Preservation of Force Output Through Progressive Reduction of Stimulation Frequency in Human Quadriceps Femoris Muscle

The purpose of this study was to determine the effects of a reduction in the pulse frequency on the fatigue rate of human quadriceps femoris muscle during intermittent (8-second) contractions. Twelve healthy subjects each participated in two experimental sessions. Thirty cycles (cycle time: 8 seconds "on"/12 seconds "off") were applied during each session. During one session, a frequency of 60 pulses per second (pps) was used for all trains. During the other session, the subjects were stimulated with 60 pps for the first train. The stimulating frequency of each train was then progressively reduced, in 5-pps steps, for contractions 2, 3, 5, 8, 12, and 20. By the fifth contraction, the differences in average force produced by the 60-pps trains and the reduced-frequency trains were significant. The difference between the two conditions increased, with the variable-frequency protocol producing 46% more force than the constant-frequency protocol during the last contraction. These results showed that, compared with a constant pulse frequency, reducing the pulse frequency during a fatiguing contraction can markedly decrease the rate of force fatigue of skeletal muscle. This finding suggests that a variable-frequency protocol, similar to the one used in this may prove to be a more effective pattern of stimulation for activation of skeletal muscle than the traditionally used constant-frequency protocol.