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A Biologically Plausible Hypothesis

I read with interest the recent article by Laufer et al titled "Quadriceps Femoris Muscle Torques and Fatigue Generated by Neuromuscular Electrical Stimulation With Three Different Waveforms" (July 2001). In the study, one perplexing finding was the "dramatic difference between the fatiguing effect of the polyphasic waveform and that of the other 2 waveforms." The authors venture to suggest that because the polyphasic waveform consists of 25 alternating cycles in one pulse train, it may provide a higher stimulation frequency, culminating in greater electrical fatigue. Although a higher stimulation frequency is associated with a greater rate of fatigue, a higher stimulation frequency may produce a higher force output.¹ The latter findings would appear to render the authors' argument internally inconsistent, in view of the comparatively weaker initial maximal tolerated contraction (MTC) induced by the polyphasic waveform. On the basis of the results of the study, an alternative explanation may be that the polyphasic waveform used in the study did not stimulate more type II muscle fibers as suggested, but conversely that the monophasic and biphasic waveforms are perhaps capable of stimulating a greater number and proportion of fatigue-resistant type IIa fibers (or even some type I oxidative fibers) and fewer type IIb fibers than the polyphasic waveform, resulting in greater MTC from the beginning and the lower rate of electrical fatigue.

I am fully aware that the purpose of the authors' study was to investigate whether stimulation-related differences exist between gender and waveforms, not to study the potential mechanisms for any such differences. Using the time-honored glycogen-depletion method on muscle fibers obtained from biopsy of the vastus lateralis muscle, Sinacore and colleagues² presented results of a single-case study to show that the polyphasic waveform selectively activate type II skeletal muscle fibers. A similar approach could perhaps be undertaken comparing the muscle activation patterns produced by the 3 waveforms to test the proposed biologically plausible hypothesis. Such an approach and that of Laufer and colleagues would serve to enhance the development of the most effective treatment regimens.

Yong-Hao Pua

Physiotherapist
Alexandra Centre for Exercise and Sports Medicine (ACES)
Rehabilitation Department
Alexandra Hospital
Republic of Singapore
(Yong_Hao_PUA{at}Alexhosp.com.sg)

References

1. Jones DA, Bigland-Ritchie B, Edwards RH. Excitation frequency and muscle fatigue: mechanical responses during voluntary and stimulated contractions. Exp Neurol.1979; 64:401–143.[ISI][Medline]
2. Sinacore DR, Delitto A, King DS, Rose SJ. Type II fiber activation with electrical stimulation: a preliminary report. Phys Ther.1990; 70:416–422.[Abstract/Free Full Text]

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