Variable-Frequency Trains Offset Low-Frequency Fatigue in Human Skeletal Muscle.

Variable-Frequency Trains Offset Low-Frequency Fatigue in Human Skeletal Muscle

Russ DW, Binder-Macleod SA (Department of Physical Therapy, University of Delaware, Newark, Del), Muscle Nerve. 1999;22:874-882.

This study examined force augmentation using a variable-frequency train stimulation in the presence of differing amounts of low-frequency fatigue (LFF). The authors were testing the hypothesis that the variable-frequency train would produce greater force augmentation than would a comparable constant-frequency train in the presence of LFF.

Twelve subjects who were healthy (6 men, 6 women; age range = 19-37 years) participated in the study. The right quadriceps femoris muscle was fatigued using a standardized, low-frequency electrical stimulation protocol (6-pulse, 14.3 Hz trains delivered at 1 pps for 3.0 minutes). According to the authors, this protocol has been shown to produce significant LFF immediately following stimulation. The isometric force-generating capacity of the quadriceps femoris muscle was determined at 90 degrees of knee flexion before LFF; immediately after LFF was induced; and at 2, 13, and 38 minutes into recovery from LFF. These recovery times were chosen because they are known to exhibit different amounts of LFF. The testing protocol used to determine the augmented force-generating capacity consisted of 5 different constant-frequency trains (8.3-100 Hz) and 1 variable-frequency pattern. The order in which the 6 different testing trains were performed was determined randomly for each subject.

The fatiguing protocol resulted in average declines of approximately 44% in both peak force and the force-time integral of the quadriceps femoris muscle. The augmentation of the force output produced by variable-frequency training was not significantly different from the 12.5 Hz constant-frequency train prior to the use of the fatiguing protocol. The variable-frequency stimulation protocol, however, resulted in an augmentation of the force output that was significantly greater than the augmentation produced by each of the constant-frequency trains at every time point after the onset of LFF. The amount of force augmentation provided by the variable-frequency stimulation pattern was inversely related to the amount of LFF present.

The authors suggested that these results are consistent with a common mechanistic explanation for the existence of LFF and the force augmentation produced with a variable-frequency stimulation protocol. The authors believed that the results of this study may have significant implications for the use of functional electrical stimulation; that is, variable-frequency stimulation may be better suited to augment the force output of muscles during the acute stages of fatigue than constant-frequency stimulation may.

Brad Stockert, PT, PhD University of the Pacific Stockton, Calif