Cervical range of motion (ROM) has been measured in many disorders of the cervical spine. It has been conventionally measured using a goniometer or cervical range of motion device (CROM; Performance Attainment Associates, St. Paul, MN, USA). Although these conventional methods are accurate enough to measure static state [1], continuous monitoring during dynamic motion is impossible. Therefore, those patients with dynamic cervical disorders such as cervical dystonia who would benefit from objective continuous cervical monitoring for evaluating disease severity and treatment outcome are evaluated with subjective scales [2,3].

Recent studies have shown promising results in measuring dynamic cervical motion using methods such as in vitro measurement using implantable micro sensors on spine model [4] and in vivo measurements using fluoroscopy [5], biplane radiograph [6], and 3D computed tomography [7]. However, these devices are not clinically applicable for long duration due to radiation hazard, invasiveness, and the need of bulky imaging devices. To overcome these shortcomings, various new devices not conventionally considered as medical devices have been used in attempt to measure human motions. Microsoft Kinect (Kinect) is one of the most attractive options on the market. It uses in-depth camera information and probabilistic model for recognizing body part in real time [8]. It doesn't require any additional equipment that may interfere with motions. Acceptable accuracy [9,10] and affordable price (US$199) has made Kinect popular in various fields of research [11,12,13,14]. Another device of interest is the recently developed attitude and heading reference system (AHRS) based on microelectromechanical systems (MEMS) technology. It has benefit of small in size and light in weight. It is becoming popular for various applications such as underwater navigation, aircraft guidance control, and human motion capture [15,16,17].

However, the validity of AHRS and Kinect for measuring cervical ROM is currently unknown. The objective of this study was to compare AHRS and Kinect to the optical motion capture (MoCap) system that has been used extensively in researches for continuously obtaining human motion data without loss of precision [18] and for continuous measurement of cervical ROM [19].

MoCap and ARHS showed fair agreement (95% LoA<10°) for measuring ROM in all directions. MoCap and Kinect system showed less favorable agreement (95% LoA>10°) with negative average difference (–7° to –7.5°) for measuring ROM in all directions (Table 1, Fig. 2). Distribution of difference in measurement between each sensor and MoCap for every 10° of measurement by MoCap is shown in Fig. 3. Axial rotation showed large measurement error distribution in both sensors and absolute measurement difference at the end of rotation. It was as high as 12.82° for AHRS (4.70°±3.04°) and 14.25° for Kinect (4.75°±3.28°). ICC values between MoCap and AHRS in –40° to 40° range were excellent for flexion/extension and lateral bending (ICC>0.9). ICC values were fair for axial rotation (ICC>0.8). ICC values between MoCap and Kinect system in –40° to 40° range were fair for all motions (ICC>0.8) (Table 2).

This is the first study to investigate the feasibility of the AHRS and the Kinect system compared to MoCap for monitoring cervical motion. Our results showed that AHRS could be used to measure ROM during continuous movement in all directions with acceptable range of measurement error.

Kinect showed larger measurement error. It mostly had smaller measurement values compared to MoCap. For monitoring continuous motion, AHRS showed excellent correlation with MoCap while Kinect showed good correlation with MoCap for flexion/extension and lateral bending in –40° to 40° range with larger measurement errors for axial rotation in both devices. However, there is no clear cutoff value for interpreting the LoA or correlation coefficient to decide whether a device is valid or useful compared to the gold standard [20]. Instead, the validity of a device depends on clinical setting of the usage and how much error can be tolerated.

These new devices have larger measurement errors compared to the results obtained from fluoroscopy or X-rays [5,6,7]. This error may limit sensor efficacy. Accuracy of AHRS is known to be affected by time, velocity, acceleration, and magnetic distortion. To minimize these measurement errors, various algorithms and filters have been studied [21,22,23,24]. In our study, peak angular velocity and acceleration were larger in axial rotation than those in flexion/extension and lateral bending. This might be the reason for the larger measurement error in axial rotation. Accuracy of Kinect can be affected by distance between subject and the camera and recognition precision. In our study, subjects were placed 100 cm in front of the camera with anticipated measurement error of <0.03 cm [10]. However, face detection algorithm failed to accurately detect the face in extreme angles. In these angles, simultaneous exposure of tip of the mandible and middle of forehead for appropriate facial recognition were not satisfied. Other factor that might have contributed to negative systemic error shown on Kinect measurement could be inadequate initialization or error in the process of converting coordinates to angles.

During data analysis, we noted that lateral bending, flexion/extension, and axial rotation were not independent motions. Small amplitude of lateral bending with flexion/extension accompanied axial rotation has been shown in previous study results [25,26]. This phenomenon indicates that the axis of the axial rotation can tilt during axial movement in relation to the body. Moreover, the axis of each cervical motion is not static. Instead, it is rather complex and dynamic [6,27]. A multi joint model rather than a single ball and socket joint model is needed to describe it mathematically. Another limitation of our study is the small number of participants with limited range in age distribution. Age has been found to have negative correlation with active cervical ROM [28,29]. It might be more suitable to apply these devices in older populations.

In conclusion, our study showed the feasibility of using AHRS to measure cervical ROM during continuous motion with an acceptable range of error. AHRS and Kinect system can also be used for continuous monitoring of flexion/extension and lateral bending in ordinary ROM. These systems are ubiquitous, they have minimal contact with the body and relatively low cost compared to MoCap. Therefore, they can be used to monitor various diseases and activities in ordinary range.