Japan has the highest proportion of elderly persons in the world. It is called a “super-aging society”. Falls and bone fracture are some of the main causes for requiring care. The ability of knee joint extension plays a key role in preventing falls. The purpose of this study was to evaluate the function of knee joint extension on elderly persons from the view point of frequency components of surface electromyography (SEMG) and mechanomyogram (MMG) activities for lower limb muscles. The frequency of SEMG and MMG activities were evaluated by using of Wavelet Transform (WT). The WT of SEMG is informative in understanding the characteristics of muscle activity, which include the motor unit (MU) firing frequency, number of recruited MUs, and a kind of fiber type. In this study, we compared the SEMG with the MMG about frequency components to estimate the functions of knee joint extension in the elderly persons.