Center of Mass Controls Balance

An elegant new model of balance control suggests the brain only cares about one thing: the body’s center of mass.

Bumped from behind, a person may step forward to avoid falling. Perhaps her arms fly out as well. To the untrained eye, these movements seem like the result of the brain controlling individual nerve and muscle reflexes. Yet an elegant new model of balance control suggests the brain only cares about one thing: the body’s center of mass. This possibility, modeled for the first time, could help rehabilitation experts design better treatments to suit the specific needs of each balance-impaired patient.

 

Biomedical engineer Lena Ting (right) prepares to measure how a volunteer's muscles react when her balance is disturbed by a moving platform.  Courtesy of Georgia Institute of Technology.“People had theories about the center of mass being important, but they hadn’t actually demonstrated in a causal sense that it was critical,” says Lena Ting, PhD, assistant professor of biomedical engineering at Georgia Institute of Technology and Emory University and co-author of the work published in the October 2007 issue of Nature Neuroscience. “We’ve shown that the nervous system controls the arms and legs to regulate center of mass motion.”

 

Neuroscientists have tested a variety of hypotheses such as whether balance originates from motions of the head or the ankle. But these hypotheses have not consistently predicted which muscles would spring into action when a person loses balance. Ting’s previous experiments found that the only way to foretell muscle reaction accurately was to monitor the direction of the fall, not individual joint angles. This suggested that the body’s reflexes during a fall involve a higher level of control: If the center of mass is off-kilter, the nervous system will act to bring it back to balance.

 

To explore this idea in action, the researchers placed cats on a moving platform that made them lose their balance. The team also induced sensory damage in the cats that triggered balance-control problems. A computer simulation created by the researchers accurately predicted the reactions of the cats’ muscles based on the motions of their centers of mass.

 

In addition, the cats with sensory damage regained balance within a few days. They used different sensory path- ways to do the same balancing tasks, resulting in unique patterns of muscle activity. While these muscular adjustments were clinically “abnormal,” they were close to optimum for the balancing task at hand. This result should earn notice from balance rehabilitation professionals, Ting says; they now have an accurate, unique goal toward which they can aim each patient’s rehabilitation efforts.

 

Ting's group has provided an attractive, simple model of posture control, says Fay Horak, PhD, a senior scientist at the Neurological Sciences Institute of the Oregon Health & Science University. “The big implication is that something this complicated, that involves many, many joints and muscles, could be controlled by the nervous system regulating a single parameter,” she says.

 

Horak believes researchers need more data before applying Ting's results to humans with balance disorders. Ting concurs, noting that her team has started using the moving platform to test human balance reactions.



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