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1.
Ann Biomed Eng ; 43(8): 1865-76, 2015 Aug.
Article in English | MEDLINE | ID: mdl-25527318

ABSTRACT

The objective of this study was to evaluate whether an adapted Hill-type model of muscle energetics could account for the relatively high energy turnover observed during low frequency isometric Neuromuscular Electrical Stimulation (NMES). A previously validated Hill-based model was adapted to estimate the energy consumption due to muscle activation, force maintenance and internal shortening of the muscle during isometric NMES. Quadriceps muscle model parameters were identified for 10 healthy subjects based on the experimentally measured torque response to isometric stimulation at 8 Hz. Model predictions of torque and energy consumption rates across the stimulation range 1-12 Hz were compared with experimental data recorded from the same subjects. The model provided estimates in close agreement with the experimental values for the group mean energy consumption rate across the frequency range tested, (R adj (2) = 0.98), although prediction of individual data points for all frequencies and all subjects was more variable, (R adj (2) = 0.70). The model suggests that approximately one-third of the energy between 4 and 6 Hz is due to shortening heat. The model provides a means of identifying optimal therapeutic stimulation patterns for sustained incremental oxygen uptake at minimum torque output for a given muscle and provides insight into the energetic components involved.


Subject(s)
Isometric Contraction , Models, Neurological , Muscle, Skeletal , Adult , Electric Stimulation/methods , Humans , Male
2.
Ann Biomed Eng ; 37(6): 1141-51, 2009 Jun.
Article in English | MEDLINE | ID: mdl-19319681

ABSTRACT

Most bioelectric signals are not only functions of time but also exhibit a variation in spatial distribution. Surface EMG signals are often "summarized" by a large electrode. The effect of such an electrode is interpreted as averaging the potential at the surface of the skin beneath the electrode. We first introduce an electrical equivalent model to delineate this principle of averaging. Next, in a realistic finite element model of EMG generation, two outcome variables are evaluated to assess the validity of the averaging principle. One is the change in voltage distribution in the volume conductor after electrode application. The other is the change in voltage across the high impedance double layer between tissue and electrode. We found that the principle of averaging is valid, once the impedance of the double layer is sufficiently high. The simulations also revealed that skin conductivity plays a role. High-density surface EMG provided experimental evidence consistent with the simulation results. A grid with 120 small electrodes was placed over the thenar muscles of the hand. Electrical nerve stimulation assured a reproducible compound muscle response. The averaged grid response was compared with a single electrode matching the surface of the high-density electrodes. The experimental results showed relatively small errors indicating that averaging of the surface potential by the electrode is a valid principle under most practical conditions.


Subject(s)
Electrodes , Electromyography/instrumentation , Galvanic Skin Response , Models, Biological , Muscle, Skeletal/physiology , Anisotropy , Electric Conductivity , Electric Impedance , Humans , Surface Properties
3.
Article in English | MEDLINE | ID: mdl-18002558

ABSTRACT

During fatiguing contractions muscle fiber conduction velocity (CV) decreases progressively. The exact cause of this is not yet fully known, although recent studies suggest that changes in extracellular potassium concentration play an important role. A model was developed to examine the effect of accumulation of extracellular potassium ions on the muscle fiber action potential and its CV. As the extracellular potassium concentration was increased, the action potential progressively broadened, reduced in peak-peak amplitude and its CV decreased. However, when the inward rectifier channels were blocked, the changing shape of the action potential and the reduction of its CV with increasing extracellular potassium were dramatically reduced. The simulation results support the hypothesis that the reduction in muscle fiber CV observed during sustained fatiguing contractions may be due in part, to increased accumulation of extracellular potassium ions and suggest that the inward rectifier currents play an important role on the relationships observed.


Subject(s)
Models, Biological , Muscle Fibers, Skeletal/physiology , Muscle, Skeletal/physiology , Potassium/metabolism , Animals , Cations, Divalent , Computer Simulation , Extracellular Fluid/metabolism , Humans , Muscle Contraction/physiology , Muscle Fatigue/physiology
4.
Prosthet Orthot Int ; 27(1): 48-54, 2003 Apr.
Article in English | MEDLINE | ID: mdl-12812327

ABSTRACT

The effect of subcutaneous fat on myoelectric signal amplitude and cross-talk was studied using finite element (FE) models of electromyogram (EMG) signal propagation. A FE model of the upper arm consisted of skin, fat, muscle and bone tissues in concentric layers. Single muscle fibre action potentials were simulated for muscle fibres at a variety of depths and combined to simulate surface EMG interference patterns. As fat layers of 3, 9 and 18 mm were added to the model, the RMS (root mean square) amplitude of the surface EMG signal directly above the centre of the active muscle decreased by 31.3, 80.2 and 90.0%, respectively. Similarly, surface EMG cross-talk above the region of inactive muscle increased as the fat layer thickness increased. The surface EMG RMS amplitude fell below 5% of its value above the centre of the muscle at 14 degrees, 17 degrees, 34 degrees and 47 degrees from the edge of the active muscle with fat layers of 0, 3, 9 and 18 mm, respectively. An additional model was developed with the subcutaneous fat layer thinned from 9 mm to 3 mm in a small, focal region under a pair of recording electrodes. Reducing the fat layer in this manner caused the surface EMG amplitude at the electrodes to increase by 241% and decreased the EMG cross-talk by 68%; this was near the values for the 3 mm uniform fat layer. This demonstrates that fat reduction surgery can increase surface EMG signal amplitude and signal independence for improved prosthesis control.


Subject(s)
Adipose Tissue/physiology , Finite Element Analysis , Muscle, Skeletal/physiology , Action Potentials , Computer Simulation , Electromyography , Humans , Signal Processing, Computer-Assisted
5.
IEEE Trans Rehabil Eng ; 8(3): 353-61, 2000 Sep.
Article in English | MEDLINE | ID: mdl-11001515

ABSTRACT

Spectral compression of the electromyographic (EMG) signal, due largely to decreasing muscle fiber conduction velocity, is commonly used as an indication of muscle fatigue. Current methods of estimating conduction velocity using characteristic frequencies such as the median frequency of the power spectrum, are based on an assumption of uniform spectral compression. To examine changes in the EMG frequency spectrum during fatigue, muscle fiber conduction velocity was measured during sustained, isometric contractions of the biceps brachii. Compression of the EMG power and amplitude spectra was simultaneously examined using the median frequency and an alternative method-the spectral distribution technique. The spectral distribution technique consistently gave a better estimate of the relative change in muscle fiber conduction velocity than either of the median frequencies. This was further examined using a physiologically based EMG simulation model, which confirmed these findings. The model indicated that firing statistics can significantly influence spectral compression, particularly the behavior of characteristic frequencies in the vicinity of the firing rates. The relative change in the median frequency, whether of the amplitude or frequency spectrum, was consistently greater than the relative change in conduction velocity. The most accurate indication of the relative change in conduction velocity was obtained by calculating the mean shift in the midfrequency region of the EMG amplitude spectrum using the spectral distribution technique.


Subject(s)
Computer Simulation , Electromyography/methods , Isometric Contraction/physiology , Models, Neurological , Muscle Fatigue/physiology , Muscle, Skeletal/physiology , Myofibrils/physiology , Neural Conduction/physiology , Signal Processing, Computer-Assisted , Action Potentials , Adult , Analysis of Variance , Bias , Confounding Factors, Epidemiologic , Female , Humans , Male , Sensitivity and Specificity
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