According to current scientific evidence, the most suitable waveform in order to obtain muscle contraction, using low energy impulses, is the symmetrical two-phase impulse.
The impulse must be wide enough to allow a transfer of energy which is sufficient to excite the muscle fibres. A width which is greater does not give any advantage and may even create unnecessary discomfort for the patient, by stimulating painful nerve fibres.
The ideal width for the stimulation impulse (WIDTH) corresponds to the CHRONAXIE of the muscle to stimulate.
Each muscle is characterised by a chronaxy value, which essentially expresses the neuromuscular composition and the functional destination. CHRONAXY, expressed in μsec, can be defined as “the width of the impulse of current, able to cause a minimal (perceptible) muscle contraction, when it is administered at an intensity which is double the RHEOBASE value”. The RHEOBASE, in turn expressed in mA, is defined as “the level of current at which an impulse lasting longer than 100 ms must be administered in order to cause a minimal (perceptible) contraction near the motor end plate”.
In the example table below, we give the minimum and maximum chronaxy values which can be verified statistically in certain muscles.