## 1 Introduction  
The ground reaction force (GRF) and center of pressure (COP)--which is the application point of the GRF during gait--are essential parameters for kinetic gait analyses such as joint torque estimation. Kinetic gait analysis evaluates movement function, which is important for the assessment and rehabilitation of patients with gait disorders such as stroke [1], Parkinson's disease [2] and osteoarthritis [3].  
The GRF and COP are commonly measured using laboratory-grade force plates, which are the gold standard for highly accurate analysis. However, these devices are not always available in general clinical settings (such as a rehabilitation center) because they are expensive, difficult to transport, and require special training for their operation. For these reasons, the Nintendo Wii Balance Board (WBB) (Nintendo, Kyoto, Japan) has been recently suggested as an inexpensive, portable and accessible alternative to the cost-prohibitive force plates [4; 5; 6; 7; 8; 9; 10; 11; 12]. The WBB can measure uniaxial force and the COP using four force sensors (called strain gauge load cells) located near each of the four corners of the usable surface of the device.  
To date, the WBB has mainly been tested as a tool for the assessment of postural instabilities [12; 13; 14; 15; 16; 17]. Many previous studies have investigated the validity and reliability of using the WBB to measure the COP or its path velocity during static single- and double-leg standing [4; 5; 6]. In addition, several recent studies have examined the validity of using the WBB to measure weight-bearing asymmetry or the vertical GRF (vGRF) during dynamic motions such as sit-to-stand (squat) [7; 8] and ski-jumping [9] tasks.  
However, as the device has gained popularity in research, some researchers have argued against its use for postural control [6; 10]. The focus of their discussions has been on the uncertainty and reliability across multiple-WBB setups, an inconsistent sample rate and a low signal-to-noise ratio. From these backgrounds, the accuracy of the vGRF and COP trajectory has not been examined for walking. Although walking is a common movement in daily life and is an important task in clinical gait disorder rehabilitation settings, the magnitude of the vGRF and COP positions in a walking stance change more dynamically within a short period of time than they do for other static motions.  
Thus, to overcome the limitations of the WBB, calibrating or resampling approaches have been proposed to enhance the measurement accuracy of the COP during static-stance tasks for the assessment of postural sway [6; 11]. These techniques are also expected to improve the accuracy during more dynamic tasks.  
Therefore, the purpose of this study is to determine the accuracy of the vGRF and COP trajectory measurements taken with the WBB while walking. In the study, we examined the accuracy while people walked along straight and curved paths, because these motions are natural and occur frequently in daily life. In addition, measuring the force distribution in these motions is useful when evaluating potential advantages during the rehabilitation of patients with gait disorders [18; 19].