLAINS: NOTHING BEATS GRIT

Les Mills Creative Erin Maw says nothing beats LES MILLS GRIT, because it’s backed by science and each workout is carefully crafted to deliver the HIIT effect.

“Every quarter, we test LES MILLS GRIT workouts using heart rate monitors on 8+ participants. If the average heart rate is lower than 85% for 20 minutes of the 30-minute workout, we adjust specific sections of the workout accordingly. This creates a workout that is demanding and vigorous, and true HIIT. If your goal is to improve fitness, try 2x LES MILLS GRIT workouts a week for six weeks and you will absolutely notice a difference – it will get you really fit, really fast.

LES MILLS GRIT also builds mental toughness and discipline. I am a runner, and during my hard runs I think about how LES MILLS GRIT helps enable me to stay in the ‘pain cave’ for longer. It has trained me mentally to stay endure challenging physical fitness, so I can stay in my ‘dark place’ and push harder!

Yes, it’s challenging and uncomfortable, but it’s highly effective. The moves are simple and you can modify as you need to suit your level!”

One person exercising indoors wearing a black sports bra with neon green trim and navy leggings. A wooden bench with a tablet and resistance bands is in the background.

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