Medical devices, namely, pulse generators for muscle stimulation comprised of portable and non-portable tissue stimulators and programmable tissue stimulators to deliver interferential stimulation waveforms; electrical stimulation apparatus for treatment of muscles for physical therapy purposes and for treatment of intractable pain syndromes for pain management; medical devices, namely, pulse generators for muscle stimulation comprised of a dynamic hybrid muscle stimulation and interferential stimulation system in the nature of tissue stimulators, an electrical current generator for direct and interferential current generation and electrodes; medical devices, namely, an electrical stimulator programmed to provide for muscle stimulation and for pain management through a pulse generator that delivers interferential stimulation waveforms