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1 Centro di Bioingegneria, Dipartimento di Elettronica, Politecnico di Torino, Torino 10129; 2 Dipartimento Fisiologia Umana e Farmacologia, and 3 Scuola di Medicina dello Sport, Università di Roma, and 5 Facoltà di Ingegneria, Dipartimento INFOCOM and 6 Centro Interdipartimentale Sistemi Biomedici, Università degli Studi, La Sapienza, Roma 00185; and 4 Facoltà di Scienze Motorie, Istituto Universitario di Scienze Motorie, Roma 00194, Italy
Amplitude and
frequency content of the surface electromyographic (EMG) signal
reflect central and peripheral modifications of the neuromuscular
system. Classic surface EMG spectral variables applied to
assess muscle functions are the centroid and median power
spectral frequencies. More recently, nonlinear tools have been
introduced to analyze the surface EMG; among them, the recurrence quantification analysis (RQA) was shown to be particularly promising for the detection of muscle status changes. The purpose of this work
was to analyze the effect of motor unit short-term synchronization and
conduction velocity (CV) on EMG spectral variables and two variables
extracted by RQA, the percentage of recurrence (%Rec) and determinism
(%Det). The study was performed on the basis of a simulation model,
which allowed changing the degree of synchronization and mean CV of a
number of motor units, and of an experimental investigation of the
surface EMG signal properties detected during high-force-level
isometric fatiguing contractions of the biceps brachii muscle.
Simulations and experimental results were largely in agreement and show
that 1) spectral variables, %Rec, and %Det are influenced
by CV and degree of synchronization; 2) spectral variables
are highly correlated with %Det (R =
0.95 in the
simulations and
0.78 and
0.75 for the initial values and normalized
slopes, respectively, in the experimental signals), and thus the
information they provide on muscle properties is basically the same;
and 3) variations of %Det and %Rec in response to changes
in muscle properties are significantly larger than the variations of
spectral variables. This study validates RQA as a means for fatigue
assessment with potential advantages (such as the higher sensitivity to
changes of muscle status) with respect to the classic spectral analysis.
surface electromyography; motor unit short-term synchronization; surface electromyographic modeling; recurrence plot analysis
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