Reliability of measurements of body center-of-mass momentum during sit-to-stand in healthy adults

Program in Physical Therapy, Northwestern University Medical School, Chicago, IL 60611.

BACKGROUND AND PURPOSE. The purpose of this study was to determine the reliability of measurements of momentum of the body's center of mass (CM) during a sit-to-stand (STS) transfer in healthy adults. SUBJECTS. Nineteen healthy adults aged 25 to 38 years (mean = 31.7, SD = 4.2) participated. METHODS. Horizontal and vertical components of CM momentum were computed for STS transfers made at three movement speeds (fast, natural, and slow) with the aid of a motion analysis system. Two force platforms detected the time when the subject lost contact with the chair and the propulsive and braking impulses in the horizontal and vertical directions. Separate intraclass correlation coefficients (ICCs) were calculated for three temporal variables (time to peak horizontal and vertical momentum and time to when the subject lost contact with the chair) and two magnitude variables (peak horizontal and vertical momentum). RESULTS. The ICCs for magnitude variables were > or = .81 for all speeds of movement. The ICCs for temporal variables ranged from .28 for fast movements to .75 for slow movements. CONCLUSION AND DISCUSSION. Measurement of peak vertical and horizontal momentum magnitudes is highly reproducible during STS transfers. Measurement of temporal variables exhibits a range of reliability estimates. Implications include consideration of the speed at which STS transfer is performed and its effect on reliability estimates and the potential differences between reliability estimates for magnitude measurements versus temporal measurements.

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