According to the scientific actual evidence, the most appropriate waveform to get a muscular contraction it is the impulse symmetrical biphasic.
The width of the impulse must be such to allow a transfer of sufficient energy to excite the muscular fibers. A wider pulse-width doesn't carry to any advantage, it can instead create an useless bother to the patient, through the stimulation of the nervous fibers nervous preposed to carry the pain signal.
The ideal pulse-width (WIDTH) corresponds to the value of CHRONAXY of the muscle to be stimulated.
Each muscle is characterized by a value of chronaxy, that intrinsically expresses the neuro-muscular composition and the functional destination of the same muscle. The CHRONAXY, express in µsec, could be definite like "the pulse-width, of an impulsive current able to provoke a least (perceptible) muscular contraction, when he comes administered to an intensity double as regards the value of RHEOBASE." The RHEOBASE, in turn, express in mA, is defined like "the level of current to which an impulse having a pulse-width greater than 100 ms must be administered, to provoke, in correspondence of the motor point, a least (perceptible) contraction."