Alzheimer's disease is the most common form of dementia and the sixth-leading cause of death in the USA. Currently, 5.4 million Americans live with Alzheimer's disease, and this number is set to quadruple to as many as 16 million by the year 2050.

With September World Alzheimer's Month and September 21 World Alzheimer's Day, we've rounded up some of the best exercises to slow cognitive decline and reduce the risk of Alzheimer's.

Aerobic exercise

Whether you choose walking, dancing, or even gardening, two studies published earlier this year found that virtually any type of aerobic exercise can be beneficial for improving brain health, increasing brain volumes and helping reduce the risk of developing Alzheimer's by 50 percent.

Research carried out by a team from UCLA, California, USA, also found that physical activity increased brain volume, especially in those aged 75 and older, leading the team to suggest that it is never too late get involved in physical activity.

And a new European study published just earlier this month also suggested that moderately vigorous physical activity during middle age -- described as activity more strenuous than walking -- is associated with a lower risk of cognitive impairment later in life.

Yoga

It seems that cardio isn't the only brain-boosting exercise, with a small-scale study from UCLA and Australia's University of Adelaide finding participants who attended classes of Kundalini yoga and practiced 20 minutes of meditation every day showed bigger improvements in visual-spatial memory skills, which help with recalling locations and navigating, than those who instead attended memory enhancement training and practiced memory exercises daily.

Those in the yoga and meditation group also showed larger improvements in levels of depression, anxiety, coping skills and resilience to stress, which are important when coming to terms with the onset of cognitive impairment.

Weightlifting

Although few studies have so far looked at the brain benefits of lifting weights, a Canadian study found that lifting weights twice a week helps to maintain brain health.

The team looked at the effect of weight training on the brain's white matter, which is particularly susceptible to lesions (holes) as we age, causing problems with memory and thinking skills.

The results showed that those who did balancing and stretching exercises or weight trained just once a week showed a significant increase in the number of white matter lesions, but those who weight trained twice a week showed less shrinkage of their white matter, suggesting that if the minimum threshold of working out twice a week is achieved, lifting weights can have a positive impact on the structure of the brain.