Calibration of an instrumented treatment table for measuring manual therapy forces applied to the cervical spine.

Manual therapy techniques are commonly used to treat musculoskeletal neck disorders, but little is known about the manual forces applied during cervical spine treatment. Forces may vary between practitioners, and this may affect patient outcomes. This study reports the development of an instrumented treatment table and its calibration for measuring posteroanterior-directed forces applied during cervical spine mobilisation. A treatment table surface was instrumented with seven biaxial load cells to measure manually applied forces in three planes. Accuracy of the system was evaluated using known weights (unloaded and loaded to represent a patient's body weight), selected to be consistent with the level of forces expected to be applied during cervical mobilisation. Recorded force values strongly correlated with known weights (Pearson's r=0.999 to 1.000 for forces applied in different directions and locations, unloaded and loaded). The accuracy of forces in the unloaded condition was very good for vertical forces (mean absolute error 1.1N, SD 1.5), and reasonably good for horizontal forces (2.8N, SD 2.4 for mediolateral, 3.4N, SD 1.5 for caudad-cephalad). In the loaded condition absolute error increased slightly for horizontal forces. The accuracy of measured forces indicates the instrumented table is acceptable for measuring cervical mobilisation forces. Using it allows practitioners to perform manual techniques using their usual clinical technique, however interpretation of force data is limited because it represents force applied to the table rather than at a specific joint.