A Variance Distribution Model of Surface EMG Signals Based on Inverse Gamma Distribution

Objective: This paper describes the formulation of a surface electromyogram (EMG) model capable of representing the variance distribution of EMG signals. Methods: In the model, EMG signals are handled based on a Gaussian white noise process with a mean of zero for each variance value. EMG signal variance is taken as a random variable that follows inverse gamma distribution, allowing the representation of noise superimposed onto this variance. Variance distribution estimation based on marginal likelihood maximization is also outlined in this paper. The procedure can be approximated using rectified and smoothed EMG signals, thereby allowing the determination of distribution parameters in real time at low computational cost. Results: A simulation experiment was performed to evaluate the accuracy of distribution estimation using artificially generated EMG signals, with results demonstrating that the proposed model's accuracy is higher than that of maximum-likelihood-based estimation. Analysis of variance distribution using real EMG data also suggested a relationship between variance distribution and signal-dependent noise. Conclusion: The study reported here was conducted to examine the performance of a proposed surface EMG model capable of representing variance distribution and a related distribution parameter estimation method. Experiments using artificial and real EMG data demonstrated the validity of the model. Significance: Variance distribution estimated using the proposed model exhibits potential in the estimation of muscle force.

I. Introduction

Surface electromyogram (EMG) signals, which can be measured from the skin surface, reflect muscular activity. Examples of their application in a wide range of fields, such as rehabilitation, prosthesis control, and motion analysis [1]– [11], include work by Song et al. [1] who conducted myoelectrically controlled upper-limb training for poststroke patients and showed resulting improvement of upper-limb function. Fukuda et al. [4] and Shenoy et al. [5] also achieved multifunctional prosthesis control using EMG signals measured from multiple electrodes.