A study led by engineers at the University of Boulder (CU Boulder) has pinpointed why people lose speed and agility as they get older.  The research, published in the Journal of Neuroscience, found that people over the age of 65 slow down partially because it costs them more energy to move than someone younger.

According to Study Finds, the researchers divided participants into two groups: ages 18 to 35 and in ages 66 and 87. They asked both groups to reach for a target on a screen with a robotic arm, like playing a video game. The team analyzed the patterns of these reaches and found that older adults modified their motions to conserve energy.

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“All of us, whether young or old, are inherently driven to get the most reward out of our environment while minimizing the amount of effort to do so,” explained co-lead author Erik Summerside, an integrative physiologist at CU Boulder.

The researchers proposed two theories for why we slow down as we age. The first suggests that our muscles work less efficiently as we get older. The second offers the hypothesis that aging alters the reward circuitry in the brain producing less dopamine, the brain chemical that signals satisfaction upon completing a task.

According to a CU Boulder news release the study participants were then asked to sit down and grab the handle of a robotic arm that operated the cursor on a computer screen. They were told to reach forward and move the cursor toward a target. If they were successful, they received a small reward, such as the targets exploding and making a noise.

Both groups arrived at their targets about 5% sooner when they knew they would hear the sounds. The younger group simply moved their arm faster, while the older group improved their reaction times by beginning their reaches 17 milliseconds sooner on average.

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When the researchers added an 8-pound weight to the robotic arm for the younger participants, the differences vanished. They concluded that the brain could detect very small changes in energy expenditure and can adjust our movements accordingly.

“Even when moving with just a few extra pounds, reacting quicker became the energetically cheaper option to get the reward, so the younger adults imitated the older adults and did just that,” explained Robert Coulter, co-lead of the study and a graduate student of integrative physiology at CU Boulder.

The researchers say their findings suggest that both young and old have no difficulty perceiving rewards, but the brain slows down our movements when the effort becomes tiresome. They noted that if future research can tease out where and how these changes occur, scientists may be able to develop treatments to reduce the toll of aging and diseases such as Parkinson’s, multiple sclerosis, depression, and schizophrenia.