We defined the better wave shape to be used, and a single pulse could also produce a momentary contraction like a “wriggle”; any way, to obtain a sustained contraction, we must supply a pulse train, characterised by a “frequency” (at least 30 pulses per second). Frequency is the repetition rate of pulses in a time unit (in a second).
The frequency to be used will depend on the type of fibres to be stimulated. Any muscle includes slow fibre, characterised by an aerobic metabolism, and fast fibres, characterised bi anaerobic metabolism. If we want to stimulate a muscle were slow fibres are dominant, we have to use low frequencies (1 to 30 Hz). If we want to stimulate muscles having fast fibres dominance, we have to use higher frequencies.
The frequency of pulses can also define the aim of the electro stimulation, as passive training. Physiological and metabolic characteristics of a muscle in fact, can be modified by the electro stimulation.
The high frequency stimulation can partially "convert" the tonic fibres in phasic fibres, these last characterised by a higher development of instantaneous force, to the prejudice of the muscular resistance.
A very low frequency (1 to 10 Hz) will be always accepted, like a massage and it will not produce fatigue. The physiological effect is related to the blood flow improvement, bringing to a better tissue feeding and facilitation in the catabolites drainage (absorption).