Effects of subject position on balance platform measurements

The high cost of commercially available force plates instrumented to quantify postural sway can be prohibitive to both research and clinical institutions. Therefore custom plate design and construction within the institutional environment is not uncommon. Some plate designs, however, may give erroneous measurements depending upon subject position. We report here on the existence of a kern boundary on the plate surface. Loads placed inside this boundary result in support compression, while those placed outside cause at least one support to experience tension. The findings indicate that the type of force transducer used for corner support (unidirectional or bidirectional), the type of connection between the plate and transducers, plate weight, subject weight, plate dimensions and transducer preloads are all critical to accurate measurements.