Three basic modulations are common in commercially available stimulators. One is termed interpulse interval, the second "burst of pulses" and the third "interrupted pulses" (or "ON and OFF" modes). Modulation of the inter-pulse interval simultaneously modulates pulse (rate) frequency. Binder-Macleod and colleagues identified a pattern of modulation termed catch-like property of human muscle where manipulation of the inter-pulse intervals resulted in greater force production and less muscle fatigue compared to using constant pulse rate (Binder-Macleod and Lee, 1996; Russ and Binder-Macleod, 1999). Having a burst of three pulses decreases the amplitude of peak current at a given level of excitation while concurrently causing a threefold increase in RMS current amplitude compared to a single biphasic pulse (Kantor et al., 1994). Consequently the use of burst of pulses can be justified only if it makes the stimulation more comfortable.
The option of interrupted pulses is mandatory in most neurorehabilitation clinical applications. Each stimulation program for different clinical applications is likely to require somewhat different contraction (on) and relaxation (off) times, also known as the duty cycle. The greater the duty cycle, the more profound the problem with muscle fatigue, since the time of rest is reduced.
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