The Probability Density Function (PDF) of surface EMG with increasing force: a comparison between the tibialis anterior and the vastus lateralis

Objectives

The process by which the surface EMG signal is progressively filled up with motor unit potentials has so far been investigated only in the quadriceps muscles. However, the sEMG filling process is influenced by anatomical, physiological, and neural factors, and thus may be different for each muscle. Here, we sought to characterize the sEMG filling process of the tibialis anterior (TA) and compare it to that of the vastus lateralis (VL).

Methods

Surface EMG signals were recorded from the TA and VL muscles of healthy subjects as force was gradually increased from 0 to 80% MVC. The sEMG filling process was analyzed by measuring the EMG filling factor (FF), an index determined by the shape of the probability density function (PDF) of the sEMG signal.

Results

(1) The sEMG filling process showed significant differences between the TA and VL muscles (p < 0.05).

(2) In the TA, the degree of sEMG filling at the onset of the contraction was low (FF < 0.45) for 72 % of male subjects and 53 % of female subjects, whereas, in the VL, the degree of initial sEMG filling was low for 89 % of male subjects, but only in 12 % of female subjects.

(3) In the TA, the sEMG at high forces (>40 % MVC) contained spikes with largely different amplitudes (FF ∼ 0.5), whereas, in the VL, the sEMG contained spikes with comparable amplitudes (FF ∼ 0.63).

Conclusions

(1) At high forces, the TA PDF was close to Laplacian, whilst the VL PDF was nearly Gaussian; (2) The sEMG filling curves are more informative in the TA than in the VL; (3) The sEMG filling process is muscle and gender specific.