Errors in three dimensional spinal motion measurement using skin surface motion sensors

Measurement of spinal motion using skin surface measurements is important in gaining a better understanding of the possible factors and risks associated with back pain in a variety situations and the development of effective rehabilitation and ergonomic programmes. In addition, this kind of system can enable activity monitoring outside the laboratory environment. However, skin surface measurements are prone to errors due to sensor misalignment and skin movement. In this study these two potential errors were being investigated using two electromagnetic tracking sensors. By using a hinged joint to study the effect of sensor misalignment, it was found that a 5˚ misalignment produced a measurement error of between 2.6° and 5.8°. This error grew when the degree of misalignment increased. In the sensor attachment experiment, two participants were recruited and requested to perform six physiological movements with four different attachment methods. It was observed that insecure sensor attachment would also produce large artefacts (20.1° to 23.6° of coupled motions compared to 3.3°/3.9° to 5.8°/5.7° with secured attachment methods during flexion) during spinal motion measurements. In conclusion, ensuring secure sensor attachment and alignment using base plates and external strapping was found to minimise error due to skin movement and contraction of muscles, thus contributing to a more reliable and valid study in spinal motion measurement using external motion sensors.