Accuracy of force and center of pressure measures of the Wii Balance Board
Introduction
The Nintendo Wii Balance Board (WBB; Nintendo, Kyoto, Japan) is increasingly used in the rehabilitation and assessment of postural control. Its low cost (<$100 USD) and portability make it an attractive alternative to current methods of measuring balance in research settings that typically require expensive force plates and/or complex instrumentation that is limited to specialized labs. The device is a 23 × 43 cm platform that wirelessly transmits vertical ground reaction forces from under each corner as a user stands or moves on its surface. Commercial games using the WBB have been investigated as potential balance test substitutes [1], [2], possible interventions for balance rehabilitation [3], [4], [5], [6], [7], [8], and to reduce fear of falling [9], [10]. However, the measured forces and COP are not directly accessible to the researcher in consumer video games. To overcome this, a few studies have used custom software to interface a computer to the WBB to measure vertical forces and COP [11], [12] as well as provide real-time visual feedback for balance training [13], [14], [15], [16].
The wide availability and apparent benefits of the WBB are tempered by the lack of sufficient information about accuracy and reliability of the WBB force and COP, which make it difficult to evaluate its appropriateness as a device for measuring balance control based on recommended criteria. The WBB has been reported to sample each force channel at ∼100 Hz, which is above the 40 Hz minimum recommended for recording COP in postural sway [17]. The force sensors have been reported to be linear [18] with COP noise levels of approximately ±0.5 mm [19]. Further validation attempts have compared the COP path length during standing balance between the WBB and laboratory grade force plates [12], [18], [20]. However, information that is typically available for laboratory-grade force plates, such as the measurement uncertainty and reliability across a range of conditions, multiple devices, and the accuracy of the device's calibration values is currently unavailable.
Our goal was to provide the basic uncertainty metrics for static loads typically provided with calibrated force plates to allow researchers to evaluate whether the WBB is suitable for studying human balance [17] or other applications. We present a standard measurement uncertainty analysis that quantifies the repeatability and accuracy of a single sample of the WBB force and COP measurements. We also present the effects of wear from prolonged use in a student laboratory setting.
Section snippets
Wii Balance Board description, data interface, and devices tested
The WBB consists of a rigid platform with four uni-axial vertical force transducers located in the feet at the corners of the board, one transducer per foot (Fig. 1). Each transducer is a load cell consisting of a cantilevered metal bar with a strain gauge that converts applied force into a voltage that is digitized and transmitted wirelessly by electronics in the WBB [21].
To collect data from the WBB, data was streamed to a computer (Apple MacBook Pro, OSX 10.6) using the Bluetooth HID
Uncertainty of force measurements
In the detailed protocol on a single WBB, similar force estimates were found using the regression-generated calibration values compared to the factory-stored calibration measure, thus factory-stored calibration values were used in all further analyses. Using regression-generated calibration values, the RSS combined values of the four force sensors had a max absolute error of 9.9 N, discretization error of 2 mN, linearity error of 4.0 N, and repeatability error of 3.5 N across tested operating
Discussion
Here we have provided information about the accuracy and uncertainty of the WBB that may be used to evaluate the suitability of the WBB for various applications, and more specifically for human postural control measures. We found the total uncertainty of force measurements to be within ±9.1 N and COP location within ±4.1 mm across boards, which is much higher than the recommended uncertainty of 0.1 mm for posturography applications [17]. Repeatability of a single measurement within a board was
Conflict of interest statement
The authors have no conflict of interest and have no financial connection with Nintendo or AMTI.
Acknowledgements
We would like to thank Essy Behravesh and the GT BMED physiology lab for use of the heavily used boards and Stacie Chvatal, Seyed Safavynia, and Shawn Burns for contributing lightly used WBBs. This work was funded by NIH R01 HD04922 to LHT.
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