Standing on a bus that lurches forward. Catching a stumble on an icy sidewalk. For younger people, these moments of lost balance are handled almost automatically. Fast, lower-level brainstem circuits fire off corrective signals before anyone even realizes what happened. But as people age, the brain has to recruit backup. A new study finds that older adults and those with Parkinson’s disease rely more on the brain’s higher-level regions just to stay upright, and that this shift can be detected without ever scanning the brain itself.
Researchers at Georgia Institute of Technology, Emory University, Ohio University, and the University of Padova built a computer model that breaks down the muscle responses people produce when knocked off balance into separate parts, each linked to a different level of the nervous system. By analyzing how quickly and intensely leg muscles fire after a person is physically pushed off balance, the team could infer which pathways were likely contributing, whether fast, automatic circuits deep in the brain or slower circuits linked to higher-level brain regions.
Falls are among the leading causes of injury and death in older adults, and balance problems are a hallmark of Parkinson’s disease. Yet doctors have had limited tools to understand why a particular person’s balance is faltering at a brain level. This study, published in eNeuro, offers a potential new way to peer inside the nervous system’s balance-control chain of command using nothing more than muscle recordings and a moving platform.
How Researchers Tested Balance in Older Adults and Parkinson’s Patients
Nineteen older adults without Parkinson’s disease and 17 individuals with Parkinson’s were recruited for the study. Participants with Parkinson’s were asked to stop taking their medications for at least 12 hours before testing so the researchers could observe the disease’s effects without the masking influence of drugs. Each person’s neurologist approved this temporary medication pause.
Participants stood barefoot on a motorized platform that could slide forward or backward without warning. Each person received 48 pushes across six conditions that varied by direction and intensity. Medium and large pushes were adjusted based on each person’s height so the physical challenge was comparable across participants. Everyone was told to cross their arms, stare at a fixed point about 15 feet away, and try to recover their balance without taking a step.
While participants wobbled and corrected, sensors tracked their body motion and electrodes on their lower legs recorded electrical activity in two muscles: one along the shin and one in the calf. These two muscles work in opposition. When one contracts to pull the body forward, the other resists. Together, they offer a window into how the nervous system orchestrates a balance correction.
A Math Model That Separates Fast Brain Circuits From Slow Ones
At the centerpiece of the study was a mathematical model the researchers call a “neuromechanical model.” It reconstructs recorded muscle activity by combining information about how the body moved with time delays that correspond to known speeds of different brain pathways. Faster signals, arriving roughly 120 milliseconds after the push, are consistent with processing in lower, more automatic brain regions. Slower signals, arriving around 200 milliseconds, match the timing expected when information has to travel up to the brain’s outer layer, the cortex, and back down again.
Splitting each balance-correcting muscle response into two components, the model found the first kicked in around 119 milliseconds in older adults without Parkinson’s and 116 milliseconds in those with the disease. The second arrived around 200 milliseconds in older adults and 215 milliseconds in Parkinson’s participants. Both components grew stronger as the pushes got bigger, which makes intuitive sense: a harder shove demands a bigger correction. And the two-loop model was far more accurate than a simpler, single-loop version, with reconstruction accuracy improving by roughly 10 percent when the second, slower loop was included.
Why Muscles Sometimes Fight Against Their Own Balance
Beyond the main muscle that corrects balance, the researchers also examined what happens in the opposing muscle, the one that sometimes fires at exactly the wrong time. This kind of opposing muscle activity is commonly observed in older adults and people with Parkinson’s, creating a tug-of-war in the legs.
Carved into two pieces by the model, opposing muscle activity breaks down into a “destabilizing component” that actually works against balance recovery and a “stabilizing component” that helps the body adjust as the platform slows down. That destabilizing burst arrived at about 181 milliseconds in older adults and 173 milliseconds in those with Parkinson’s, a timing window consistent with circuits running through a cluster of brain structures heavily damaged in Parkinson’s disease.
Among older adults without Parkinson’s, higher levels of this destabilizing muscle activity were linked to worse scores on a clinical balance test that evaluates things like sensory awareness, dynamic walking, and the ability to recover from a push. Put simply, the people whose muscles fought hardest against their own balance corrections tended to be the ones who struggled most on standardized assessments.
That correlation did not hold in the Parkinson’s group. Researchers suggest this disconnect may reflect the broader neurological changes that accumulate as Parkinson’s progresses, making the relationship between any single muscle-activity pattern and overall balance ability harder to pin down.
One difference did emerge between groups: people with Parkinson’s showed a larger acceleration-driven component of the destabilizing opposing muscle activity during forward pushes compared to older adults without the disease. This fits with a well-known feature of Parkinson’s in which muscles contract inappropriately when they should be relaxing.
How Aging Shifts the Balance Burden to Higher Brain Regions
When comparing their findings to previously published data from younger adults, a notable pattern emerged. Older adults, with or without Parkinson’s, appeared to engage the slower, thinking-brain response at smaller push intensities than younger adults did. In younger people, the faster automatic circuits seemed to handle mild disturbances on their own, only calling in reinforcements from higher brain regions when the challenge ramped up. Older adults, by contrast, seemed to need that backup even for gentler pushes.
This observation lines up with a well-established idea in brain science: as people age, the brain compensates for declining automatic processes by calling on additional resources from its outer layers. It is the neural equivalent of needing reading glasses. The hardware still works, but it needs more help to do the same job.
Worth noting is that direct statistical comparisons between the older adult data and the previously published younger adult data were not possible because the experimental setups differed. Still, the pattern is consistent with evidence suggesting that this kind of brain over-recruitment is a signature of aging balance control.
Source : https://studyfinds.com/older-brains-work-harder-falling-muscle-test/
