This study presents a novel method for detection of the onset time of muscle activity using surface electromyogram (EMG) signals. The method takes advantage of the nonlinear properties of the Teager-Kaiser energy (TKE) operator, which simultaneously considers the amplitude and instantaneous frequency of the surface EMG, and therefore increases the prospects of muscle activity detection. To detect the onset time of muscle activity, the surface EMG signal was first processed by the TKE operator to highlight motor unit activities of the muscle. Then a robust threshold-based algorithm was developed in the TKE domain to locate the onset of muscle activity. The validity of the proposed method was illustrated using various surface EMG simulations as well as experimental surface EMG recordings.