The two-stage step-frequency decoupling filter separates human step-frequency harmonics through low-band (1.5–3.5 Hz) moving spectral autocorrelation, then uses scipy.signal.filtfilt to apply a fourth-order zero-phase high-pass Butterworth filter (cutoff frequency 6.0 Hz) to attenuate human swaying while preserving mechanical shock signals in the 12–28 Hz band.