Spectral analysis of surface motor unit action potentials and surface interference electromyogram

One-Channel Surface Electromyography Decomposition for Muscle Force Estimation

Estimating muscle force by surface electromyography (sEMG) is a non-invasive and flexible way to diagnose biomechanical diseases and control assistive devices such as prosthetic hands. To estimate muscle force using sEMG, a supervised method is commonly adopted. This requires simultaneous recording of sEMG signals and muscle force measured by additional devices to tune the variables involved. However, recording the muscle force of the lost limb of an amputee is challenging, and the supervised method has limitations in this regard. Although the unsupervised method does not require muscle force recording, it suffers from low accuracy due to a lack of reference data. To achieve accurate and easy estimation of muscle force by the unsupervised method, we propose a decomposition of one-channel sEMG signals into constituent motor unit action potentials (MUAPs) in two steps: (1) learning an orthogonal basis of sEMG signals through reconstruction independent component analysis; (2) extracting spike-like MUAPs from the basis vectors. Nine healthy subjects were recruited to evaluate the accuracy of the proposed approach in estimating muscle force of the biceps brachii. The results demonstrated that the proposed approach based on decomposed MUAPs explains more than 80% of the muscle force variability recorded at an arbitrary force level, while the conventional amplitude-based approach explains only 62.3% of this variability. With the proposed approach, we were also able to achieve grip force control of a prosthetic hand, which is one of the most important clinical applications of the unsupervised method. Experiments on two trans-radial amputees indicated that the proposed approach improves the performance of the prosthetic hand in grasping everyday objects.

Relation between the size of motor units and the spectral characteristics of their action potentials. kazuseki@iname.com

OBJECTIVE

The purpose of this study was to clarify the influence of the size of an individual motor unit on the spectral characteristics of its motor unit action potentials (MUAPs).

METHODS

In 4 human subjects, we first averaged the isometric force and the surface EMG signals triggered by the intramuscularly recorded action potentials of each motor unit during voluntary isometric contraction. Then, we obtained averaged twitch contraction curves, averaged MUAPs derived from surface EMG signals (S-MUAPs), and the power spectrum of the S-MUAPs. Finally, we tested for correlations among these results in each subject.

RESULTS

There was a positive correlation among the mean power frequency of the S-MUAPs, the maximal amplitude of the S-MUAPs, and the maximal twitch tension of their motor unit in each subject (r = 0.58-0.85; P < 0.05).

CONCLUSIONS

Since the twitch tension and the amplitude of the S-MUAPs are well-known predictors of the size of motor units, we conclude that the spectral characteristics of the S-MUAPs reflect the size of the motor unit from which they originate.

AAEE minimonograph #29: automatic quantitative electromyography

The present status of different computerized methods of automatic quantitative electromyography are reviewed. Interference pattern methods-turns analysis, spectral analysis-are efficient, but the results usually cannot be directly related to the physiological properties of the motor units. Integration analysis does not currently have a major role in diagnostic electromyography. Traditional measurement of single motor unit action potentials during weak contraction can be facilitated and made more objective with computer assistance, but only the lowest-threshold motor units in the muscle are amenable to study. A new class of methodologies under development permit the decomposition of interference patterns into their constituent motor unit action potentials for measurement of configurational and behavioral properties. Patient data from these various methods can be statistically compared with normative data bases available on-line in computerized electromyographs. Both quantitative and quantitative electromyography have applications in the neuromuscular electrodiagnostic examination.

Automatic analysis of surface EMG (preliminary findings in healthy subjects and in patients with neurogenic motor diseases)

Supramaximal electrical stimulation of a motor nerve produces a full contraction of a muscle and the corresponding compound action potential can be recorded. Recent studies appear to support the view that all the motor units are activated during voluntary maximal contraction, at least in the tibialis anterior muscle. The compound action potential and the EMG interference pattern in the tibialis anterior are regarded as two different manifestations of the activation of all the motor units. A method has been developed which compares these EMG activities, by automatic analysis, in order to obtain useful parameters for clinical applications.