The old workout mantra of “no pain, no gain” may be truer than first thought, based on new research from scientists in the US.

A study published recently in the EMBO Journal found that the intensity of a workout, rather than the overall energy expended, is key to achieving the best results.

Scientists at the Scripps Research Institute in Florida reported – through experiments on mice, it should be added – that intense exercise changes the body and muscles at a molecular level in ways that milder physical activity cannot replicate.

The scientists were examining the sympathetic nervous system – the portion that triggers the fight or flight response in animals, including people, when faced with peril or stress. Such situations prompt the release of catecholamines – biochemicals like adrenaline, which set the heart racing and increase alertness. Catecholamines interact with another by-product of stress – protein CRTC2 – with the scientists finding that the more interaction that occurs (hastened by intense physical activity), the greater the adaptations in terms of muscle size and endurance.

The findings add credence to the current popularity of high intensity workouts like CrossFit, as well as bringing into focus the importance of accurately gauging the intensity of a workout.

One of the scientists behind the study, Michael Conkright, concluded that there is “some truth to that idea of ‘no pain, no gain,” adding that the periodic evaluation of workout intensity is vital to continually improving fitness, as the body quickly adapts to new exertion levels.