Fatigue-induced changes in muscle fiber action potentials estimated by wavelet analysis T. Vukova * , M. Vydevska-Chichova, N. Radicheva Institute of Biophysics, Bulgarian Academy of Sciences, Akad. G. Bontchev Str., bl. 21, Sofia 1113, Bulgaria Received 6 October 2005; received in revised form 31 August 2006; accepted 11 September 2006 Abstract We aimed to investigate fatigue-induced changes in the spectral parameters of slow (SMF) and fast fatigable muscle fiber (FMF) action potentials using discrete wavelet (DWT) and fast Fourier (FFT) transforms. Intracellular potentials were recorded during repet- itive stimulation of isolated muscle fibers immersed in Ca 2+ -enriched medium, while extracellular potentials were obtained from muscle fibers pre-exposed to electromagnetic microwaves (MMW, 2.45 GHz, 20 mW/cm 2 ). The changes in the frequency distribution of the action potentials during the period of uninterrupted fiber activity were used as criteria for fatigue assessment. The wavelet coefficients’ changes in the calculated frequency scales demonstrated a contribution of the increased [Ca 2+ ] 0 to an earlier compression of the fre- quency spectrum towards lower ranges. Root mean square (RMS) analysis of the wavelet coefficients calculated from SMF potentials showed a reduction of the higher frequencies (scale 1) by 90% in elevated [Ca 2+ ] 0 vs. 55% in controls and an increase of low frequencies (scale 5) by 323% vs. 187%, respectively. For FMF potentials a decrease of 71% vs. 59% for high frequencies (scale 1, elevated [Ca 2+ ] 0 vs. control) and an increase of 386% vs. 295% in scale 5, respectively, were observed. MMW pre-exposure resulted in increased muscle fiber resistance to fatigue. The fatigue-induced decrease of potential high frequencies (SMF: 59% vs. 96%, MMW vs. control; FMF: 30% vs. 92%, respectively), and the increase of low frequencies (SMF: 200% vs. 207%, MMW vs. control; FMF: 93% vs. 314%, respectively) were significantly smaller and delayed in exposed muscle fibers. Data from RMS analysis indicate that DWT provides a reliable method for estimation of muscle fatigue onset and progression. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Muscle fiber fatigue; Extracellular calcium ions increased; Microwaves; Intra- and extracellular action potentials; Discrete wavelet transform; Fast Fourier transform 1. Introduction The fatigue-related decline in membrane excitability leads to an incomplete excitation-contraction coupling and loss of force. The mechanisms underlying peripheral fatigue may differ depending on the muscle (Bigland- Ritchie et al., 1978) or its fiber composition (Colliander et al., 1988). Intra- and extracellular recordings of sarcolemmal action potentials from isolated muscle fibers have shown a decrease of amplitude and an increase in time parameters during continuous activity. Parameter changes have been referred to different factors: muscle fiber type, fatiguing protocols (Mileva et al., 1998; Vydevska-Chichova et al., in press), ionic composition of the surrounding medium (Balog and Fitts, 2001; Cairns et al., 1998; Radicheva et al., 2000; Radicheva and Mileva, 2001), electromagnetic irradiation (Radicheva et al., 2001, 2002a,b), etc. Repetitive stimulation of skeletal muscles results in spe- cific alterations in ionic composition of the t-tubular sys- tem: Na + depletion (Bezanilla et al., 1972; Garcia et al., 1991), K + (Almers, 1980; Kirsch et al., 1970) and Ca 2+ (Bianchi and Narayan, 1982; Howell and Oetliker, 1987; Howell et al., 1987) accumulation. These ionic changes result in a decrease of action potential amplitude, deceler- ation of membrane repolarization and depolarization pro- cesses. The depolarization could be sufficiently large to 1050-6411/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jelekin.2006.09.014 * Corresponding author. Tel.: +359 2 9792128; fax: +359 2 9712493. E-mail address: tivukova@bio.bas.bg (T. Vukova). Available online at www.sciencedirect.com Journal of Electromyography and Kinesiology 18 (2008) 397–409 www.elsevier.com/locate/jelekin