POWER SPECTRUM ANALYSIS OF THE RECTIFIED ELECTROMYOGRAM DURING GAIT FOR NORMALS AND PATIENTS

The aim of the present study was to investigate the power spectrum of the rectified electromyographic (EMG) activity during gait in order to achieve an optimal cut-off frequency before calculating the EMG envel ope. The frequency content of the rectified EMG signals was found to d iffer substantially between healthy subjects and patients. For the gas trocnemius lateralis muscle in young healthy subjects, 95% of the powe r was found to be below 3.8 Hz, whereas the 95% limit was 5.6 Hz for e lderly subjects, 13.1 Hz for parkinsonians, and 11.5 Hz for patients w ith cerebellar ataxia. For the hamstring lateralis muscle, the 95% lim it was 4.5 Hz for the young healthy subjects, 12.8 Hz for the elderly subjects, 33.8 Hz for the parkinsonians, and 26.0 Hz for the cerebella r ataxia patients. This indicates that the low-pass cut-off frequencie s generally used today (<3 Hz) for analysis of normal and pathologic g ait are far too low. Individual filter setting is therefore recommende d for both normals and the different groups of patients. For some freq uencies of energy accumulation, we found a significant difference betw een the young subjects and either the elderly subjects or the parkinso nians. The power spectrum of the rectified EMG itself could therefore be suggested as a tool to distinguish between normals and patients.