The study led by the University of Sydney showed that increased muscle strength leads to improved brain function in adults with MCI.
MCI defines people who have noticeably reduced cognitive abilities such as reduced memory but are still able to live independently, and is a precursor to Alzheimer's disease.
Participants were asked to carry out a mix of weight lifting and brain training.
Findings from the Study of Mental and Resistance Training (SMART) trial show, for the first time, a positive causal link between muscle adaptations to progressive resistance training and the functioning of the brain among those over 55 with MCI.
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By 2050, around 135 million people are expected to have dementia, said researchers.
An earlier study by the same team behind this study showed the participant's global cognition improved significantly after the weight training, whereas cognitive training did not do this.
"What we found in this follow-up study is that the improvement in cognition function was related to their muscle strength gains," said Yorgi Mavros, researcher at Sydney University in Australia.
"The stronger people became, the greater the benefit for their brain," said Mavros.
They were divided into four groups doing either: resistance exercise and computerised cognitive training; resistance exercise and a placebo computerised training (watching nature videos); brain training and a placebo exercise programme (seated stretching/calisthenics); or placebo physical exercise and placebo cognitive training.
Participants doing resistance exercise prescribed weight lifting sessions twice a week for six months, working to at least 80 per cent of their peak strength.
As they got stronger, the amount of weight they lifted on each machine was increased to maintain the intensity at 80 per cent of their peak strength.
"The key however is to make sure you are doing it frequently, at least twice a week, and at a high intensity so that you are maximising your strength gains. This will give you the maximum benefit for your brain," said Mavros.
The study was published in the Journal of the American Geriatrics Society.