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Fatigability and Variable-Frequency Train Stimulation of Human Skeletal Muscles

CS Bickel, PT, PhD, is Post-doctoral Research Fellow, Department of Exercise Science, The University of Georgia, 300 River Rd, Athens, GA 30602 (USA) (cbickel{at}coe.uga.edu).
JM Slade, MA, is a doctoral student in the Department of Exercise Science, The University of Georgia
GL Warren, PhD, is Associate Professor, Department of Physical Therapy, Georgia State University, Atlanta, Ga
GA Dudley, PhD, is Distinguished Research Professor and Professor of Exercise Science, The University of Georgia, and Research Scientist, Shepherd Center, Atlanta, Ga

Address all correspondence to Dr Bickel

Background and Purpose. The quadriceps femoris (QF) and tibialis anterior (TA) muscles are often activated through the use of electrical stimulation by physical therapists. These 2 muscles are fundamentally different in regard to their fiber-type composition. Whether protocols developed using a given muscle can be applied to another muscle has seldom been questioned, even if they differ in fiber type. The purpose of this study was to test the hypothesis that torque augmentation during variable-frequency train (VFT) stimulation as compared with constant-frequency train (CFT) stimulation in the fatigued state would not differ between these muscles, even though the TA muscle has 50% relatively more slow fibers than the QF muscle relative to each muscle's overall composition. Subjects. Ten recreationally active men with no history of lower-extremity pathology participated in the study (mean age=25 years, SD=4, range=19–31; mean height=179 cm, SD=5, range=170–188; mean body mass=80 kg, SD=15, range=63–111). Methods. The subjects' TA and QF muscles were stimulated with CFTs (six 200-microsecond square waves separated by 70 milliseconds) or VFTs (first interpulse interval=5 milliseconds) that evoked an isometric contraction. Results. After potentiation, the torque-time integral and peak torque were not different for the VFT and CFT stimulation. Rise time was longer for the TA muscle than for the QF muscle and for CFT stimulation versus VFT stimulation (both approximately 40%). After 180 CFTs (50% duty cycle), peak torque decreased 56% overall, with no differences between muscles. Enhancement of the torque-time integral (25%) by VFT stimulation was not different between fatigued QF and TA muscles. Torque augmentation was due to the VFT stimulation evoking 27% greater peak torque and less slowing of rise time than the CFT stimulation (15% versus 30%). Discussion and Conclusion. The results indicate that the QF muscle may not necessarily fatigue more than the TA muscle. The results suggest that VFTs augment the force of fatigued, human skeletal muscle irrespective of fiber type.

Key Words: Catchlike property • Electrical stimulation • Fatigue • Quadriceps femoris muscle • Tibialis anterior muscle

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				C. M Gregory and C S. Bickel Recruitment Patterns in Human Skeletal Muscle During Electrical Stimulation Physical Therapy, April 1, 2005; 85(4): 358 - 364. [Abstract] [Full Text] [PDF]