Comparison of four methods for determining the cut-off frequency of accelerometer signals in able-bodied individuals and ACL ruptured subjects

Abstract

Background: Selecting the appropriate cut-off frequency to filter triaxial accelerometric data is a challenging issue in gait analyses. It reduces soft tissues artifacts and the variability in the kinematic data waveform from able-bodied and physically impaired gaits.

Research question: Are cut-off frequencies estimated by four filtering methods similar along each axis of a triaxial accelerometer and for able-bodied subjects and those with an anterior cruciate ligament rupture (ACLR)?

Methods: After walking on a treadmill, the cut-off frequency for the tibial accelerations was calculated using 95 and 99 per cent of the energy spectrum (E), residual analysis (RA) and, the method (Yu) proposed by Yu et al. [1]. The coefficient of variation was used to express the variability of the cut-off frequencies estimated by the four methods. t-Test and repeated measure ANOVA were applied to examine the effects of healthiness and acceleration axis on cut-off frequencies.

Results: On average, E95 and E99 gave the lowest and the highest cut-off frequencies respectively. The results demonstrated the effect of ACL injury and axes on the cut-off frequencies, especially on the RA method. There was a significant difference in the cut-off frequencies between healthy and ACLR subjects for the vertical axis with the RA method and for the anterior-posterior (AP) axis with the Yu method. Similar cut-off frequencies were obtained for all axes with the E99 method for within groups' comparison. The E95 and E99 methods gave the least and most variable outputs respectively. Significant within group differences between cut off frequencies calculated by four methods, led to disappearing peaks in the more fluctuating portion of the acceleration data.

Significance: A single cut-off frequency is not recommended for all individuals and axes. In cases where a single cut-off frequency is necessary for all individuals or axes, RA or E99 methods are suggested respectively.

Keywords: Accelerometer; Anterior cruciate ligament rupture; Cutoff frequency; Gait analysis; Power spectrum; Residual analysis.