Because the frequencies that are passed on to the body with the impulses are the decisive factor whether your muscles are only “stimulated” or
really “trained”.

The previously known Whole-Body EMS Tens training systems (up to 100Hz) merely irritate the muscle by causing it to react to a twinge. This tenses the muscle, but at a superficial depth such as during a fear response. This does not mean that low frequencies are bad. Quite the contrary. But for them to fully develop, the frequencies must be carried deep into the muscle.