Reliability and Repetition Effect of the Center of Pressure and Kinematics Parameters That Characterize Trunk Postural Control During Unstable Sitting Test.

BACKGROUND: Although unstable seat methodology has been used to assess trunk postural control, the reliability of the variables that characterize it remains unclear.

OBJECTIVE: To analyze reliability and learning effect of center of pressure (COP) and kinematic parameters that characterize trunk postural control performance in unstable seating. The relationships between kinematic and COP parameters also were explored.

DESIGN: Test-retest reliability design.

SETTINGS: Biomechanics laboratory setting.

PARTICIPANTS: Twenty-three healthy male subjects.

METHODS: Participants volunteered to perform 3 sessions at 1-week intervals, each consisting of five 70-second balancing trials. A force platform and a motion capture system were used to measure COP and pelvis, thorax, and spine displacements. Reliability was assessed through standard error of measurement (SEM) and intraclass correlation coefficients (ICC2,1 ) using 3 methods: (1) comparing the last trial score of each day; (2) comparing the best trial score of each day; and (3) calculating the average of the three last trial scores of each day.

MAIN OUTCOME MEASUREMENTS: Standard deviation and mean velocity were calculated to assess balance performance.

RESULTS: Although analyses of variance showed some differences in balance performance between days, these differences were not significant between days 2 and 3. Best result and average methods showed the greatest reliability. Mean velocity of the COP showed high reliability (0.71 < ICC < 0.86; 10.3 < SEM < 13.0), whereas standard deviation only showed a low to moderate reliability (0.37 < ICC < 0.61; 14.5 < SEM < 23.0). Regarding the kinematic variables, only pelvis displacement mean velocity achieved a high reliability using the average method (0.62 < ICC < 0.83; 18.8 < SEM < 23.1). Correlations between COP and kinematics were high only for mean velocity (0.45 <r < 0.89).

CONCLUSIONS: A familiarization session should be performed to overcome learning effect. COP parameters showed a better reliability than kinematics ones. Specifically, mean velocity of COP showed the highest test-retest reliability, especially for the average and best methods. Although correlations between COP and mean joint angular velocity were high, the few relationships between COP and kinematic standard deviation suggest different postural behavior can lead to a similar balance performance during an unstable sitting protocol.

LEVEL OF EVIDENCE: III.