Study Shows Difficulty of Adapting Movements to Obstacles

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by Forest Ray PhD |

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People with Parkinson’s disease are less able to adapt their movements before crossing obstacles such as mounting a curb, according to a recent study.

This type of adaptation, called locomotor synergy, may explain why crossing obstacles is more difficult for those with Parkinson’s and could serve as a new biomarker for early detection of Parkinson’s-related motor dysfunction.

The study, “Step length synergy while crossing obstacles is weaker in patients with Parkinson’s disease,” was published in the journal Gait & Posture.

Locomotor synergy involves combining different factors, such as speed and foot position, into a single movement. Although gait instability means that Parkinson’s patients fall more often than other people, the synergy involved in this phenomenon had not been studied yet.

Researchers associated with the Human Movement Laboratory at São Paulo State University, in Brazil, measured the step-length synergy of 13 Parkinson’s patients and 11 healthy controls, while walking up to and stepping over a 15 centimeter (cm)-high rubber foam block. (Fifteen cm is almost 6 inches, which is the standard curb height throughout Brazil.)

Overall, participants with Parkinson’s tended to show lower step-length synergy than the healthy controls and more caution approaching the block.

The foam block was placed 4 meters (4.3 yards) down an 8.5 meter-long, 3.5 meter-wide path. Participants walked this path at their own pace with or without glasses or hearing aids, but without assistance.

Although the researchers did not instruct participants to step over the obstacle with a particular leg, they positioned the block in such a way that each volunteer would cross with their right leg, in order to minimize person-to-person variation.

Trials in which a participant contacted the rubber block or stepped over it with their left leg were repeated until each participant had successfully crossed the obstacle 20 times with their right leg.

The investigators measured step length synergy with eight motion-capture cameras, mapping movements based on the positions of reflective markers placed on each participant.

“While the subject is walking along the gangway toward the obstacle and crossing it, the cameras emit infrared light, which is reflected by the markers. The cameras capture the position of the markers, enabling us to determine step length and duration. Gait analysis software does the other calculations,” explained Fabio Barbieri, PhD, the study’s senior author, in a press release.

Overall, the Parkinson’s group showed 53% lower step-length synergy than the healthy controls. Their average step lengths were shorter, speed slower, and overall stability lower. They also tended to place their front heel closer to the block.

Participants with Parkinson’s also appeared to approach the block more cautiously than people in the control group, slowing down and showing less variance in their steps both before and after crossing the obstacle.

The group established a synergy index based on their results, to be used as a metric in performing future synergy studies.

“We are cautiously optimistic that, after studying large cohorts, and consistent with similar results in prehensile and postural studies, our synergy index will be useful in early detection of locomotor problems in PD patients,” they wrote.

The authors said this study was the first time their methodology had been used for gait analysis.

“In conclusion,” the researchers wrote, “we demonstrated weaker step-length synergies in PD patients. The measurement of this synergy requires tracking one landmark per foot, and a stationary obstacle, making it clinically feasible. The step-length synergy index will augment assessment, and possibly early detection of locomotor disability in PD patients.”

The group now plans to study how obstacle height affects step-length synergy.

“We want to know if this synergy changes because the obstacle is higher or lower,” Barbieri said. “This concerns the environment in which the patient moves. If there are obstacles of a certain height in the area, they may cause problems and lead to falls, so we can modify the environment to facilitate locomotion.”

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