Pulse rate or frequency determines the rate of nerve depolarization. Increasing pulse frequency provides for temporal summation of force output. A pulse rate that ranges between 1 and 10 pulses per second (pps) induces twitch contractions of skeletal muscles. Faster pulse rates (15-25 pps) induce incomplete tetanic contractions and pulse rates higher than 45-50 pps typically induce fused tetanic contraction. For most applications it is desirable to have a tetanic contraction of muscle. There is considerable inter-subject variability with regard to fusion frequency. Pulse rate is also associated with stimulation comfort during tetanic contractions (Naaman et al., 2000). Increasing pulse rate is known to cause a greater degree of muscle fatigue during tetanic contractions (Binder-Macleod and Guerin, 1990; Cestari et al., 1995; Binder-Macleod and Lee, 1996;
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