HIIT: Benefits, Risks, And Who It Helps Most, Explained

Medical comment provided by Dr Giuseppe Aragona, GP and medical adviser for Prescription Doctor.

When I first began working out about 10 years ago, high-intensity interval training, or HIIT, was everywhere.

Now, it seems to have fallen out of fashion.

Is that deserved? We looked at the research into HIIT to see what science says, and asked GP and medical adviser Dr Giuseppe Aragona for his thoughts, too.

HIIT, which involves short bursts of incredibly high-intensity exercise interspersed with rest periods, makes you reach about 80-100% of your maximum heart rate.

We’re talking burpees. We’re talking squat jumps. Rowing counts, too (HIIT is considered a form of cardio).

“High-intensity interval training, or HIIT, can be a very effective and time-efficient form of exercise, but like most things in medicine, the benefits and risks depend heavily on the individual, their baseline fitness, and how the training is implemented, rather than the concept itself being inherently good or bad,” Dr Aragona told us.

Some argue that HIIT can be hard on your body, especially if your form isn’t perfect (and given that some very fast-paced HIIT workouts demand multiple complex moves, like jumping lunges, in short intervals, it might be hard to perform all of them to the ideal standard).

But a 2026 study from University Hospital Rigshospitalet and the University of Copenhagen found that HIIT workouts were well-tolerated by (meaning they didn’t cause harm to) people with mild to moderate knee osteoarthritis over a 12-week period. Training sessions happened three times a week.

“From a musculoskeletal point of view, poor technique, insufficient warm-up, or pre-existing joint issues can increase the risk of strain or injury,” Dr Aragona said.

If structured well, though, HIIT can sometimes be tolerated by those with e.g. sore knees, “as the intermittent nature of the exercise reduces sustained load through the joint compared with continuous activity”.

Some research says excessive HIIT may harm our mitochondria, which help to generate energy from cells, and possibly lower our glucose tolerance.

But per the GP, “the idea that HIIT ‘damages mitochondria’ is somewhat overstated in the mainstream discussion, as in reality moderate amounts of high-intensity exercise tend to stimulate mitochondrial biogenesis and improve metabolic health”.

“Excessive volumes without recovery may contribute to oxidative stress and fatigue, which is where the perception likely comes from, so the issue is less about HIIT itself and more about overtraining,” he said.

Some research suggests that doing HIIT too close to bed can ruin your sleep, and that it might be a lot worse than lower-intensity activity for helping older people with existing poor sleep.

But others say it can improve sleep efficiency and perceived sleep quality.

Dr Argaona agreed that negative changes to sleep are only expected if you do HIIT “late in the evening or on consecutive days”: otherwise, you’re likely in the clear.

A common critique of HIIT is that it doesn’t build muscle, as it’s a predominantly cardiovascular exercise. But one paper found 12 weeks of HIIT increased lean leg muscle in participants.

“It is generally less effective than structured strength training for building significant muscle mass or bone density, where progressive overload and mechanical loading are more important,” Dr Aragona said ― in other words, it’s true that HIIT won’t build as much lean muscle as strength training.

However, “weight-bearing HIIT can still play a supportive role in maintaining bone health, especially when combined with resistance exercise,” he added.

“HIIT is very effective at improving aerobic capacity and VO2 max because it repeatedly pushes the cardiovascular system close to its upper limits,” Dr Aragona told us. That can help us to use oxygen more efficiently when we work out.

“It is also worth noting that high-intensity training has broader systemic benefits, including improvements in insulin sensitivity and vascular function,” said the GP.

That can even have benefirs for those with erectile dysfunction (ED) “by enhancing cardiovascular health, boosting testosterone, and increasing blood flow… regular, intense, short-duration workouts performed perhaps twice per week can strengthen the pelvic floor and improve vascular function”.

It depends on your fitness levels, said the doctor.

He explained: “I generally suggest that for most people two to three HIIT sessions per week is sufficient to gain cardiovascular benefits, particularly improvements in VO2 max, which is one of the strongest predictors of long-term health and mortality… beyond that point the incremental benefit diminishes while the risk of overuse injury and burnout increases.”

If you already do cardio and strength training, “it should be viewed as a supplement rather than a replacement, perhaps used once or twice a week to improve cardiovascular fitness and add variety”.

Many of the harms linked to HIIT come about when it’s overdone. Dr Aragona said that some of the signs you’ve been going too hard include:

• persistent tiredness
• declining performance
• irritability
• sleep disturbance.

There are “clear groups in whom caution is warranted, including individuals with uncontrolled hypertension, significant cardiovascular disease, advanced joint problems, or those who are completely unaccustomed to exercise,” said the doctor.

Consider speaking to your doctor and starting slowly, if deemed appropriate, if you’re considering HIIT.

If you’re new to working out, “suddenly introducing very intense activity can increase the risk of cardiac events or musculoskeletal injury, and in these patients a gradual build-up with lower intensity work is far safer and more sustainable”.