Muscle function loss can result from multiple nervous system diseases including spinal cord injury (SCI), stroke, and multiple sclerosis (MS). Electrical muscle stimulation is clinically employed for rehabilitative and therapeutic purpose and typically requires mA-level stimulation current. Here, we report electrical muscle stimulation, which is directly powered by a stacked-layer triboelectric nanogenerator (TENG) through a flexible multiple-channel intramuscular electrode. This multiple-channel intramuscular electrode allows mapping of motoneurons that is sparsely distributed in the muscle tissue and thus enables high efficiency TENG muscle stimulation, although the short-circuit current of the TENG is only 35 μA. With a stimulation efficiency matrix, we find the electrical muscle stimulation efficiency is affected by two factors, namely, the electrode-motoneuron position, and the stimulation waveform polarity. To test whether it is a universal phenomenon for electrical stimulation, we then further investigate with the conventional square wave current stimulation and confirm that the stimulation efficiency is also affected by these two factors. Thus, we develop a self-powered direct muscle stimulation system with a TENG as power source and waveform generator, and a multiple-channel intramuscular electrode to allow motoneuron mapping for stimulation efficiency optimization. We believe such self-powered system could be potentially used for rehabilitative and therapeutic purpose to treat muscle function loss.