Characteristics of the Post-Activation Effect of Human Skeletal Muscles Using Spectral and Non-Linear Parameters of Interference Electromyogram

The post-activation effect (PAE) is a special kind of involuntary muscle tone (tonic automatism) that is generated in the “tonogenic” structures of the brain, presumably without involving the mechanisms of the “sensory copy” and “motor command.” In this regard, the PAE electromyogram (EMG) signal may have a simpler temporal structure compared to the PAE-inducing random activity. The purpose of this work is to characterize the temporal structure and complexity of interference EMG (iEMG) of the deltoid and biceps muscle of the human shoulder using the fractal (D) and correlation dimension (D_c). It was found that in deltoid muscles the D value was 1.78–1.81 both during PAE and random effort (p > 0.05). D_c ranged between 4.0–4.2 and also did not differ between PAE and voluntary effort, although the mean frequency of the iEMG spectrum during PAE was 15–16 Hz (p < 0.05) higher compared to random effort. In the biceps brachii D was 1.8 during PAE and 1.68 during voluntary effort (p < 0.05), and D_c was not different (4–4.8). Thus, despite the supposed difference in the central organization of PAEs and voluntary effort, the temporal structure of their iEMGs did not differ, indicating that isometric random effort and involuntary tone in the form of PAEs share a common principle of EMG signal generation. At the same time, the difference in the frequency of the iEMG spectrum indicates the peculiarities of the organization of PAE and arbitrary reduction at the level of the motoneuronal pool.