However, not only are carcasses interrupted by tracts of fat with a high resistance and by bones with a variable resistance, but muscles themselves are electrically anisotropic . Resistivity is inversely proportional to meat temperature, it tends to be greater across rather than along muscle fibers, and it may show a transient increase post mortem followed by a progressive decline. Resistance of a whole carcass is modified by factors such as, the time lapse between animal exsanguination and muscle stimulation, the distance between electrodes, electrode surface area in contact with meat or connective tissue and not blocked by fat, whether or not the carcass is whole, eviscerated or split, and electrode location. Continuous DC currents only elicit strong stimulation when they are first applied, but interrupted DC currents of square wave impulses may be used to prolong this initial response. However, DC currents of any type soon cause Polarization at the electrode-tissue interface.
Polarization increases resistance and decreases responses.
A muscle that has almost ceased responding to unidirectional square waves may respond with original intensity if polarity is reversed, as shown below. . To avoid polarization, square waves of alternating polarity may be used but apparatus to produce these at high voltage may be expensive. Thus, there are some advantages to using apparatus which supplies modified sine wave impulses derived from the regular 50 or 60 Hz commercial power supply.