Inertia artefacts and their effect on the parameterisation of keyboard reaction forces

Abstract

Reaction force measurements collected during typing on keyboard trays contain inertia artefacts due to dynamic movements of the supporting work surface. To evaluate the effect of these artefacts, vertical forces and accelerations were measured while nine volunteers touch-typed on a rigid desk and a compliant keyboard tray. Two signal processing methods were evaluated: 1) low pass filtering with 20 Hz cut-off; 2) inertial force cancellation by subtracting the accelerometer signal. High frequency artefacts in the force signal, present on both surfaces, were eliminated by low pass filtering. Low frequency artefacts, present only when subjects typed on the keyboard tray, were attenuated by subtracting the accelerometer signal. Attenuation of these artefacts altered the descriptive statistics of the force signal by as much as 7%. For field measurements of typing force, reduction of low frequency artefacts should be considered for making more accurate comparisons across groups using work surfaces with different compliances. Direct measures of physical risk factors in the workplace can improve understanding of the aetiology of musculoskeletal disorders. Findings from this study characterise inertia artefacts in typing force measures and provide a method for eliminating them. These artefacts can add variability to measures, masking possible differences between subject groups.

Keywords:

• exposure assessment tools
• physical risk factors
• work-related musculoskeletal disorders

Acknowledgements

This study was supported by the National Institute of Occupational Safety and Health (NIOSH)/Centers for Disease Control and Prevention (CDC) Grant (R01 OH003997) at the Harvard School of Public Health. We would also like to acknowledge Jim Ploger for his assistance in designing the keyboard force platform and conducting force measurements.