We developed a technique to study temporal discharge patterns of single motor units using 1/f process model, whose fractional parameter gamma was shown to be a useful indicator for distinguishing between discharge behaviors of single motor units of normal subjects and patients with upper motor neuron lesions. We have studied a total of 47 motor units in 3 normal subjects, and 41 in 3 patients with upper motor neuron lesions from biceps and extensor digitorum communis muscles during steady contraction. The parameter gamma was estimated with an algorithm based on wavelet analysis. The mean value of gamma in patients was 1.52, and the mean value of gamma in normal subjects was -0.06. These results suggest that 1/f process can be used to document the impaired motor control mechanisms at single motor unit level.