Relationship between body segment movements and center of pressure shifts during trunk lean movements while sitting in healthy adults.
Boqun Liu, Shintaro Uehara, Akiko Yuasa and 3 others
PMID 42358806WHAT IT FOUND
Pelvic tilt was the only segment whose movement was significantly associated with center-of-pressure shift during side-to-side sitting lean in 28 healthy young adults.
In forward-backward lean, head, trunk, and hip also showed significant associations. The head moved opposite to the trunk in nearly everyone.
Key findings
01During left-right trunk lean, only pelvic tilt angle change was significantly associated with COP displacement (β = 0.18 cm per degree, p < 0.001). Head (p = 0.064) and trunk (p = 0.121) did not reach significance.
02During forward-backward lean, all four segments were significantly associated with COP displacement: pelvic tilt (β = 0.25 cm/°), hip (β = 0.25 cm/°), trunk (β = 0.19 cm/°), all p < 0.001; head (β = −0.07 cm/°, p = 0.001) was negative, reflecting its countermovement.
03The head moved in the opposite direction to the trunk lean in almost all participants, regardless of leaning direction, consistent with a balance-maintaining countermovement strategy.
STILL TO COME
How it was doneWhat they foundWhat it means for PTs
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What it does not show
All participants were healthy adults aged 21–31. The paper itself notes that older adults and post-stroke individuals show smaller reaching distances and lower COP displacements, so these normative values cannot be assumed to apply to those populations. Each segment was modeled separately rather than all at once, because of the small sample size and the risk of multicollinearity. The independent contribution of any single segment, once the others are accounted for, is not known. Pose estimation was two-dimensional. Out-of-plane movement or axial rotation during the lean could introduce projection error in the segment angles. COP was sampled at 5 Hz, which may miss small rapid oscillations during the two-second hold at maximum lean. The study is correlational. It shows which segments move together with COP displacement; it does not show that changing one segment will change the others.
Declared interests
Funded by the Japanese Ministry of Health, Labour and Welfare and JST SPRING (Grant JPMJSP2191), a government programme for assistive products. No industry sponsor is named.
The easy way to misread this
Do not read the pelvic-tilt association as proof that training pelvic control will improve sitting balance. This is a correlational study in 28 healthy young adults, and each segment was modeled separately, so the independent contribution of any one segment cannot be determined. The findings have not been tested in older adults or in neurological populations, and the authors themselves flag the results as correlational and exploratory.