Have you just joined a gym and already started a high-intensity "sprint training" workout? A new study has found signs of stress in the muscle tissues of non-athletes and untrained people after ultra-intense leg and arm cycling exercises which might not be healthy.

Researchers found untrained people had a weakened ability to fight off free radicals -- molecules that can alter DNA and harm healthy cells -- that may increase the risk of cancer, premature aging and organ damage.

Beginners start slowly and gradually increase intensity over time, under the supervision of a trained professional or kinesiologist, the authors suggested.

"Our study raises questions about what the right dose and intensity of exercise for the average person really is," said senior study author Robert Boushel from University of British Columbia's school of Kinesiology in Canada.

"We need to be cautious about supporting sprint training in the general population," Boushel added in the paper published in the Federation of American Societies for Experimental Biology Journal.

The researchers analysed tissue samples from their test the participants and found that their mitochondria -- the powerhouse of cells -- were only firing at half-power post-training, reducing their capacity to consume oxygen and their ability to fight off damage from free radicals.

"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," said Boushel.

Seasoned athletes and those who are well trained have built up antioxidant enzymes in their bodies to protect against free radicals, said Boushel.

The study was carried out in a dozen male volunteers in Sweden, all of whom were in good health but self-identified as untrained or only moderately active.

The men participated in high-intensity training over the course of two weeks that involved repeated 30-second all-out sprints, followed by rest periods.

The study, high-intensity sprint training inhibits mitochondrial respiration through aconitase inactivation.