Force output during fatigue with progressively increasing stimulation frequency.

There is currently a controversy over whether stimulation frequencies should increase or decrease to optimize force output over time. This study compared changes in thenar muscle force and M-wave amplitude during progressively increasing (20-40 Hz), decreasing (40-20 Hz) and constant (20 Hz) frequency stimulation of the median nerve continuously for 3 min. Twenty-three individuals participated in three sets of experiments. There was no significant difference in the force-time integrals between the three fatigue tasks. The rate of fatigue was not correlated to the number of stimulation pulses delivered (20 Hz: 3,600, 20-40 and 40-20 Hz: 5,400). All fatigue tasks caused a significant reduction in M-wave amplitude and the reduction was largest for the 20-40 Hz protocol. However, multiple linear regression analysis revealed that the M-wave amplitude could not predict the changes in force over time for the 20 Hz or 20-40 Hz protocols. Thus during sustained evoked contractions with stimulation frequencies within the physiological range, frequencies can vary significantly without changing the overall force-time integral.