It’s often said that health clubs don’t sell fitness, they sell motivation. There’s no greater driver of motivation than a gym-goer feeling like they’re making serious progress towards their goals. Fitness is a results game, so members making headway are less likely to leave and more likely to bring their friends along.

How can you help more members get faster results? The secret isn’t in adding volume, but intensity. High-intensity interval training (HIIT) offers remarkable fitness benefits to participants via shorter workouts, so it’s easy to see why HIIT has become so popular with time-starved, experience-hungry members, especially Millennials and Gen Z.

A HIIT workout is designed with bursts of intensity followed by periods of recovery, which drives the body into overdrive, increasing aerobic fitness, unleashing fast-twitch muscle fibers and growing lean muscle tissue and reducing fat. Previous research has demonstrated HIIT workouts generate 9 percent more fat loss (1) and are 17 percent more effective than steady state training (2).

Perfect for people with a base level of fitness, HIIT workouts can boost results and transform the fitness of regular exercisers. Studies have shown that replacing an hour a week of a normal training routine with 1-2 HIIT sessions showed vast improvements in lean body mass, glucose tolerance and excess post-exercise oxygen consumption (EPOC). There was also a startling reduction in cardiovascular disease risk factors (3). For elite athletes, it can reduce training time while still improving performance (4).

HIIT workouts are commonly associated with weight-bearing exercises like burpees, lunges and tuck jumps, which can sometimes be problematic for exercisers with foot, knee or lower back pain. But there are a growing number of low-impact HIIT alternatives available, such as high-intensity interval cycling, rowing and aquatic workouts that allow exercisers to repeatedly push their heart rate to the max training zone without risk of injury (5).

But, as transformative as high-intensity interval training can be, ground-breaking new research (6) highlights the danger of too much HIIT. The short-term release of cortisol that is produced during a HIIT workout can help enhance strength, improve immunity and reduce inflammation, but long-term increases in cortisol can cause fatigue, joint pain and mood disturbance. The recovery period after a HIIT session is when all of the positive adaptations and developments take place, and insufficient recovery can reduce the effectiveness of the training. Despite feeling as though they’ve pushed their body to maximum performance, exercisers won’t be getting a good return on their hard work and may be doing more harm than good. Research shows that incorporating just 30-40 minutes of HIIT above 90% heart rate max in a weekly exercise regimen will bring optimal results. Looking to offer members truly transformative results in record time? High-intensity interval workouts will take their fitness to the next level.