An active lifestyle may be a healthy lifestyle, but some more extreme forms of workout are best left to those who are already fit, according to new research in the journal of The Federation of American Societies for Experimental Biology (FASEB) Journal.

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Work out to keep fit, but build up gradually.

Whether it is the desire for a "super-lean body" or the lure of burning calories quickly - some researchers have claimed that a person "can burn an extra 200 calories per day by putting in only 2.5 minutes of work" - high-intensity "sprint training" has been gaining popularity at gyms.

Intense exercise training can stimulate the growth of mitochondria and increase the body's capacity to use oxygen, enhancing cardiovascular fitness and strength and keeping cardiovascular disease and obesity at bay.

However, working out without short-term preparation, such as warming up, or long-term preparation, such as building up intensity over time, can cause damage to the body.

Intense workouts damage muscles, reduce mitochondrial functionality

Canadian and European researchers, led by Robert Boushel, director of the University of British Columbia's School of Kinesiology in Canada, analyzed tissue samples from 12 male volunteers in Sweden.

All the participants were healthy but described themselves as either untrained or only moderately active.

The men took part in high-intensity training over a 2-week period. The exercise regimen involved repeated 30-second all-out sprints, followed by rest periods.

The researchers observed signs of stress in the muscle tissues of the participants after carrying out ultra-intense leg and arm cycling exercises.

Tests showed that their mitochondria, the "powerhouse of cells," were only functioning at half their capacity after training, reducing their ability to consume oxygen and to defend against damage from free radicals.

Free radicals are molecules that can modify DNA and cause harm to healthy cells. High levels of free radicals appear to be a risk factor for a range of medical conditions, including premature aging, organ damage and cancer.

Boushel says the findings raise questions about what constitutes appropriate dosage and intensity of exercise for the average individual. He urges caution when encouraging the general population to participate in sprint training.

Boushel explains that experienced and well-trained athletes accumulate antioxidant enzymes in their bodies, and these offer protection against free radicals.

For beginners, however, he recommends starting slowly and building up intensity over time. Exercise should also take place under the eye of a trained professional or kinesiologist.

He warns:

"If you're new to going to the gym, participating in high-intensity sprint classes may increase your performance but might not be healthy for you."

The potential long-term adverse effects of high-intensity sprint training are unknown, but ongoing studies are looking at different levels of exercise and evaluating quantities and intensity of training against different biomarkers for health.

Medical News Today reported last year that aerobic exercise does not offset the health effects of obesity.