Before the electrical signals reach your muscles and cause them to contract, the current first needs to overcome the resistance of your skin. The following principle applies here: The higher the frequency – i.e. the greater the number of impulses per second, measured in the unit hertz – the easier this resistance is to overcome.
Many EMS systems use low-frequency (LF) impulses that are significantly lower than 1000 hertz (Hz). The skin resistance represents a significant barrier for this frequency, which means that the impulses do not penetrate as deep. This can result in an unpleasant effect in which the low-frequency impulses feel like little pinpricks while training.