determining the optimal system-specific cut-off frequencies for filtering in-vitro upper extremity impact force and acceleration data by residual analysis

the fundamental nature of impact testing requires a cautious approach to signal processing, to minimize noise while preserving important signal information. however, few recommendations exist regarding the most suitable filter frequency cut-offs to achieve these goals. therefore, the purpose of this investigation is twofold: to illustrate how residual analysis can be utilized to quantify optimal system-specific filter cut-off frequencies for force, moment, and acceleration data resulting from in-vitro upper extremity impacts, and to show how optimal cut-off frequencies can vary based on impact condition intensity. eight human cadaver radii specimens were impacted with a pneumatic impact testing device at impact energies that increased from 20 j, in 10 j increments, until fracture occurred. the optimal filter cut-off frequency for pre-fracture and fracture trials was determined with a residual analysis performed on all force and acceleration waveforms. force and acceleration data were filtered with a dual pass, 4th order butterworth filter at each of 14 different cut-off values ranging from 60 hz to 1500 hz. mean (sd) pre-fracture and fracture optimal cut-off frequencies for the force variables were 605.8 (82.7) hz and 513.9 (79.5) hz, respectively. differences in the optimal cut-off frequency were also found between signals (e.g. fx (medial–lateral), fy (superior–inferior), fz (anterior–posterior)) within the same test. these optimal cut-off frequencies do not universally agree with the recommendations of filtering all upper extremity impact data using a cut-off frequency of 600 hz. this highlights the importance of quantifying the filter frequency cut-offs specific to the instrumentation and experimental set-up. improper digital filtering may lead to erroneous results and a lack of standardized approaches makes it difficult to compare findings of in-vitro dynamic testing between laboratories.