Reconstructing an accelerometer-based pelvis segment for three-dimensional kinematic analyses during laboratory simulated tasks with obstructed line-of-sight.

Close interface between humans and inanimate objects (furniture, assistive devices, and external loads) can obstruct line-of-sight in biomechanics studies that utilize optoelectronic motion capture systems. This specific problem is frequently encountered with the pelvis segment. This study sought to compare joint and pelvis angles computed from a pelvis-fixed local coordinate system (LCS) that was constructed from optically tracked pelvis landmarks (gold standard) and landmarks derived from angular deviations calculated from triaxial accelerometer data. One participant performed seven tasks: sitting, forward bend, sit-to-stand-to-sit, forward lunge, symmetrical squat, asymmetrical squat, and gait. The root mean square error (RMSE) and coefficient of determination (R2) were examined for the pelvis, lumbar spine, and hip joint angles calculated using the standard and accelerometer-based methods for creating a LCS. The RMSE values for global pelvis angles ranged from 2.2° (gait; R2 = 0.47) to 4.9° (sit-to-stand-to-sit; R2 = 0.98), 0.6° (sitting; R2 = 0.88) to 7.4° (gait; R2 = 0.39), and 1.5° (forward bend; R2 = 0.99) to 2.9° (sit-to-stand-to-sit; R2 = 0.99) for motion about the X, Y, and Z axes, respectively. The magnitude of error observed for adjacent joint motion was lowest about the Z axis for all tasks. In conclusion, the accelerometer-based LCS offers an alternative method for computing pelvis and adjacent joint angles without the reliance on a visual line-of-sight. For motion about the X and Y axes, time-series data derived with the accelerometer-based method may be less representative of discrete events, particularly for gait and lunging tasks.