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 envelope. The frequency content of the rectified EMG signals was found to differ substantially between healthy subjects and patients. For the gastrocnemius lateralis muscle in young healthy subjects, 95% of the power was found to be below 3.8 Hz, whereas the 95% limit was 5.6 Hz for elderly subjects, 13.1 Hz for parkinsonians, and 11.5 Hz for patients with cerebellar ataxia. For the hamstring lateralis muscle, the 95% limit 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 cerebellar ataxia patients. This indicates that the low-pass cut-off frequencies generally used today (<3 Hz) for analysis of normal and pathologic gait are far too low. Individual filter setting is therefore recommended for both normals and the different groups of patients. For some frequencies of energy accumulation, we found a significant difference between the young subjects and either the elderly subjects or the parkinsonians. The power spectrum of the rectified EMG itself could therefore be suggested as a tool to distinguish between normals and patients.