Relationship between stimulation train characteristics and dynamic human skeletal muscle performance

Aim:  The purpose of the present study was to investigate the effect of activation frequency on dynamic human muscle performance for a range of train durations and number of pulses during free limb movement. Methods:  The quadriceps femoris muscles of 10 subjects were activated with stimulation trains with different activation frequency, train durations and number of pulses. The peak excursion produced in response to each train was the dependent measure of muscle performance. Results:  The excursion–frequency (for a 300-ms train duration) and excursion–train duration (for trains with frequencies of 10, 30 or 59 Hz) relationships could each be fit with a two-parameter exponential equation (R2 values > 0.97). Because the number of pulses in a stimulation train is a function of both train duration and frequency, the excursion produced as a function of the number of pulses was characterized by a three-parameter exponential equation that represented this combined relationship. The relationship between the measured and predicted excursions in response to a wide range of stimulation trains had a R2 = 0.96. In addition, one-way repeated measures analyses of variance (anovas) showed that the frequency at which the maximum excursion was produced increased with an increase in the number of pulses in the trains tested. Conclusion:  These results show the importance of train duration and the number of pulses contained within a train on the relationship between activation frequency and human skeletal muscle performance.